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

[Gerhard Pircher]

Am 11.05.20 um 22:24 schrieb John Paul Adrian Glaubitz:
> On 5/11/20 10:12 PM, Christian König wrote:
>> I unfortunately only have an x86 Mac to test this on, but as far as I know the Radeon
>> AGP support for PowerPC is disabled for years already because it didn't worked reliable.
>
> I have a current Debian unstable running on an iBook G4 with Radeon graphics enabled
> on a current kernel and graphics stack and it runs fine. Not sure though whether
> it currently employs all AGP features, but I would like to be able to continue
> using it and so are the users on the debian-powerpc mailing list.
I didn't have much luck with agp_uninorth and radeon DRM on my MacMini
G4 (2xx) with current Debian unstable (yet have to send an installation
report, sorry!). The X server didn't even come up with AGP enabled.
The only thing that works is, if the radeon driver is forced to use
PCIGART, which was even the default after install, if I remember
correctly.
Problem is that graphics acceleration over PCI is very, very slow... :-(

>>>> 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.
>>>>
>>>> Please comment what you think about this.
>>> I would be against such a move as AGP graphics is still used by people running the powerpc
>>> and ppc64 Debian ports on their vintage hardware [1].
>>
>> Please note that at least the Mac I was able to test with Radeon hardware just continuous
>> to work. But it is certainly possible that some pre r3xx generation hardware will break with this.
>>
>> We just stop using this bogus idea of trying to use uncached system memory as "extension" of the
>> on board video memory and instead switch to the reliable device internal GART.
>
> Well, the title "Remove AGP support" in the subject implied something else to me which
> is why I wrote this mail. If this just applies to the mechanism to allow system memory
> to be used as graphics memory, the results may be different.
I was also following the discussion about AGP some time ago on the
PowerPC Linux mailing list and honestly also understood it in the
way that AGP would be entirely removed. :-)

But I guess what is planned is to use the radeon/TTM drivers PCIGART
functionality while preserving the setup of the fast AGP transfer modes
(maybe by using a static AGPGART, if necessary?). Is that what Linux
developers have in mind with "Remove AGP support"?

>> Maybe we should just deprecate the configuration option first?
>
> Would this change imply the removal of CONFIG_AGP_*? If yes, I assume that would
> kill CONFIG_AGP_UNINORTH which we have enabled on our PowerPC kernels for powerpc
> and ppc64.
>
> Adrian
>
Gerhard