hi I'm really interested in Linux kernel development but I'm having trouble understanding a few big concepts.
First, is a device driver programmer and embedded programmer two completely differnet jobs??
I mean I know embedded programmers get in to firmwares and circuits and stuff like that
but do device driver programmers also do the same thing?
Second, I heard that it's good to start linux kernel development by writing device drivers.
Does this mean you have to master device drivers?
Frankly, i want to know exactly what each of these programmers do
what I really want is to understand ARM and x86 based linux kernel and get in to development
and I'm just curious do I really have to know all the circuits and stuff like that.
P.S, is a system programmer also a differnet job??
Some concepts within today's Linux kernel are really complex: scheduling, memory management (MM), locking, stuff specific to each architecture, security, etc.
However, it is generally true that device drivers are somewhat simple (that is, compared to the rest), because their job is usually to act as bridges between userspace interfaces and the actual drived device. Consequently, they seldom play with the internal mechanics of the kernel, except for the drivers API, of course. Also, the kernel community is much more inclined to accept device drivers contributions since they affect only specific use cases (whereas contributions to the core, like MM or security, affect everyone).
Now, before trying anything on the kernel side, make sure to understand the userspace properly. Because, as mentioned above, device drivers register functions that get called when a user calls specific system calls (syscalls) on the special file representing the device. You must then understand very well those system calls.
Before actually writing code, go read a few books mentioned here. They are technical books, but you will need to understand the subject properly.
Also, go read actual code. That's probably the best way to learn: looking at what others did (which is, after all, the very essence of free software). You can start by looking at simple drivers, like the ones in drivers/leds.
About job titles: an "embedded programmer" (or embedded computer engineer) is of course a good candidate for writing device drivers. Embedded computer engineers may also write firmwares, microcontroller programs and actual userspace applications that act close to hardware components (device control, serial protocols, etc.).
You don't need to master circuits to write device driver code, but you certainly need to understand core hardware concepts like interrupts, memory-mapped I/O, timing, buses, locking, power management and possibly some assembly language.
Related
So, I know Linux kernel is quite "heavy" when considering lower scale embedded systems, but currently but we're a 2 man team trying to see how to create our own embedded system.
I'm the one in charge of all software (the other guy is a HW guy), and thus I would like to re-use existing libraries and frameworks as much as possible, and I would like to bounce off some ideas with gurus around here.
I am fairly comfortable in Linux, but the booting and initialization process is new to me, and I need to dive in to that soon enough. Any book recommendations are welcome as well!
I haven't designed any embedded systems before.. Only own some ARM dev boards (beagleboard and raspberry pi).
Current I have prototype of the software running on beagleboard already, and now we're thinking how to minimize the cost, and to create something our own..
It's a system connected to the internet, and I need to run a tiny web server with some scripting support. Performance wise I don't think it needs to be too powerful.
I would like to minimize all bootloader etc work, since I'm a one man SW team, and just concentrate on the application itself.
Of course I understand that I need to configure our kernel for this, but this is indeed why I thought selecting some SoC would be good, since they usually have some linux and bootloaders ready..
First I thought that Cirrus EP9301 would be perfect, since it seems to be a good package, and not very expensive.. But it seems that it's already in end-of-life, and also support for this is very bad (people on the cirrus forums constantly complain about it).
Are there some good choices for this kind of project, which would enable us "easily" to get linux kernel up and running, with still maintaining some kind of decent BOM (hopefully 20USD or so) ?
Your hardware guy should already know this, but go with an existing reference design. Take the raspberry pi, the beagleboard/bone, open-rd, or any number of other existing systems and clone the part you need. As a result the linux porting will be a matter of removing what you are not using from the reference design instead of adding new stuff and hoping it works. If you go with flat pack parts you can do the work in your garage, if you go with bgas you need the equipment for that or pay someone to do it. (can you tell yet that I hate bgas?).
