[Xorg-driver-geode] Disable compression patch? huh?

[Jordan Crouse]

On 15/02/08 11:03 -0500, Warren Togami wrote:
> Hi folks,
> 
> I am now attempting to pull in the latest upstream drv-amd into Fedora.
> 
> http://cvs.fedoraproject.org/viewcvs/devel/xorg-x11-drv-amd/
> We have very old patches in our Fedora 8 driver.  I have confirmed with 
> Dan Williams that two patches are no longer needed.
> 
> http://cvs.fedoraproject.org/viewcvs/devel/xorg-x11-drv-amd/lx-disable-compression.patch?rev=1.1&view=log
> But this patch we are uncertain.  Ajax recalls that it was framebuffer 
> compression was definitely broken in some way in the past thus it was 
> completely disabled in our driver.  Interestingly, he also mentions that 
> "compression" actually uses more memory than without in this case.

Yes - compression doesn't mean what you think it means </princess_bride>

In this particular case, what is being compressed is the data traveling
across the internal bus between the memory controller and the display
controller.  In order to do this in the hardware, the pitch in the
framebuffer needs to be a power of two; so in the case of the OLPC
instead of a pitch of (1200x2=2400), the pitch would be 4096, which
ended up expending very nearly 2 MB more of memory.

Compression is most useful for situations where this is a enormous 
amount of traffic on the internal bus combined with high display
resolutions - like, say streaming video from a PCI attached NIC in 720p
resolution.  Most other people wouldn't use it, though they probably
wouldn't notice or care if it was enabled.

I do remember some issues with compression in the past, but we've had
it enabled by default in the driver for some time now, and I haven't
heard any complaints.  Part of the reason why we patched it out in
the Fedora driver was because we wanted it disabled for the memory
hogging reason, and the OLPC guys preferred to have it disabled in the
code rather then manually disabling it in the configuration file.

Jordan

-- 
Jordan Crouse
Systems Software Development Engineer 
Advanced Micro Devices, Inc.