Re: Hardware Abstraction Layer

[Havoc Pennington <hp@redhat.com>]

Hi,

I just learned from a comment on gnomedesktop.org that I missed most of
this thread, it landed in the wrong mail folder or something. ;-)

On Fri, 2003-09-19 at 20:12, Greg KH wrote:
> > Things aren't handled properly today in the sense that there's no UI on
> > it, there's no integration with applications. To add that on the
> > desktop/app level, we first have to have a single API that will work
> > with different kinds of device and on different operating systems.
> 
> Hm, for a lot of this a UI isn't needed, right?  The user doesn't need
> to know that a driver was just automatically loaded for their device,
> correct?

That's right. The bulk of the UI should be about using devices rather
than getting them connected and working.

> Again, there are very notable exceptions, the digital cameras supported
> by gphoto2 and the USB scanners supported by xsane, both using userspace
> only code.  I'm just pretty skeptical that something can be created that
> would allow OEMs to plop into a userspace location that would enable
> device support across multiple operating systems.  But please, prove me
> wrong, I will be very happy.

Ah, I don't really mean that the OEM interface would be OS-independent;
either the file plopped would have to contain OS-specific portions, or
there would be a per-OS file. That's OK since we can expect an OEM to
deal with each OS independently, as they only have 1 device * N OSes.
It's the app developers that have to be insulated from M devices * N
OSes.

The OS independent portion is the part the app sees. Some properties
exported by the HAL are probably even OS-specific, but the point is
there's an OS-independent core/subset. (i.e. the idea is to let you
write portable code, not to keep you from punching down to OS details if
you want)

> I understand this.  The kernel doesn't contain #ifdef hell for that very
> reason.  But this is my main point as to how this might get very tough
> for having cross platform support.  The BSDs support lots of USB devices
> today in a very different manner than Linux does.

I think of it as "portability by subdirectory" ;-)

You have an API that apps talk to, and then some generic code, and then
a per-OS backend with a subdirectory for each backend:

 hal/
   include/
     hal/hal.h
   generic/
     hal.c
   linux/
     usb.c
   freebsd/
     usb.c

It isn't so much that the freebsd/ subdir is populated immediately, but
rather that when someone appears with the skills and motivation they can
add that subdir without having to rearchitect everything.

The desktop userspace doesn't have a list of required platforms so much
as the requirement that "if someone is actively maintaining a platform
they can keep that platform working"

> I'm happy to see this work happening, and if there's anything that you
> need from the kernel, hotplug scripts, or udev that I can help out with,
> please let me know.

I really appreciate your interest/help, btw.

I'm curious how many iterations of all this we'll go through before
we're happy with it. Wait, I've worked on software long enough to know
the answer is "infinite"... ;-)

Havoc




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