Is linux a requirement, if not that opens the door to a lot more devices using freertos or chibios or a number of other solutions. the stm32f4 discovery board for example is $20, uses what can barely be called a microcontroller for all the features it has (cortex-m4). Supposedly possible to run uclinux on a cortex-m, but definitely possible to run any number of rtoses and have an ip stack, etc. stellaris (ti.com) has a number of eval boards, one/some with ethernet already (use as a reference design). You can also take the wiznet approach (or a spi ethernet) and use any microcontroller (puts you into the avr/msp430 level and price range). Bang for buck the cortex-m's are good, arm based so comfortable to work with, etc.
Using linux if you are already not an experienced at porting to an embedded platform, and dont want to learn that on this go around, I would definitely go with a clone of an existing design, leverage as much as you can from a project with folks that are experienced at porting linux to a platform. If need be take an existing board (beagle/raspi/openrd) and go through the motions of porting to the platform with the cheat sheet of having access to an existing port, see if you cant get uboot ported and linux booting, etc. (dont really need uboot at all, that is possibly an unnecessary complication, just get dram up and pass the atags, etc to linux and just branch to it, pretty easy to launch linux from bare metal).
You could probably do worse than taking the Broadcom BCM2835 - used on the Raspberry Pi - as your starting point - especially if you want to avoid kernel and boot-loader work and a source of reference schematics. If this proves too expensive, check out other devices in the Broadcom range.
A few bits of advice
You probably want some flash rather than a MMC card interface for production use. eMMC is an option. NAND flash is a nightmare due to rapid component obsolescence and the need to get own and dirty with the MTD drivers.
USB Ethernet will be easier to integrate than a controller hanging off a general purpose bus, but won't perform as well. SmSC seems to be popular source for either
You could also have a look at the work that Olimex is doing with their linux boards. Perhaps even order a som and then combine it with other external components.
I am looking at some pointers for understanding how the Linux kernel implements the setting up of various hardware clocks. This basically relates to working with setting up the various clocks that hardware features like the LCD, UART etc will use. For example when Linux boots how does it handle setting up the clocks for UART or USB. Maybe something like a Clock manager or something.
I am basically trying to implement something similar for a different OS on a new hardware that i am working on. Any help would be really appreciated.
[Edit]
Thanks for the replies and the links. So here is what i have implemented up until now. This should give you an idea of where I'm headed.
I looked up the Hardware Reference Manual for the particular system I'm targeting and wrote some code to monitor/modify the signals/pins of the peripherals I am interested in i.e. turning them ON/OFF from the command line.Now a collection of these clocks/signals together control a peripheral.The HRM would say that if you want to turn on the UART or something then turn on such and such signals/pins. And #BjoernD yes I am using something like a mmap() function to talk to the peripherals.
The meat of my question is that I want to understand the design and implementation of a Clock/Peripheral Manager which uses the utility that I have already written. This Clock/Peripheral Manager would give me the control of enabling/disabling the peripherals I want.Basically this Manager would enable me to make changes in the init code that is right now running. Also during run time processes can call this Manager to turn ON/OFF the devices so that power consumption is optimized. It might not have made perfect sense but I'm myself trying to wrap my head around this.
Now I'm sure something like this would have been implemented in Linux or for that matter any OS for performance issues (nobody would want to waste power by turning on all peripherals at boot time). I want to understand the Software Architecture of it. Reference from any OS would do as of now to atleast get a headstart. Also I am not writing my own OS, there is an OS in place but Im looking more at a board level software aka BSP to work on. But thanks for the OS link anyways, they are really good. Appreciate it.
Thanks!
What you want to achieve is highly specific to a) the platform you are using and b) the device you want to use. For instance, on x86 there are 3 ways to communicate with a device:
Interrupts allow the device to signal the CPU. The OS usually provides mechanisms to register interrupt handlers - functions that are called upon occurrence of an interrupt. In Linux see request_irq() and friends in linux/include/interrupt.h
Memory-mapped I/O is physical memory of the device that the platform's BIOS makes available in the same way you also access plain physical memory - simply by writing to a memory address. What exactly is behind such memory (e.g., network interface config registers or an LCD frame buffer) depends on the device and is usually specified in the device's data sheet.
I/O ports are accessed through a special address space and special instructions (INB/OUTB & co.). Other than that they work similar to I/O memory.
