Nouveau - Mar 2016 - [Bug 94757] New: powerpc64 & 64Kb kernel pagesize not working with nouveau

https://bugs.freedesktop.org/show_bug.cgi?id=94757

            Bug ID: 94757
           Summary: powerpc64 & 64Kb kernel pagesize not working with
                    nouveau
           Product: xorg
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/nouveau
          Assignee: nouveau at lists.freedesktop.org
          Reporter: mathieu.malaterre at gmail.com
        QA Contact: xorg-team at lists.x.org

The recent Debian kernels for powerpc64 are configured with a 64Kb kernel
pagesize.
This works slightly better in terms of performance, however the nouveau display
driver does not currently work kernel pagesizes other than 4Kb size when
running XOrg and powerpc64. It works ok if only using the console though.

Currently, users must recompile the kernel unfortunately and configure with 4Kb
kernel pagesize as this is what nouveau will only work with for now.
This is a hindrance to making people want to adopt debian as their preffered
linux distro with powerpc64 G5 powermac machines.

Im currently running vanilla 3.18.16 configured with 4Kb and nouveau can be
made to work without too many dramas.

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--- Comment #1 from Mathieu Malaterre <mathieu.malaterre at gmail.com> ---
Original bug report is: https://bugs.debian.org/790690

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--- Comment #2 from Mathieu Malaterre <mathieu.malaterre at gmail.com> ---
As explained here:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=790694#27

[...]
I saw that bug report as well.  I'm not sure what to do about it -
other distributions were also using 64K pages for 64-bit PowerPC the
last time I looked, and there may be good reasons to do that.
[...]

So it would make sense to make nouveau capable of handling 64K pages for
64-bits PowerPC.

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--- Comment #3 from Ilia Mirkin <imirkin at alum.mit.edu> ---
Unfortunately it's not just a switch to flip on. A lot of the current code
is
very confused about cpu pages vs gpu pages (which are always 4K). Also one
would need a bunch of handling code to deal with sub-cpu-page-sized allocations
on the GPU (or conversely just disallow those entirely).

Nouveau is known to not work properly on non-4K-cpu-page-size, at least with
some GPUs (maybe even all).

AFAIK the benefits of using 64K-pages are virtually non-existent outside the
HPC space (each cached 1 byte file takes up 64K of memory, so it goes a lot
faster). But that's certainly not an argument for not supporting 64K pages
in
nouveau.

The argument for not supporting 64K pages is -ENOPATCH :) AFAIK benh tried for
a while and failed. This was ~2y ago.

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--- Comment #4 from Mathieu Malaterre <mathieu.malaterre at gmail.com> ---
Understood. I've reported that back to the Debian kernel team, let's see
if we
can have 4K page back for PPC64:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=790694#52

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--- Comment #5 from Mathieu Malaterre <mathieu.malaterre at gmail.com> ---
As a side note, would it be possible to have a better error message. Something
along:

printk( "Sorry nouveau only support 4K pages" );

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--- Comment #6 from Ben Skeggs <skeggsb at gmail.com> ---
I have a WIP chunk of work which will hopefully address this issue as a
side-effect.  It's not ready for general consumption yet, but I'll
update the
bug once it is.

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Mathieu Malaterre <mathieu.malaterre at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |https://bugs.debian.org/790
                   |                            |690

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--- Comment #7 from Benjamin Herrenschmidt <benh at kernel.crashing.org>
---
There's an old patch of mind floating on the lists somewhere that did make
it
somewhat work on my G5's nv43 but it only addressed part of the problem (the
kernel framebuffer basically).

There's also an issue in that the per-context registers that are meant to be
mapped into userspace are only 4K appart.

nVidia binary driver on POWER8 (for compute) works around that by using the
_PAGE_4K_PFN hack we have which allows a driver to create a user mapping for a
64K page that is made of 16 times the same underlying HW 4K page.

That requires unfortunately some trickery to deal with on both side of the
user/kernel interface, so it's not a "drop-in" fix in the existing
stack.

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--- Comment #8 from Benjamin Herrenschmidt <benh at kernel.crashing.org>
---
s/mind/mine ;-)

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--- Comment #9 from Jeremy Linton <jeremy.linton at arm.com> ---
This problem happens on Fedora/RHEL & Centos for ARM64 as well, because
those
distributions are using 64k pages.

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Jeremy Linton <jeremy.linton at arm.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|powerpc64 & 64Kb kernel     |powerpc64, arm64 & 64Kb
                   |pagesize not working with   |kernel pagesize not working
                   |nouveau                     |with nouveau

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Martin Peres <martin.peres at free.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |MOVED
             Status|NEW                         |RESOLVED

--- Comment #10 from Martin Peres <martin.peres at free.fr> ---
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