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

[Dave Airlie]

On Tue, 12 May 2020 at 06:28, Alex Deucher <alexdeucher at gmail.com> 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.
> >
> > > Please comment what you think about this.
> >
> > > Regards,
> > > Christian.
> >
> > Christian,
> >
> > I would respectfully ask that this change be rejected.
> >
> > I'm  currently  an  end user on both Intel (32-bit and 64-bit) and PPC
> > (Macs, IBM Power - BE and LE).
> >
> > Linux is not just used for modern hardware. There is also a subset of
> > the user base that uses it for what is often termed retro-computing.
> > No it's not commercial usage, but no one can seriously claim that that
> > Linux is for business only.
> >
> > Often the old hardware is built far batter than the modern junk, and
> > will continue to run for years to come. This group of folks either has
> > existing hardware they wish to continue to use, or are acquiring the
> > same because they are tired of generic locked-down hardware.
> >
> > A significant percentage of the video hardware that falls in the retro
> > category uses the AGP video bus. Removing that support for those cases
> > where it works would severely limit performance and in some cases
> > functionality. This can mean the difference between being able to run
> > an application, or having it fail.
> >
>
> 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.
>

>From memory there is quite a loss in speed though, like pretty severe.

The radeon PCI GART has a single slot TLB, if memory serves.

I think this is going to be a hard sell at this stage, I'm guessing
users will crawl out of the woodwork, I'm sure with 2 hours after I'm
able to access the office, I can boot the 865 AGP box with an rv350 in
it on a modern distro.

Maybe we can find some way to compartmentalise AGP further?

Dave.