[RFC] Remove AGP support from Radeon/Nouveau/TTM

[Al Dunsmuir]

On Monday, May 11, 2020, 4:27:55 PM, Alex Deucher wrote:
> On Mon, May 11, 2020 at 4:22 PM Al Dunsmuir <al.dunsmuir at sympatico.ca> wrote:
>>
>> On Monday, May 11, 2020, 1:17:19 PM, "Christian König" wrote:
>> > Hi guys,
>>
>> > Well let's face it AGP is a total headache to maintain and dead for at least 10+ years.
>>
>> > We have a lot of x86 specific stuff in the architecture independent
>> > graphics memory management to get the caching right, abusing the DMA
>> > API on multiple occasions, need to distinct between AGP and driver specific page tables etc etc...
>>
>> > So the idea here is to just go ahead and remove the support from
>> > Radeon and Nouveau and then drop the necessary code from TTM.
>>
>> > For Radeon this means that we just switch over to the driver
>> > specific page tables and everything should more or less continue to work.
>>
>> > For Nouveau I'm not 100% sure, but from the code it of hand looks
>> > like we can do it similar to Radeon.

> Note there is no loss of functionality here, at least on radeon
> hardware.  It just comes down to which MMU gets used for access to
> system memory, the AGP MMU on the chipset or the MMU built into the
> GPU.  On powerpc hardware, AGP has been particularly unstable, and AGP
> has been disabled by default on radeon on powerpc for years now.  In
> fact, this will probably make older hardware more reliable as it takes
> AGP out of the equation.

Alex,

Thanks for the clarification.

If the actual performance impact is little to none, and we end up with
more reliable operation that is certainly beneficial then the change
is indeed for the good.

The  Phoronix  title screaming "AGP Graphics Card Support Proposed For
Removal  From  Linux  Radeon/NVIDIA Drivers" was neither accurate, nor
helpful.

Al