Re: [git pull] drm for 5.8-rc1

[Karol Herbst <kherbst@redhat.com>]

On Wed, Aug 12, 2020 at 7:03 PM James Jones <jajones@nvidia.com> wrote:
>
> On 8/12/20 5:37 AM, Ilia Mirkin wrote:
> > On Wed, Aug 12, 2020 at 8:24 AM Karol Herbst <kherbst@redhat.com> wrote:
> >>
> >> On Wed, Aug 12, 2020 at 12:43 PM Karol Herbst <kherbst@redhat.com> wrote:
> >>>
> >>> On Wed, Aug 12, 2020 at 12:27 PM Karol Herbst <kherbst@redhat.com> wrote:
> >>>>
> >>>> On Wed, Aug 12, 2020 at 2:19 AM James Jones <jajones@nvidia.com> wrote:
> >>>>>
> >>>>> Sorry for the slow reply here as well.  I've been in the process of
> >>>>> rebasing and reworking the userspace patches.  I'm not clear my changes
> >>>>> will address the Jetson Nano issue, but if you'd like to try them, the
> >>>>> latest userspace changes are available here:
> >>>>>
> >>>>>     https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3724
> >>>>>
> >>>>> And the tegra-drm kernel patches are here:
> >>>>>
> >>>>>
> >>>>> https://patchwork.ozlabs.org/project/linux-tegra/patch/20191217005205.2573-1-jajones@nvidia.com/
> >>>>>
> >>>>> Those + the kernel changes addressed in this thread are everything I had
> >>>>> outstanding.
> >>>>>
> >>>>
> >>>> I don't know if that's caused by your changes or not, but now the
> >>>> assert I hit is a different one pointing out that
> >>>> nvc0_miptree_select_best_modifier fails in a certain case and returns
> >>>> MOD_INVALID... anyway, it seems like with your patches applied it's
> >>>> now way easier to debug and figure out what's going wrong, so maybe I
> >>>> can figure it out now :)
> >>>>
> >>>
> >>> collected some information which might help to track it down.
> >>>
> >>> src/gallium/frontends/dri/dri2.c:648 is the assert hit: assert(*zsbuf)
> >>>
> >>> templ is {reference = {count = 0}, width0 = 300, height0 = 300, depth0
> >>> = 1, array_size = 1, format = PIPE_FORMAT_Z24X8_UNORM, target =
> >>> PIPE_TEXTURE_2D, last_level = 0, nr_samples = 0, nr_storage_samples =
> >>> 0, usage = 0, bind = 1, flags = 0, next = 0x0, screen = 0x0}
> >>>
> >>> inside tegra_screen_resource_create modifier says
> >>> DRM_FORMAT_MOD_INVALID as template->bind is 1
> >>>
> >>> and nvc0_miptree_select_best_modifier returns DRM_FORMAT_MOD_INVALID,
> >>> so the call just returns NULL leading to the assert.
> >>>
> >>> Btw, this is on Xorg-1.20.8-1.fc32.aarch64 with glxgears.
> >>>
> >>
> >> So I digged a bit deeper and here is what tripps it of:
> >>
> >> when the context gets made current, the normal framebuffer validation
> >> and render buffer allocation is done, but we end up inside
> >> tegra_screen_resource_create at some point with PIPE_BIND_SCANOUT set
> >> in template->bind. Now the tegra driver forces the
> >> DRM_FORMAT_MOD_LINEAR modifier and calls into
> >> resource_create_with_modifiers.
> >>
> >> If it wouldn't do that, nouveau would allocate a tiled buffer, with
> >> that it's linear and we at some point end up with an assert about a
> >> depth_stencil buffer being there even though it shouldn't. If I always
> >> use DRM_FORMAT_MOD_INVALID in tegra_screen_resource_create, things
> >> just work.
> >>
> >> That's kind of the cause I pinpointed the issue down to. But I have no
> >> idea what's supposed to happen and what the actual bug is.
> >
> > Yeah, the bug with tegra has always been "trying to render to linear
> > color + tiled depth", which the hardware plain doesn't support. (And
> > linear depth isn't a thing.)
> >
> > Question is whether what it's doing necessary. PIPE_BIND_SCANOUT
> > (/linear) requirements are needed for DRI2 to work (well, maybe not in
> > theory, but at least in practice the nouveau ddx expects linear
> > buffers). However tegra operates on a more DRI3-like basis, so with
> > "client" allocations, tiled should work OK as long as there's
> > something in tegra to copy it to linear when necessary?
>
> I can confirm the above: Our hardware can't render to linear depth
> buffers, nor can it mix linear color buffers with block linear depth
> buffers.
>
> I think there's a misunderstanding on expected behavior of
> resource_create_with_modifiers() here too:
> tegra_screen_resource_create() is passing DRM_FORMAT_MOD_INVALID as the
> only modifier in non-scanout cases.  Previously, I believe nouveau may
> have treated that as "no modifiers specified.  Fall back to internal
> layout selection logic", but in my patches I "fixed" it to match other
> drivers' behavior, in that allocation will fail if that is the only
> modifier in the list, since it is equivalent to passing in a list
> containing only unsupported modifiers.  To get fallback behavior,
> tegra_screen_resource_create() should pass in (NULL, 0) for (modifiers,
> count), or just call resource_create() on the underlying screen instead.
>
> Beyond that, I can only offer my thoughts based on analysis of the code
> referenced here so far:
>
> While I've learned from the origins of this thread applications/things
> external to Mesa in general shouldn't be querying format modifiers of
> buffers created without format modifiers, tegra is a Mesa internal
> component that already has some intimate knowledge of how the nouveau
> driver it sits on top of works.  Nouveau will always be able to
> construct and return a valid format modifier for unorm single sampled
> color buffers (and hopefully, anything that can scan out going forward),
> both before and after my patches I believe, regardless of how they were
> allocated.  After my patches, it should even work for things that can't
> scan out in theory.  Hence, looking at this without knowledge of what
> motivated the original changes, it seems like
> tegra_screen_resource_create should just naively forward the
> resource_create() call, relying on nouveau to select a layout and
> provide a valid modifier when queried for import.  As Karol notes, this
> works fine for at least this simple test case, and it's what nouveau
> itself would be doing with an equivalent callstack, excepting the
> modifier query, so I find it hard to believe it breaks some application
> behavior.  It'll also end up being equivalent (in end result, not quite
> semantically) to what dri3_alloc_render_buffer() was doing prior to the
> patch mentioned that broke things for Karol, so certainly for the DRI3
> usage it's the right behavior.
>
> Ilia, what in the nouveau DDX (As in Xfree86 DDX?) assumes linear
> buffers?  It sounds like you don't think it will interact poorly with
> this path regardless?  Thierry, do you recall what motivated the
> force-linear code here?
>
> As to why this works for Thierry and not Karol, that's confusing.  Are
> you both using the same X11 DDX (modesetting I assume?) and X server
> versions?  Could it be a difference in client-side DRI library code somehow?
>

it's X. 1.20.99.1 works, 1.20.8 is broken.

> Thanks,
> -James
>
> >    -ilia
> >
>

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