There's a multitude of ways to find out what resources a device provies and where the BIOS mapped them. Some platforms use ACPI tables (google yourself for the 1,000k page spec), PCI provides info on devices in a standardized way through the PCI config space, USB has similar ways of discovering devices attached to the bus, and some devices, e.g., UARTS, are simply specified to be available at a pre-configured I/O range that is fixed for your platform.
As a start for understanding Linux, I'd recommend "Understanding the Linux kernel". For specifics on how Linux handles devices and what is there to write drivers, have a look at Linux Device Drivers. Furthermore, you will need to have a look at the peculiarities of your platform and the device you want to drive.
If you want to start an own OS, a UART is certainly something that will be veeery helpful to print debug output, so you might want to go for this first.
Now that I wrote down all this, it seems that your actual question is: How to get started with Operating System design. This question should be highly valuable for you: What are some resources for getting started in operating system development?
The two big power users in most computers are the CPU and the disks. Both of these have capabilities for power saving in Linux. The CPU clock can be slowed down when the system is not busy, and the disk motors can be stopped when no I/O is happening. For a UART, even if you save all of the power that it uses by turning off its clock, it is still tiny compared to the others because a UART doesn't have much logic in it.
Best ways to save power are
1) more efficient power supply
2) replace rotating disk with SSD
3) Slow down the CPU and memory bus
I plan to develop a nice little application that will run on an arm-based embedded Linux platform; however, since that platform will be battery-powered, I'm searching for relevant information on how to handle power save.
It is kind of important to get decent battery time.
I think the Linux kernel implemented some support for this, but I can't find any documentation on this subject.
Any input on how to design my program and the system is welcome.
Any input on how the Linux kernel tries to solves this type of problem is also welcome.
Other questions:
How much does the program in user space need to do?
And do you need to modify the kernel?
What kernel system calls or APIs are good to know about?
Update:
It seems like the folks involved with the "Free Electrons" site have produced some nice presentations on this subject.
http://free-electrons.com/services/power-management/
http://free-electrons.com/docs/power
http://free-electrons.com/docs/optimizations
But maybe someone else has even more information on this subject?
Update:
It seems like Adam Shiemke's idea to go look at the MeeGo project may be the best tip so far.
It may be the best battery powered Embedded Linux project out there at this moment.
And Nokia is usually kind of good at this type of thing.
Update:
One has to be careful about Android since it has a "modified" Linux kernel in the bottom, and some of the things the folks at Google have done do not use baseline/normal Linux kernels. I think that some of their power management ideas could be troublesome to reuse for other projects.
I haven't actually done this, but I have experience with the two apart (Linux and embedded power management). There are two main Linux distributions that come to mind when thinking about power management, Android and MeeGo. MeeGo uses (as far as I can tell) an unmodified 2.6 kernel with some extras hanging on. I wasn't able to find a lot on exactly what their power management strategy is, although I suspect more will be coming out about it in the near future as the product approaches maturity.
There is much more information available on Android, however. They run a fairly heavily modified 2.6 kernel. You can see a good bit on the different strategies implemented in http://elinux.org/Android_Power_Management (as well as kernel drama). Some other links:
https://groups.google.com/group/android-kernel/browse_thread/thread/ee356c298276ad00/472613d15af746ea?lnk=raot&pli=1
http://www.ok-labs.com/blog/entry/context-switching-in-context/
I'm sure that you can find more links of this nature. Since both projects are open source, you can grab the kernel code, and probably get further information from people who actually know what they are talking about in forms and groups.
At the driver level, you need to make sure that your drivers can properly handle suspend and shut devices off that are not in use. Most devices aimed at the mobile market offer very fine-grained support to turn individual components off, and to tweak clock settings (remember, power is proportional to clock^2).
Hope this helps.
You can do quite a bit of power-saving without requiring any special support from the OS, assuming you are writing (or at least have the source code for) your application and drivers.
