Hi Daniel,
On Wednesday 10 July 2013 13:54:33 Daniel Vetter wrote:
> I've checked both implementations (radeon/nouveau) and they both grab
> the page array from ttm simply by dereferencing it and then wrapping
> it up with drm_prime_pages_to_sg in the callback and map it with
> dma_map_sg (in the helper).
Have you checked drm_gem_cma_prime_get_sg_table (in drm_gem_cma_helper.c) as
well ?
> Only the grabbing of the underlying page array is anything we need to
> be concerned about, and either those pages are pinned independently,
> or we're screwed no matter what.
>
> And indeed, nouveau/radeon pin the backing storage in their
> attach/detach functions.
>
> The only thing we might claim it does is prevent concurrent mapping of
> dma_buf attachments. But a) that's not allowed and b) the current code
> is racy already since it checks whether the sg mapping exists _before_
> grabbing the lock.
>
> So the dev->struct_mutex locking here does absolutely nothing useful,
> but only distracts. Remove it.
>
> This should also help Maarten's work to eventually pin the backing
> storage more dynamically by preventing locking inversions around
> dev->struct_mutex.
>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> drivers/gpu/drm/drm_prime.c | 3 ---
> 1 file changed, 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 85e450e..64a99b3 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -167,8 +167,6 @@ static struct sg_table *drm_gem_map_dma_buf(struct
> dma_buf_attachment *attach, if (WARN_ON(prime_attach->dir != DMA_NONE))
> return ERR_PTR(-EBUSY);
>
> - mutex_lock(&obj->dev->struct_mutex);
> -
> sgt = obj->dev->driver->gem_prime_get_sg_table(obj);
>
> if (!IS_ERR(sgt)) {
> @@ -182,7 +180,6 @@ static struct sg_table *drm_gem_map_dma_buf(struct
> dma_buf_attachment *attach, }
> }
>
> - mutex_unlock(&obj->dev->struct_mutex);
> return sgt;
> }
--
Regards,
Laurent Pinchart
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