[PATCH] drm/gem: Internally test import_attach for imported objects

Tue Apr 15 13:14:39 UTC 2025

Hi

Am 15.04.25 um 14:52 schrieb Christian König:
> Am 15.04.25 um 14:40 schrieb Thomas Zimmermann:
>> Hi
>>
>> Am 15.04.25 um 14:19 schrieb Christian König:
>>> Am 15.04.25 um 12:45 schrieb Thomas Zimmermann:
>>>> Hi
>>>>
>>>> Am 15.04.25 um 11:39 schrieb Christian König:
>>>>> Am 15.04.25 um 11:20 schrieb Thomas Zimmermann:
>>>>>> Test struct drm_gem_object.import_attach to detect imported objects
>>>>>> during cleanup. At that point, the imported DMA buffer might have
>>>>>> already been released and the dma_buf field is NULL. The object's
>>>>>> free callback then does a cleanup as for native objects.
>>>>> I don't think that this is a good idea.
>>>>>
>>>>> The DMA-buf is separately reference counted through the import attachment.
>>>> I understand that it's not ideal, but testing for import_attach to be set is what we currently do throughout drivers. Putting this behind an interface is already a step forward.
>>>>
>>>>>> Happens for calls to drm_mode_destroy_dumb_ioctl() that eventually
>>>>>> clear the dma_buf field in drm_gem_object_exported_dma_buf_free().
>>>>> That is for exported DMA-buf and should *NEVER* be used for imported ones.
>>>> Did you look at the discussion at the Closes tag? Where else could dma-buf be cleared?
>>> Yeah, I've seen that. The solution is just completely wrong.
>>>
>>> See for the export case obj->dma_buf points to the exported DMA-buf and causes a circle dependency when not set to NULL when the last handle is released.
>>>
>>> But for the import case obj->dma_buf is actually not that relevant. Instead obj->import_attach->dma_buf should be used.
>> So if I understand correctly, the tests in that helper should be done by looking at import_attach->dma_buf.
> At least in theory yes. IIRC we also set obj->dma_buf to the same value as import_attach->dma_buf for convenient so that code doesn't need to check both.
>
> But it can be that obj->dma_buf is already NULL while the import attachment is still in the process of being cleaned up. So there are a couple of cases where we have to look at obj->import_attach->dma_buf.

Ok, that should also solve the problem for now. The point here is to 
limit exposure of import_attach.

>
>> The long-term goal is to make import_attach optional because its setup requires a DMA-capable device.
> HUI WHAT?
>
> Dmitry and I put quite some effort into being able to create an import_attach without the requirement to have a DMA-capable device.
>
> The last puzzle piece of that landed a month ago in the form of this patch here:
>
> commit b72f66f22c0e39ae6684c43fead774c13db24e73
> Author: Christian König <christian.koenig at amd.com>
> Date:   Tue Feb 11 17:20:53 2025 +0100
>
>      dma-buf: drop caching of sg_tables
>      
>      That was purely for the transition from static to dynamic dma-buf
>      handling and can be removed again now.
>      
>      Signed-off-by: Christian König <christian.koenig at amd.com>
>      Reviewed-by: Simona Vetter <simona.vetter at ffwll.ch>
>      Reviewed-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>
>      Link: https://patchwork.freedesktop.org/patch/msgid/20250211163109.12200-5-christian.koenig@amd.com
>
> When you don't create an import attachment the exporter wouldn't know that his buffer is actually used which is usually a quite bad idea.
>
> This is for devices who only want to do a vmap of the buffer, isn't it?

Yeah, the vmap needs the sgtable, which needs import_attach IIRC. 
Somewhere in there a DMA device is required. But it's not of high 
priority as we have workarounds.

Best regards
Thomas

>
> Regards,
> Christian.
>
>> Best regards
>> Thomas
>>
>>> Regards,
>>> Christian.
>>>
>>>> Best regards
>>>> Thomas
>>>>
>>>>> Regards,
>>>>> Christian.
>>>>>
>>>>>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>>>>>> Fixes: b57aa47d39e9 ("drm/gem: Test for imported GEM buffers with helper")
>>>>>> Reported-by: Andy Yan <andyshrk at 163.com>
>>>>>> Closes: https://lore.kernel.org/dri-devel/38d09d34.4354.196379aa560.Coremail.andyshrk@163.com/
>>>>>> Tested-by: Andy Yan <andyshrk at 163.com>
>>>>>> Cc: Thomas Zimmermann <tzimmermann at suse.de>
>>>>>> Cc: Anusha Srivatsa <asrivats at redhat.com>
>>>>>> Cc: Christian König <christian.koenig at amd.com>
>>>>>> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
>>>>>> Cc: Maxime Ripard <mripard at kernel.org>
>>>>>> Cc: David Airlie <airlied at gmail.com>
>>>>>> Cc: Simona Vetter <simona at ffwll.ch>
>>>>>> Cc: Sumit Semwal <sumit.semwal at linaro.org>
>>>>>> Cc: "Christian König" <christian.koenig at amd.com>
>>>>>> Cc: dri-devel at lists.freedesktop.org
>>>>>> Cc: linux-media at vger.kernel.org
>>>>>> Cc: linaro-mm-sig at lists.linaro.org
>>>>>> ---
>>>>>>     include/drm/drm_gem.h | 8 +++++++-
>>>>>>     1 file changed, 7 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
>>>>>> index 9b71f7a9f3f8..f09b8afcf86d 100644
>>>>>> --- a/include/drm/drm_gem.h
>>>>>> +++ b/include/drm/drm_gem.h
>>>>>> @@ -589,7 +589,13 @@ static inline bool drm_gem_object_is_shared_for_memory_stats(struct drm_gem_obje
>>>>>>     static inline bool drm_gem_is_imported(const struct drm_gem_object *obj)
>>>>>>     {
>>>>>>         /* The dma-buf's priv field points to the original GEM object. */
>>>>>> -    return obj->dma_buf && (obj->dma_buf->priv != obj);
>>>>>> +    return (obj->dma_buf && (obj->dma_buf->priv != obj)) ||
>>>>>> +           /*
>>>>>> +        * TODO: During object release, the dma-buf might already
>>>>>> +        *       be gone. For now keep testing import_attach, but
>>>>>> +        *       this should be removed at some point.
>>>>>> +        */
>>>>>> +           obj->import_attach;
>>>>>>     }
>>>>>>       #ifdef CONFIG_LOCKDEP

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)