Your drivers need to be able to disable their associated devices and bring them back up without requiring a restart or introducing system instability. If your devices are connected to a PCI/PCIe bus, research which power states they support (D0 - D3) and what your driver needs to do to transition between these low-power modes. If you are selecting hardware devices to use, look for devices that adhere to the PCI Power Management Specification or have similar functionality (such as a sleep mode and a "wake up" interrupt signal).
When your device boots up, every device that has the ability to detect whether it is connected to anything needs to do so. If any ports or buses detect that they are not being used, power them down or put them to sleep. A port running at full power but sitting unused can waste more power than you might think it would. Depending on your particular hardware and use case, it might also be useful to have a background app that monitors device usage, identifies unused/idle resources, and acts appropriately (like a "screen saver" for your hardware).
Your application software should make sure to detect whether hardware devices are powered up before attempting to use them. If you need to access a device that might be placed in a low-power mode, your application needs to be able to handle a potentially lengthy delay in waiting for the device to wake up and respond. Your applications should also be considerate of a device's need to sleep. If you need to send a series of commands to a hardware device, try to buffer them up and send them out all at once instead of spacing them out and requiring multiple wakeup->send->sleep cycles.
Don't be afraid to under-clock your system components slightly. Besides saving power, this can help them run cooler (which requires less power for cooling). I have seen some designs that use a CPU that is more powerful than necessary by a decent margin, which is then under-clocked by as much as 40% (bringing the performance down to the original level but at a fraction of the power cost). Also, don't be afraid to spend power to save power. That is, don't be afraid to use CPU time monitoring hardware devices for opportunities to disable/hibernate them (even if it will cause your CPU to use a bit more power). Most of the time, this tradeoff results in a net power savings.
One of the most important things to think of as a power aware application developer is to avoid unnecessary timers. If possible use interrupt driven solutions instead of polled solutions. If a timer must be used then use as long poll interval as is possible.
For example if something special should be done at a certain room temperature it is unnecessary to check the temperature every 100 ms since temperature in a room changes slowly. A more reasonable polling interval is could be 60 s.
This affects the power consumption in several ways. In Linux the CPUIDLE subsystem takes the CPU (SOC) to as deep power saving state as possible depending on when it predicts the next wakeup to occur. Having a lot of timers in a system will fragment the sleep making it impossible to go to the deeper sleep states for longer periods. A typical deep sleep state for CPUIDLE turns the CPU off but keeps the RAM in self refresh. When a timer triggers the CPU will boot and serve the timer of the application.
It's not actually your topic, but it might come in handy to log your progress: i was looking for testing / measuring my embedded linux system. chris desjardins from this forum recommended me this:
I have successfully used bootchart in the past:
http://elinux.org/Bootchart
Here is a list of other things that may also help:
http://elinux.org/Boot_Time
I need to produce an embedded ARM design that has requirements to do many things that embedded Linux would do. However the design is cost sensitive and does not need huge amounts of horse power. Mostly will be talking to serial interfaces. Ideally I would like to use one of the low end ARMs. What is the lowest configuration of an ARM that you have successfully used embedded Linux on.
Edit:
The application needs a file system on some kind of flash device and the ability to run applications for processing the data. Some of the applications might be written by others than myself. I also need to ability to load new applications or update old apps using the serial ports to accept the apps.
When I have looked at other embedded OSes they seem to be more of a real time threading solution than having the ability to run applications. I am open to what ever will get the job done.
I think you need to weigh your cost options here.
ARM + linux is an option but you will be paying a very high operating overhead for such a simple (from your description) set of features. You can't just look at the cost of the ARM chip but must also consider external RAM which will very likely be required as well as flash to get enough space available to run the kernel + apps.
NOTE: you may be able to avoid the external requirements with a very minimal kernel and simple apps combined with a uC with large internal resources.
A second option is a much simpler microcontroller with a light weight OS. This will cut your hardware costs on the CPU and you can likely run something like this without external RAM or flash (dependent on application RAM and program space requirement)
third option: I don't actually see anything in your requirements that demands any OS at all be used. Basic file systems are very simple, for instance there are even FAT drivers out there for 8 bit PIC's. Interfacing to an SD card only requires a SPI port and minimal external circuitry.
