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SubjectRe: [PATCH v8 2/2] drm/gem: Don't map imported GEMs
On 8/16/22 14:44, Dmitry Osipenko wrote:
> On 8/12/22 18:01, Rob Clark wrote:
>> On Fri, Aug 12, 2022 at 7:57 AM Rob Clark <> wrote:
>>> On Fri, Aug 12, 2022 at 4:26 AM Dmitry Osipenko
>>> <> wrote:
>>>> On 8/11/22 02:19, Rob Clark wrote:
>>>>> On Wed, Aug 10, 2022 at 3:23 PM Dmitry Osipenko
>>>>> <> wrote:
>>>>>> On 8/11/22 01:03, Rob Clark wrote:
>>>>>>> On Wed, Aug 10, 2022 at 12:26 PM Dmitry Osipenko
>>>>>>> <> wrote:
>>>>>>>> On 8/10/22 18:08, Rob Clark wrote:
>>>>>>>>> On Wed, Aug 10, 2022 at 4:47 AM Daniel Vetter <> wrote:
>>>>>>>>>> On Wed, Jul 06, 2022 at 10:02:07AM +0300, Dmitry Osipenko wrote:
>>>>>>>>>>> On 7/6/22 00:48, Rob Clark wrote:
>>>>>>>>>>>> On Tue, Jul 5, 2022 at 4:51 AM Christian König <> wrote:
>>>>>>>>>>>>> Am 01.07.22 um 11:02 schrieb Dmitry Osipenko:
>>>>>>>>>>>>>> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
>>>>>>>>>>>>>> handle imported dma-bufs properly, which results in mapping of something
>>>>>>>>>>>>>> else than the imported dma-buf. On NVIDIA Tegra we get a hard lockup when
>>>>>>>>>>>>>> userspace writes to the memory mapping of a dma-buf that was imported into
>>>>>>>>>>>>>> Tegra's DRM GEM.
>>>>>>>>>>>>>> Majority of DRM drivers prohibit mapping of the imported GEM objects.
>>>>>>>>>>>>>> Mapping of imported GEMs require special care from userspace since it
>>>>>>>>>>>>>> should sync dma-buf because mapping coherency of the exporter device may
>>>>>>>>>>>>>> not match the DRM device. Let's prohibit the mapping for all DRM drivers
>>>>>>>>>>>>>> for consistency.
>>>>>>>>>>>>>> Suggested-by: Thomas Hellström <>
>>>>>>>>>>>>>> Signed-off-by: Dmitry Osipenko <>
>>>>>>>>>>>>> I'm pretty sure that this is the right approach, but it's certainly more
>>>>>>>>>>>>> than possible that somebody abused this already.
>>>>>>>>>>>> I suspect that this is abused if you run deqp cts on android.. ie. all
>>>>>>>>>>>> winsys buffers are dma-buf imports from gralloc. And then when you
>>>>>>>>>>>> hit readpix...
>>>>>>>>>>>> You might only hit this in scenarios with separate gpu and display (or
>>>>>>>>>>>> dGPU+iGPU) because self-imports are handled differently in
>>>>>>>>>>>> drm_gem_prime_import_dev().. and maybe not in cases where you end up
>>>>>>>>>>>> with a blit from tiled/compressed to linear.. maybe that narrows the
>>>>>>>>>>>> scope enough to just fix it in userspace?
>>>>>>>>>>> Given that that only drivers which use DRM-SHMEM potentially could've
>>>>>>>>>>> map imported dma-bufs (Panfrost, Lima) and they already don't allow to
>>>>>>>>>>> do that, I think we're good.
>>>>>>>>>> So can I have an ack from Rob here or are there still questions that this
>>>>>>>>>> might go boom?
>>>>>>>>>> Dmitry, since you have a bunch of patches merged now I think would also be
>>>>>>>>>> good to get commit rights so you can drive this more yourself. I've asked
>>>>>>>>>> Daniel Stone to help you out with getting that.
>>>>>>>>> I *think* we'd be ok with this on msm, mostly just by dumb luck.
>>>>>>>>> Because the dma-buf's we import will be self-import. I'm less sure
>>>>>>>>> about panfrost (src/panfrost/lib/pan_bo.c doesn't seem to have a
>>>>>>>>> special path for imported dma-bufs either, and in that case they won't
>>>>>>>>> be self-imports.. but I guess no one has tried to run android cts on
>>>>>>>>> panfrost).
>>>>>>>> The last time I tried to mmap dma-buf imported to Panfrost didn't work
>>>>>>>> because Panfrost didn't implement something needed for that. I'll need
>>>>>>>> to take a look again because can't recall what it was.
>>>> Upd: I re-checked Panfrost using today's linux-next and mapping of
>>>> imported dma-buf works, I mapped imported buf from video decoder.
>>>> Apparently previously I had some local kernel change that broke the mapping.
>>>>>>>>> What about something less drastic to start, like (apologies for
>>>>>>>>> hand-edited patch):
>>>>>>>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
>>>>>>>>> index 86d670c71286..fc9ec42fa0ab 100644
>>>>>>>>> --- a/drivers/gpu/drm/drm_gem.c
