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

Alex Deucher alexdeucher at gmail.com
Mon May 11 20:27:55 UTC 2020

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.


> There are multiple active Linux distributions for PPC Macs. Many folks
> are moving from running an original Mac OS X because of the recent
> migration of most Web services to new encryption protocols (TLS 1.2,
> SSL, etc.) which can not be practically supported by browsers on that
> OS - even the open source ones. Linux on that same hardware provides
> the opportunity to work online with current protocols and ensure that
> they are reasonably secure.
> Another group that fits in the retro category is those folks with
> older PC hardware who are not migrating to the latest Microsoft OS,
> but instead want to move to Linux.
> Some folks suggest the retro folks use an older unmaintained Linux
> version. That's just not appropriate nor reasonable, again from either
> a security and protocol point of view.
> Myself? I'm a full-time C programmer and Electrical Engineer. My
> current work project is about 400KLOC. I've done some work on PC and
> embedded video hardware and software in the past.
> My aim is to start out by helping to test the video drivers (on both
> PC and MAC hardware), and as I gain more knowledge become part of the
> effort to bring the drivers to the older hardware into a better form
> (in kernel drivers VS user-space).
> Over the last few years I have been collecting a significant (~40)
> number of video cards of various flavors. Some PCI, but the majority
> evenly divided between AGP and PCI-X. Most are ATI/Radeon (Mach64+
> through R7) with some Nvidia, Matrox and a few others (Chrome and even
> a Number Nine). I try to have both PC and Mac variants of the same
> card where-ever possible to make comparisons across architectures
> easier. This year I have acquired some of the last Sapphire Radeon AGP
> cards (HD3xxx, HD4xxx). I'm a frequent flyer on eBay and Kijiji.
> I've got lots of Macs (PCI, AGP and PCI-X) and a number of full PC
> systems. I'm setting up a number of dedicated test bench rigs (and
> full systems) specifically for the video testing. Most are AMD 64-bit
> capable, but also Intel. Finally, there are a few embedded systems
> (AMD and a C3).
> From the programming side, Amazon has been a good source on older
> editions of computer graphics books, which should provide background
> knowledge to help me in my journey.
> One glitch is that I'm partway through eye surgery (cornea + cataracts),
> and my final operation is held up due to COVID-19.  After things get
> back to more of a normal state and that can be completed, my progress
> should be a lot smoother.
> I hope this puts another prospective on your suggested change, from a
> Linux enthusiast and programmer whose usage set is far different from
> your own. For that group of Linux users, I ask that you NOT go ahead
> with this change as it would be far more impactful to that groups than
> you might think.
> Respectfully,
> Al Dunsmuir
> Al
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