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SubjectRe: [PATCH] dma-buf: refcount the attachment for cache_sgt_mapping
Hi Christian,

Thanks for the quick reply.

On Wed, Jun 12, 2019 at 07:45:38AM +0000, Koenig, Christian wrote:
> Am 12.06.19 um 03:22 schrieb Nicolin Chen:
> > Commit f13e143e7444 ("dma-buf: start caching of sg_table objects v2")
> > added a support of caching the sgt pointer into an attach pointer to
> > let users reuse the sgt pointer without another mapping. However, it
> > might not totally work as most of dma-buf callers are doing attach()
> > and map_attachment() back-to-back, using drm_prime.c for example:
> > drm_gem_prime_import_dev() {
> > attach = dma_buf_attach() {
> > /* Allocating a new attach */
> > attach = kzalloc();
> > /* .... */
> > return attach;
> > }
> > dma_buf_map_attachment(attach, direction) {
> > /* attach->sgt would be always empty as attach is new */
> > if (attach->sgt) {
> > /* Reuse attach->sgt */
> > }
> > /* Otherwise, map it */
> > attach->sgt = map();
> > }
> > }
> >
> > So, for a cache_sgt_mapping use case, it would need to get the same
> > attachment pointer in order to reuse its sgt pointer. So this patch
> > adds a refcount to the attach() function and lets it search for the
> > existing attach pointer by matching the dev pointer.
>
> I don't think that this is a good idea.
>
> We use sgt caching as workaround for locking order problems and want to
> remove it again in the long term.

Oh. I thought it was for a performance improving purpose. It may
be a misunderstanding then.

> So what is the actual use case of this?

We have some similar downstream changes at dma_buf to reduce the
overhead from multiple clients of the same device doing attach()
and map_attachment() calls for the same dma_buf.

We haven't used DRM/GRM_PRIME yet but I am also curious would it
benefit DRM also if we reduce this overhead in the dma_buf?

Thanks
Nicolin

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