Re: Linux vs Windows Drivers

Bit <bit2300@xxxxxxxxx> · Wed, 26 Dec 2007 17:01:54 -0500

Luciano Rocha wrote:
On Wed, Dec 26, 2007 at 04:01:22PM -0500, Bit wrote:

 Luciano Rocha wrote:

On Wed, Dec 26, 2007 at 12:48:52PM -0500, Bit wrote:

 Thanks to both of you for the reply.  Good information, but that still  
doesn't really answer my question.  I'm more interested in the technical  
side of things.  What I really want to understand boils down to this:

 Why is it that in Windows I can install ATI drivers once and never worry  
about it again, while in Linux I may have to *reinstall* the drivers at a  
later date after a system update to get my card working with them again?   
Experience has proven to me that in Windows I can install the ATI drivers  
once, leave those same drivers on there for eternity, update the system 
over  and over with Automatic Updates, and never worry about it breaking 
my video  card.  In Linux, every time I see a kernel update, I've learned 
to be braced  for impact and just be ready with my ATI drivers to 
reinstall to get my card  working again.  I've never understood this.  I'd 
like a technical  explanation for why this is so.

Linux doesn't have a stable ABI (for drivers, userland is a different
thing), but Windows does.

That means that drivers compiled for your kernel today may not install
on newer (or older) kernels. You'll have to recompile it. Also, changes
like support for more than 4GB, how the lower 4GB is split, architecture
options, gcc version, function calling convections, etc., creates
dependencies that have to be met by the binary driver.

Windows guarantees that the exposed interface doesn't change, so there's
no need to recompile things if something internal changes.

But Linux doesn't even have a stable API, so the module may not compile
on your newer kernels.

Please see Documentation/stable_api_nonsense.txt, in the kernel sources,
or online at:
<http://scienceblogs.com/gregladen/2007/12/linux_stable_api_vs_not.php>

Note that without a stable API, there is no change of a stable ABI.

 Luciano, thank you very much.  I read your post, the link you provided, and 
 a few other things from that link, and I at least understood enough to 
 realize that it answers my question.  I think I *kind of* get it now.

 I think understanding the answer to my question really revolves around 
 understanding an API and an ABI.  Would you please read the following and 
 let me know if I at least get the gist of what these two things are?

FYI, when in doubt about these acronyms, search for "define: ABI", for
instance, in google.

 An API influences what your source code will look like.  If they change the 
 Linux kernel API, then you may need to make changes to your source code such 
 as making "myLinuxKernelAPIFunctionCall( myparam1, myparam2 )" look 
 something more like "myUpdatedLinuxKernelAPIFunctionCall( myparam1 )" in 
 order to even make your code compile.

Yes, but also more important things. Like changing the use of semaphores
to mutexes, when appropriate (they "lock" something, mutexes can have
only one accessing the something at once, while semaphores can have N
accessing); changing the way stuff is exported to userland (sysfs,
configfs, debugfs, relayfs, procfs); etc..

 The ABI is the interface between a compiled binary kernel module and the 
 kernel.  It determines if an already compiled binary will properly interface 
 with the kernel and run.  If the ABI changes and you find your kernel module 
 won't run properly, you just need to recompile from source to get a kernel 
 module that will run.  Hopefully the API hasn't changed and you won't need 
 to change your source code to make it recompile...

Yes, that's it. The compiled modules include the dependency info, so
that you won't be able to insert it in another kernel:
$ modinfo ext2.ko
filename:       ext2.ko
license:        GPL
description:    Second Extended Filesystem
author:         Remy Card and others
depends:
vermagic:       2.6.23.12lcfs1 preempt mod_unload PENTIUM4 4KSTACKS

 BOTH kinds of changes happen with some degree of frequency in Linux.

Yes. Due to the nature of the kernel (open-source, GPL), and the current
policy, changes occur *very* frequently, especially in the 2.6.x series.

 Did I get at least this much right?

I think you're doing fine. Note that this state of affairs is more due
to how the kernel people work/philosophy, than due to technical
limitations, as Les Mikesell pointed out.

Anyway, nifty things have come from this development method in the
latest Linux kernels.

Also, there are some stable apis/abis: userland driver (for simple
devices, no dma is possible, for instance), and userland access to usb
devices (mostly used for printers, scanners and non-standard usb memory
sticks/mp3 players).

Thank you again for your help!  You've given me a place to really get 
started and this doesn't seem quite so mysterious to me anymore.  I have 
one last related burning question for now that I was hoping you might be 
able to answer.

ATI drivers are proprietary and closed-source.  So, for example, on my 
current desktop, I download the Linux drivers for my card from the link 
below and run the installer as per their instructions.
http://ati.amd.com/support/drivers/linux/linux-radeon.html

It's doing *something* to make a kernel module that will insert into and 
work with my current running kernel.  At one time, I thought that it was 
compiling a module from source code, probably by invoking make, in much 
the same way I might download and install any open-source software in 
Linux from a tarball.

However, I realized that this doesn't make sense since ATI's drivers are 
proprietary and closed-source.  So the installer I download can't 
possibly be compiling anything from source code, because that would mean 
I could almost certainly read the source code, which they don't want.  
Which leaves me wondering what the installer is really doing.  Any ideas?

Thanks!
bit
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