>>>>>>>>> +++ b/drivers/gpu/drm/drm_gem.c
>>>>>>>>> @@ -1034,6 +1034,10 @@ int drm_gem_mmap_obj(struct drm_gem_object
>>>>>>>>> *obj, unsigned long obj_size,
>>>>>>>>> {
>>>>>>>>> int ret;
>>>>>>>>> + WARN_ON_ONCE(obj->import_attach);
>>>>>>>> This will hang NVIDIA Tegra, which is what this patch fixed initially.
>>>>>>>> If neither of upstream DRM drivers need to map imported dma-bufs and
>>>>>>>> never needed, then why do we need this?
>>>>>>> oh, tegra isn't using shmem helpers? I assumed it was. Well my point
>>>>>>> was to make a more targeted fail on tegra, and a WARN_ON for everyone
>>>>>>> else to make it clear that what they are doing is undefined behavior.
>>>>>>> Because so far existing userspace (or well, panfrost and freedreno at
>>>>>>> least, those are the two I know or checked) don't make special cases
>>>>>>> for mmap'ing against the dmabuf fd against the dmabuf fd instead of
>>>>>>> the drm device fd.
>>>>>> It's not clear to me what bad Android does form yours comments. Does it
>>>>>> export dma-buf from GPU and then import it to GPU? If yes, then DRM core
>>>>>> has a check for the self-importing [1].
>>>>>> [1]
>>>>>> If you're meaning something else, then please explain in a more details.
>>>>> So, android/gralloc allocates buffers externally to the driver and
>>>>> imports them into driver. (And that seems to not just be window
>>>>> surfaces, but in cases random textures, etc)
>>>>> In the normal case these should be allocated from drm/msm so it should
>>>>> hit [1].. this is the "dumb luck" I mentioned earlier. But I'm not
>>>>> confident enough to say that there is no other case.
>>>>>>> I *think* it should work out that we don't hit this path with
>>>>>>> freedreno but on android I can't really guarantee or prove it. So
>>>>>>> your patch would potentially break existing working userspace. Maybe
>>>>>>> it is userspace that isn't portable (but OTOH it isn't like you are
>>>>>>> going to be using freedreno on tegra). So why don't you go for a more
>>>>>>> targeted fix that only returns an error on hw where this is
>>>>>>> problematic?
>>>>>> That's what the first versions of the patch did and Christian suggested
>>>>>> that it's not a good approach. In fact it should be not only Tegra that
>>>>>> has a broken dma-buf mapping, but apparently OMAP driver too.
>>>>> Hmm, I guess I'm a bit more conservative when it comes to potentially
>>>>> breaking userspace.
>>>> If such userspace exists, then of course the mapping should continue to
>>>> work. Still will be great to know what that userpsace is.
>>> Definitely existing mesa does not have a special mmap path for
>>> imported dma-bufs (both in the case of panfrost and freedreno, I
>>> didn't check any others). The only question is whether there is a
>>> case where some app/test/etc imports a foreign dma-buf fd and then
>>> does something that would trigger mmap'ing, like readpix.
>> The other complication I noticed is that we don't seem to keep around
>> the fd after importing to a GEM handle. And I could imagine that
>> doing so could cause issues with too many fd's. So I guess the best
>> thing is to keep the status quo and let drivers that cannot mmap
>> imported buffers just fail mmap?
> That actually should be all the drivers excluding those that use
> DRM-SHMEM because only DRM-SHMEM uses dma_buf_mmap(), that's why it
> works for Panfrost. I'm pretty sure mmaping of imported GEMs doesn't
> work for the MSM driver, isn't it?
> Intel and AMD drivers don't allow to map the imported dma-bufs. Both
> refuse to do the mapping.
> Although, AMDGPU "succeeds" to do the mapping using
> AMDGPU_GEM_DOMAIN_GTT, but then touching the mapping causes bus fault,
> hence mapping actually fails. I think it might be the AMDGPU
> driver/libdrm bug, haven't checked yet.

To make it more clear, I imported AMDGPU dma-buf to Intel iGPU and vice

> So we're back to the point that neither of DRM drivers need to map
> imported dma-bufs and this was never tested. In this case this patch is
> valid, IMO.

Best regards,

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