Re: [PATCH 1/8] drm/gem: Write down some rules for vmap usage

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 01.12.20 um 11:00 schrieb Daniel Vetter:
>> [...]
>> For my POV, the current interfaces have no clear policy or semantics.
>> Looking through the different GEM implementations, each one seems to
>> have its own interpretation.
> 
> Yup, that's the problem really. In the past we've had vmap exclusively
> for permanently pinned access, with no locking requirements. Now we're
> trying to make this more dynamic, but in a somewhat ad-hoc fashion
> (generic fbdev emulation charged ahead with making the fbdev
> framebuffer evictable), and now it breaks at every seam. Adding more
> ad-hoc semantics on top doesn't seem like a good idea.
> 
> That's why I think we should have 2 different interfaces:
> - dma_buf_vmap, the one we currently have. Permanently pins the
> buffer, mapping survives, no locking required.

I just looked at the implementation of dma_buf_vmap() and there's no 
pinning happening AFAICT. Also, none of the callback's implementations 
does pinning (except vram helpers). Do you mean dma_buf_attach() instead?

Best regards
Thomas

> - dma_buf_vmap_local, the new interface, the one that generic fbdev
> should have used (but oh well mistakes happen), requires
> dma_resv_lock, the mapping is only local to the caller
> 
> Trying to shovel both semantics into one interface, depending upon
> which implementation we have backing the buffer, doesn't work indeed.
> 
> Also on the pin topic, I think neither interface should require
> callers to explicitly pin anything. For existing users it should
> happen automatically behind the scenes imo, that's what they're
> expecting.
> -Daniel
> 
> 
>>> I think we could use what we've done for dynamic dma-buf attachment
>>> (which also change locking rules) and just have new functions for the
>>> new way (i.e. short term vmap protected by dma_resv lock. Maybe call
>>> these dma_buf_vmap_local, in the spirit of the new kmap_local which
>>> are currently under discussion. I think _local suffix is better, for
>>> otherwise people might do something silly like
>>>
>>>       dma_resv_lock();
>>>       dma_buf_vmap_locked();
>>>       dma_resv_unlock();
>>>
>>>       /* actual access maybe even in some other thread */
>>>
>>>      dma_buf_resv_lock();
>>>      dma_buf_vunmap_unlocked();
>>>      dma_resv_unlock();
>>>
>>> _local suffix is better at telling that the resulting pointer has very
>>> limited use (essentially just local to the calling context, if you
>>> don't change any locking or anything).
>>
>> _local sounds good.
>>
>> Best regards
>> Thomas
>>
>>>
>>> I think encouraging importers to call dma_buf_pin/unpin isn't a good
>>> idea. Yes dynamic ones need it, but maybe we should check for that
>>> somehow in the exporterd interface (atm only amdgpu is using it).
>>> -Daniel
>>>
>>>
>>>
>>>
>>>
>>>> Best regards
>>>> Thomas
>>>>
>>>>
>>>>>
>>>>> Cheers,
>>>>> Christian.
>>>>>
>>>>>>
>>>>>> That's what I meant with that this approach here is very sprawling :-/
>>>>>> -Daniel
>>>>>>
>>>>>>>      */
>>>>>>>     int drm_gem_dmabuf_vmap(struct dma_buf *dma_buf, struct dma_buf_map
>>>>>>> *map)
>>>>>>> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
>>>>>>> index 5e6daa1c982f..7c34cd5ec261 100644
>>>>>>> --- a/include/drm/drm_gem.h
>>>>>>> +++ b/include/drm/drm_gem.h
>>>>>>> @@ -137,7 +137,21 @@ struct drm_gem_object_funcs {
>>>>>>>          * Returns a virtual address for the buffer. Used by the
>>>>>>>          * drm_gem_dmabuf_vmap() helper.
>>>>>>>          *
>>>>>>> +     * Notes to implementors:
>>>>>>> +     *
>>>>>>> +     * - Implementations must expect pairs of @vmap and @vunmap to be
>>>>>>> +     *   called frequently and should optimize for this case.
>>>>>>> +     *
>>>>>>> +     * - Implemenations may expect the caller to hold the GEM object's
>>>>>>> +     *   reservation lock to protect against concurrent calls and
>>>>>>> relocation
>>>>>>> +     *   of the GEM object.
>>>>>>> +     *
>>>>>>> +     * - Implementations may provide additional guarantees (e.g.,
>>>>>>> working
>>>>>>> +     *   without holding the reservation lock).
>>>>>>> +     *
>>>>>>>          * This callback is optional.
>>>>>>> +     *
>>>>>>> +     * See also drm_gem_dmabuf_vmap()
>>>>>>>          */
>>>>>>>         int (*vmap)(struct drm_gem_object *obj, struct dma_buf_map *map);
>>>>>>> @@ -148,6 +162,8 @@ struct drm_gem_object_funcs {
>>>>>>>          * drm_gem_dmabuf_vunmap() helper.
>>>>>>>          *
>>>>>>>          * This callback is optional.
>>>>>>> +     *
>>>>>>> +     * See also @vmap.
>>>>>>>          */
>>>>>>>         void (*vunmap)(struct drm_gem_object *obj, struct dma_buf_map
>>>>>>> *map);
>>>>>>> --
>>>>>>> 2.29.2
>>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> dri-devel mailing list
>>>>> dri-devel@lists.freedesktop.org
>>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>>
>>>> --
>>>> Thomas Zimmermann
>>>> Graphics Driver Developer
>>>> SUSE Software Solutions Germany GmbH
>>>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>>>> (HRB 36809, AG Nürnberg)
>>>> Geschäftsführer: Felix Imendörffer
>>>>
>>>
>>>
>>
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>> (HRB 36809, AG Nürnberg)
>> Geschäftsführer: Felix Imendörffer
>>
> 
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer


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