[Nouveau] [PATCH 1/3] drm/radeon: remove AGP support

[Christian König]

Am 12.05.20 um 23:12 schrieb Alex Deucher:
> On Tue, May 12, 2020 at 4:52 PM Roy Spliet <nouveau at spliet.org> wrote:
>> Op 12-05-2020 om 14:36 schreef Alex Deucher:
>>> On Tue, May 12, 2020 at 4:16 AM Michel Dänzer <michel at daenzer.net> wrote:
>>>> On 2020-05-11 10:12 p.m., Alex Deucher wrote:
>>>>> On Mon, May 11, 2020 at 1:17 PM Christian König
>>>>> <ckoenig.leichtzumerken at gmail.com> wrote:
>>>>>> AGP is deprecated for 10+ years now and not used any more on modern hardware.
>>>>>>
>>>>>> Old hardware should continue to work in PCI mode.
>>>>> Might want to clarify that there is no loss of functionality here.
>>>>> Something like:
>>>>>
>>>>> "There is no loss of functionality here.  GPUs will continue to
>>>>> function.  This just drops the use of the AGP MMU in the chipset in
>>>>> favor of the MMU on the device which has proven to be much more
>>>>> reliable.  Due to its unreliability, AGP support has been disabled on
>>>>> PowerPC for years already so there is no change on PowerPC."
>>>> There's a difference between something being disabled by default or not
>>>> being available at all. We may decide it's worth it anyway, but let's do
>>>> it based on facts.
>>>>
>>> I didn't mean to imply that AGP GART support was already removed.  But
>>> for the vast majority of users the end result is the same.  If you
>>> knew enough re-enable AGP GART, you probably wouldn't have been as
>>> confused about what this patch set does either.  To reiterate, this
>>> patch set does not remove support for AGP cards, it only removes the
>>> support for AGP GART.  The cards will still be functional using the
>>> device GART.  There may be performance tradeoffs there in some cases.
>> I'll volunteer to be the one asking: how big is this performance
>> difference? Have any benchmarks been run before and after removal of AGP
>> GART code on affected nouveau/radeon systems? Or is this code being
>> dropped _just_ because it's cumbersome, with no regard for metrics that
>> determine the value of AGP GART support?
>>
> I don't think anyone has any solid numbers, just anecdotal from
> memory.  I certainly don't have any functional AGP systems at this
> point.  It's mostly just cumbersome and would allow us to clean ttm
> and probably improve stability at the same time.  At least on the
> radeon side, the only native AGP cards were r1xx, r2xx, and some of
> the early r3xx boards.  Once we switched to pcie mid-way through r3xx,
> everything was native pcie and the AGP cards used a pcie to AGP bridge
> chip so they had a decent on chip MMU.  Those older cards topped out
> at maybe 32 or 64 MB of vram, so they are going to be hard pressed to
> deal with modern desktops anyway.  No idea what sort of GART
> capabilities NV AGP hardware at this time had.

I could only test with an old x86 Mac and an r3xx generation hw and in 
this case making the switch didn't had any noticeable effect at all.

But I didn't do more than playing around with the desktop effects and 
playing a video.

I do have a PC x86 AGP board lying around somewhere here, going top give 
that one a try a well.

Christian.

>
> Alex
>
>> Roy
>>
>>> Alex
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