The application bit could be simple or complex. I've built systems around PIC18 microcontollers that run a web server and allow program updates via a simple upload screen, it just stores the new program into an EEPROM or flash, reboots into a bootloader and copies the new program into internal program memory. You could likely design a way to do this without the reboot via a cooperative multitasking type of architecture. Any way you go the programmers writing the apps are going to need to have knowledge of the architecture and access to libraries / driver you write. Your best bet to simplify this is to provide as simple an API as possible and to try to automate the build process for them.
The third option will be the "cheapest" in terms of hardware as there will be very little overhead in the processing of your applications allowing you to get away with minimal processing power and memory. It likely will require some more programming/software architecting on your part but won't require nearly the research you will need to undertake to get linux up and running in addition to learning to write the needed device drivers under a linux paradigm.
As always you have to include the software development costs in the build cost of the device. If you plan to build 10,000+ of these your likely better off keeping hardware costs down and putting more man power into designing a software solution that allows that hardware to meet the design goals. If your building 10 of them, your better off spending an extra $15-20 on hardware if it can cut down on your software development costs. For example an ARM with MMU with full linux kernel support and available device drivers.
I kind of feel that your selecting the worst of both worlds at the moment, your paying extra to get a uC you can run linux on but by doing so your also selecting a part that will likely be the most complex to get linux up and running on, especially having not worked with linux on embedded platforms before.
I've had success even on ARM7TDMI, so I don't think you're going to have any trouble. If you have a low-requirements system, you could use any kind of lightweight real-time executive and have a lot better experience than you would getting Linux to work.
I've used a TS-7200 for about five years to run a web server and mail server, using Debian GNU Linux. It is 200 MHz and has 32 MB of RAM, and is quite adequate for these tasks. It has serial port built in. It's based on a ARM920T.
This would be overkill for your job; I mention it so you have another data point.
For several years I've been using a gumstix to do prototyping and testing and I've had good results with it. I don't know if the processor they are using (Intel PXA255 on my board) is considered low-cost, but the entire Verdex line seems pretty cheap to me for an adaptable device.
ucLinux is designed specifically for resource constrained targets, but perhaps more importantly for targets without an MMU.
However you have to have a good reason to use Linux on such a system rather than a small real-time executive. Out-of-the-box networking, readily available drivers and protocol stacks for complex hardware and support for existing POSIX legacy or open source code are a few perhaps. However if you don't need that, Linux is still large, and you may be squandering resources for no real benefit. In most cases you will still need off-chip SDRAM and Flash if you choose Linux of any flavour.
I would not regard serial I/O as 'complex hardware', so unless you are running a complex, but standard protocol, your brief description does not appear to warrant the use of Linux IMO
My DLINK DIR-320 router runs Linux inside.
And I know some handymen, flashing it with Optware and connecting USB-hub, HDDs, USB-flash, and much more.
It's low-cost ready for use "platform". (If you don't need mass production). But maybe more powerful than you need.
Additionally, it can be configured wirelessly via web-interface even through your pda :)
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I was always attracted to the world of kernel hacking and embedded systems.
Has anyone got good tutorials (+easily available hardware) on starting to mess with such stuff?
Something like kits for writing drivers etc, which come with good documentation and are affordable?
Thanks!
If you are completely new to kernel development, i would suggest not starting with hardware development and going to some "software-only" kernel modules like proc file / sysfs or for more complex examples filesystem / network development , developing on a uml/vmware/virtualbox/... machine so crashing your machine won't hurt so much :) For embedded development you could go for a small ARM Development Kit or a small Via C3/C4 machine, or any old PC which you can burn with your homebrew USB / PCI / whatever device.
A good place to start is probably Kernelnewbies.org - which has lots of links and useful information for kernel developers, and also features a list of easy to implement tasks to tackle for beginners.
Some books to read:
Understanding the Linux Kernel - a very good reference detailing the design of the kernel subsystems
Linux Device Drivers - is written more like a tutorial with a lot of example code, focusing on getting you going and explaining key aspects of the linux kernel. It introduces the build process and the basics of kernel modules.
