Demystifying Device Driver Development (DDDD)

It just occurred to me recently that most people don't really have a clue about what it is that Device Driver Developers (DDDs) do for a living. Since I am a DDD myself, among other things, I thought I should make an attempt of describing what I do in a way that Common People can hopefully digest.

Since I enjoy making wannabe hilarious Tom & Jerry dialogs, I will try to put it in a dialog between Jerry the Technical Interviewer and myself. I will intentionally wear the hat of the anti-social, psycho-path, under-the-hood programmer and let Jerry extract all the information from me in a criminal-interrogation like style.

Here we go...

Jerry: So you are a DDD.
Me: Yeap.
Jerry: Can you tell me what it is that you do?
Me: Yeap.
Jerry: I mean you build device drivers right?
Me: Yeap.
Jerry: What exactly is a device driver?
Me: It's just a piece of software that controls a device. Don't you know that? Are you wasting my time?
Jerry: Yes, I guess I do, ehhh I mean I do know, but what exactly is a device?
Me: That's an interesting question so I will dignify it with an answer. A device is a clever piece of hardware. To make things simple, imagine that a device has a little CPU of its own, a CPU that can execute various tasks depending on the hardware at hand.
Jerry: Wow64, there is a CPU on these things? A CPU on a PCI card that I buy off the shelf?
Me: Duh, but of course, how else do you think they would do any useful work?
Jerry: I thought the computer's CPU was the only CPU in the block.
Me: No, devices have CPUs and depending on the hardware they can be pretty sophisticated.
Jerry: That's awesome! What can you tell me about these little CPUs?
Me: For starters, they are not little. Most of them can do several tasks at the same time and they can be pretty darn fast.
Jerry: Hmmm, wherever there is a CPU, there is a program right?
Me: Yeap.
Jerry: Well, where are the programs that these CPUs execute? Firmware stored in flash memory may be?
Me: You'd wish.
Jerry: Please elaborate, I'm getting really curious here.
Me: OK, if you insist... These programs are generated by the device driver.
Jerry: Pardon me?
Me: The programs that the device CPUs execute are generated by the device drivers. Clearer now?
Jerry: Wow64!
Me: Indeed.
Jerry: Generated by the device driver... In what language?
Me: That was a good one! Machine language of course.
Jerry: You mean mov ecx,edx and the like?
Me: Well, in a sense yes. Each device CPU has its own machine language.
Jerry: You are kidding me right?
Me: No way. Don't be an idiot.
Jerry: So the device driver builds machine language programs for the device CPU?
Me: Actually, it's even worse than that. These CPUs usually have many sub-CPUs, execution contexts we call them. The device driver generates programs for several (if not all) execution contexts. Each program is called a context program.
Jerry: Wow64 once again! And where are these context programs located at runtime?
Me: Well, in most cases in main memory, the little thingy you call RAM. Sometimes devices may have onboard memory for storing these programs, but in most cases it's just RAM.
Jerry: In RAM? Wait wait... And how can the device CPU read from RAM? Can it handle virtual addresses and the like?
Me: Man, you are really something! Virtual addresses? What are you talking about? When was the last time you heard the term "Physical Address"? These are the kind of things these CPUs can swallow.
Jerry: Physical Address? What are YOU talking about? Are we talking about DOS here?
Me: LOL. Please realize NOW that devices are what's called "Close To The Metal". They don't and they won't understand virtual crap of any sort. They operate at the PCI level, so they will only understand PCI-level physical addresses.
Jerry: Then how does it all work?
Me: Well the device driver allocates some piece(s) of memory where it intends to store the context programs and then kindly asks the operating system to give it the physical addresses of these pieces of memory. The OS happily obliges and then the device driver prepares the context programs, using physical addresses wherever a jump is required. When the context programs are ready, the device driver sets the Program Counter register on the various sub-CPUs of the device and then starts these CPUs. Ain't it cute?
Jerry: Wowowowowow64!!! That's super cool.
Me: Yeap.
Jerry: If I understand well, the device driver is like a mini operating system for a device, right?
Me: You couldn't phrase it better.
Jerry: Now I realize why being a DDD is so tough! You have to write a mini OS of your own and make the device do your bidding!
Me: Yeah, but the truth is making the context programs and handling their operation is the easy part.
Jerry: Huh? And what is the difficult part?
Me: ERROR HANDLING.
Jerry: Please please, please elaborate!
Me: Well, it's no big deal for an experienced DDD to write a device driver for a well behaved device. The tough thing is to write a driver that does not malfunction or crash the OS in the presence of device errors. These do not happen often so it is really hard to test the error handling code. Actually if you write ANY decent program, user mode or kernel, error handling largely amounts to 50% of the code. If it does not amount to 50% in your code, then you haven't done enough error handling. No developer likes error handling (with the exception of myself of course). It's boring, it's nasty, it's creepy. But unless you do it and you TEST it throughly you can't write decent software. For device drivers this is even more crucial, since in the presence of device errors the best thing that you can hope for is that the device stops functioning (we call it the device is dead). However in the majority of cases (at least in Windows) you get an OS crash.
Jerry: And how do YOU test your error handling code?
Me: That's a trade secret.
Jerry: Com'on now...
Me: I keep an arsenal of malfunctioning devices at my office. Everyone in my team has strict instructions to immediately hand me any device that appears to be malfunctioning. They are my secret treasure.
Jerry: But how can you be sure that your driver handles correctly all possible errors?
Me: I am not.
Jerry: You're NOT???
Me: Well, why do you think it is that people still get BSODs when something unusual happens? Even if you get a driver developed by a team that takes error handling very seriously, they can't test everything, simply because they haven't seen everything yet. So you get BSODs.
Jerry: I am shocked. This is brutal.
Me: Well, as I've said elsewhere BSODs are a blessing. As far as your complaints are concerned, please pass them on to the device manufacturers. If they would put some "test mode" into their devices and make them fail randomly then MY job would be MUCH more easier. But these things would have to go to the chip level and then they would have a significant added cost as you realize. So practically everyone in the industry operates in a "hoping for the best" mode.
Jerry: ...
Me: You can't imagine what I have to go through to make buggy devices operate correctly. In your mind the term "bug" is most probably related to software, but let me break some news to you: There are COUNTLESS bugs in hardware chips and devices, and poor DDDs like me have to struggle hard to ship a decent driver only to have lusers scream their lungs out at ME when they get a BSOD. They scream because probably I didn't handle 100% correctly all the bugs in the hardware chips. But who am I supposed to scream at? Texas Instruments? Intel? You get the picture.
Jerry: Well, I can only say that under this new light I have a whole new appreciation for the work you DDDs are doing.
Me: Darn right you should.
Jerry: My special thanks for this enlightening interview.
Me: May the force be with you.
Jerry: Any last minute advice for Common People who use PCs?
Me: Don't add more RAM to your PC unless you know what you are doing.
Jerry: Excuse me? Don't add more RAM? What harm could additional RAM possibly cause???
Me: I will describe that in a separate post, when I feel like having mercy on the masses. For the time being I know, and you don't. Huh!

Have fun!
Dimitris Staikos

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