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

[Thomas Zimmermann]

Am 30.11.20 um 16:30 schrieb Daniel Vetter:
> On Mon, Nov 30, 2020 at 01:04:26PM +0100, Thomas Zimmermann wrote:
>> Mapping a GEM object's buffer into kernel address space prevents the
>> buffer from being evicted from VRAM, which in turn may result in
>> out-of-memory errors. It's therefore required to only vmap GEM BOs for
>> short periods of time; unless the GEM implementation provides additional
>> guarantees.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>> ---
>>   drivers/gpu/drm/drm_prime.c |  6 ++++++
>>   include/drm/drm_gem.h       | 16 ++++++++++++++++
>>   2 files changed, 22 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>> index 7db55fce35d8..9c9ece9833e0 100644
>> --- a/drivers/gpu/drm/drm_prime.c
>> +++ b/drivers/gpu/drm/drm_prime.c
>> @@ -669,6 +669,12 @@ EXPORT_SYMBOL(drm_gem_unmap_dma_buf);
>>    * callback. Calls into &drm_gem_object_funcs.vmap for device specific handling.
>>    * The kernel virtual address is returned in map.
>>    *
>> + * To prevent the GEM object from being relocated, callers must hold the GEM
>> + * object's reservation lock from when calling this function until releasing the
>> + * mapping. Holding onto a mapping and the associated reservation lock for an
>> + * unbound time may result in out-of-memory errors. Calls to drm_gem_dmabuf_vmap()
>> + * should therefore be accompanied by a call to drm_gem_dmabuf_vunmap().
>> + *
>>    * Returns 0 on success or a negative errno code otherwise.
> 
> This is a dma-buf hook, which means just documenting the rules you'd like
> to have here isn't enough. We need to roll this out at the dma-buf level,
> and enforce it.
> 

The documentation for GEM vmap callbacks point here, so it was a good 
point to start.

I know about the dependencies on dmabuf. But fixing everything now is 
unrealistic. My hope for this patch is that we can find the necessary 
rules and document them.

> Enforce it = assert_lock_held

That's probably the final step of many.

Best regards
Thomas

> 
> Roll out = review everyone. Because this goes through dma-buf it'll come
> back through shmem helpers (and other helpers and other subsystems) back
> to any driver using vmap for gpu buffers. This includes the media
> subsystem, and the media subsystem definitely doesn't cope with just
> temporarily mapping buffers. So there we need to pin them, which I think
> means we'll need 2 version of dma_buf_vmap - one that's temporary and
> requires we hold dma_resv lock, the other requires that the buffer is
> pinned.
> 
> 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
>>
> 

-- 
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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