Linux Kernel Module Programming Guide - Some more introductory material
As suggested earlier, looking at the linux code is always a good idea, especially as Linux Kernel API's tend to change quite often ... LXR helps a lot with a very nice browsing interface - lxr.linux.no
To understand the Kernel Build process, this link might be helpful:
Linux Kernel Makefiles (kbuild)
Last but not least, browse the Documentation directory of the Kernel Source distribution!
Here are some interesting exercises insolently stolen from a kernel development class:
Write a kernel module which creates the file /proc/jiffies reporting the current time in jiffies on every read access.
Write a kernel module providing the proc file /proc/sleep. When an application writes a number of seconds as ASCII text into this file ("echo 3 > /proc/sleep"), it should block for the specified amount of seconds. Write accesses should have no side effect on the contents of the file, i.e., on the read accesses, the file should appear to be empty (see LDD3, ch. 6/7)
Write a proc file where you can store some text temporarily (using echo "blah" > /proc/pipe) and get it out again (cat /proc/pipe), clearing the file. Watch out for synchronisation issues.
Modify the pipe example module to register as a character device /dev/pipe, add dynamic memory allocation for write requests.
Write a really simple file system.
An absolute must is this book by Rubini. (available both as a hardcopy or a free soft copy)
He gives implementations of several dummy drivers that don't require that you have any hardware other than your pc. So for getting started in kernel development it's the easiest way to go.
As for doing embedded work I would recommend purchasing one of the numerous SBC (single board computers) that are out there. There are a number of these that are based on x86 processors, usually with PC/104 interfaces (electrically PC/104 is identical to the ISA bus standard, but based on stackable connectors rather than edge connectors - very easy to interface custom hardware to)
They usually have vga connectors that make it easier to do debugging.
For embedded Linux hacking, simple Linksys WRT54G router that you can buy everywhere is a development platform on its own http://en.wikipedia.org/wiki/Linksys_WRT54G_series:
The WRT54G is notable for being the first consumer-level network device that had its firmware source code released to satisfy the obligations of the GNU GPL. This allows programmers to modify the firmware to change or add functionality to the device. Several third-party firmware projects provide the public with enhanced firmware for the WRT54G.
I've tried installing OpenWrt and DD-WRT firmware on it. You can check those out as a starting point for hacking on a low-cost platform.
For starters, the best way is to read a lot of code. Since Linux is Open Source, you'll find dozens of drivers. Find one that works in some ways like what you want to write. You'll find some decent and relatively easy-to-understand code (the loopback device, ROM fs, etc.)
You can also use the lxr.linux.no, which is the Linux code cross-referenced. If you have to find out how something works, and need to look into the code, this is a good and easy way.
There's also an O'Reilly book (Understanding the Linux Kernel, the 3rd edition is about the 2.6 kernels) or if you want something for free, you can use the Advanced Linux Programing book (http://www.advancedlinuxprogramming.com/). There are also a lot of specific documentation about file systems, networking, etc.
Some things to be prepared for:
you'll be cross-compiling. The embedded device will use a MIPS, PowerPC, or ARM CPU but won't have enough CPU power, memory, or storage to compile its own kernel in a reasonable amount of time.
An embedded system often uses a serial port as the console, and to lower the cost there is usually no connector soldered onto production boards. Debugging kernel panics is very difficult unless you can solder on a serial port connector, you won't have much information about what went wrong.
The Linksys NSLU2 is a low-cost way to get a real embedded system to work with, and has a USB port to add peripherals. Any of a number of wireless access points can also be used, see the OpenWrt compatibility page. Be aware that current models of the Linksys WRT54G you'll find in stores can no longer be used with Linux: they have less RAM and Flash in order to reduce the cost. Cisco/Linksys now uses vxWorks on the WRT54G, with a smaller memory footprint.
If you really want to get into it, evaluation kits for embedded CPUs start at a couple hundred US dollars. I'd recommend not spending money on these unless you need it professionally for a job or consulting contract.
I am completely beginner in kernel hacking :) I decided to buy two books "Linux Program Development: a guide with exercises" and "Writing Linux Device Drivers: a guide with exercises" They are very clearly written and provide good base to further learning.