[RFC] Deprecate AGP GART support for Radeon/Nouveau/TTM

[Christian König]

Am 20.05.20 um 18:25 schrieb Michel Dänzer:
> On 2020-05-20 4:43 p.m., Christian König wrote:
>> Am 13.05.20 um 13:03 schrieb Christian König:
>>> Unfortunately AGP is still to widely used as we could just drop
>>> support for using its GART.
>>>
>>> Not using the AGP GART also doesn't mean a loss in functionality since
>>> drivers will just fallback to the driver specific PCI GART.
>>>
>>> For now just deprecate the code and don't enable the AGP GART in TTM
>>> even when general AGP support is available.
>> So I've used an ancient system (32bit) to setup a test box for this.
>>
>>
>> The first GPU I could test is an RV280 (Radeon 9200 PRO) which is easily
>> 15 years old.
>>
>> What happens in AGP mode is that glxgears shows artifacts during
>> rendering on this system.
>>
>> In PCI mode those rendering artifacts are gone and glxgears seems to
>> draw everything correctly now.
>>
>> Performance is obviously not comparable, cause in AGP we don't render
>> all triangles correctly.
>>
>>
>> The second GPU I could test is an RV630 PRO (Radeon HD 2600 PRO AGP)
>> which is more than 10 years old.
>>
>> As far as I can tell this one works in both AGP and PCIe mode perfectly
>> fine.
>>
>> Since this is only a 32bit system I couldn't really test any OpenGL game
>> that well.
>>
>> But for glxgears switching from AGP to PCIe mode seems to result in a
>> roughly 5% performance drop.
>>
>> The surprising reason for this is not the better TLB performance, but
>> the lack of USWC support for the PCIe GART in radeon.
> I suspect the main reason it's only 5% is that PCIe GART page tables are
> stored in VRAM, so they don't need to be fetched across the PCIe link
> (and presumably it has more than one TLB entry as well). The difference
> is much bigger with native AGP ASICs with PCI GART.

Do you have some hardware you could give that a try on?

I mean the first card I put into the system obviously only worked 
correctly with AGP disabled.

While I agree that it means a performance regression, this is a rather 
high motivation to go ahead with at least the first patch.

Christian.