From: Daniel Vetter <daniel@ffwll.ch>
To: "Christian König" <ckoenig.leichtzumerken@gmail.com>,
sumit.semwal@linaro.org, linux-media@vger.kernel.org,
dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org,
linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org
Subject: Re: [PATCH 05/12] dma-buf: add explicit buffer pinning
Date: Wed, 17 Apr 2019 16:40:11 +0200 [thread overview]
Message-ID: <20190417144011.GI13337@phenom.ffwll.local> (raw)
In-Reply-To: <20190417143051.GG13337@phenom.ffwll.local>
On Wed, Apr 17, 2019 at 04:30:51PM +0200, Daniel Vetter wrote:
> On Wed, Apr 17, 2019 at 04:20:02PM +0200, Daniel Vetter wrote:
> > On Tue, Apr 16, 2019 at 08:38:34PM +0200, Christian König wrote:
> > > Add optional explicit pinning callbacks instead of implicitly assume the
> > > exporter pins the buffer when a mapping is created.
> > >
> > > Signed-off-by: Christian König <christian.koenig@amd.com>
> >
> > Don't we need this together with the invalidate callback and the dynamic
> > stuff? Also I'm assuming that pin/unpin is pretty much required for
> > dynamic bo, so could we look at these callbacks instead of the dynamic
> > flag you add in patch 1.
> >
> > I'm assuming following rules hold:
> > no pin/upin from exporter:
> >
> > dma-buf is not dynamic, and pinned for the duration of map/unmap. I'm
> > not 100% sure whether really everyone wants the mapping to be cached for
> > the entire attachment, only drm_prime does that. And that's not the only
> > dma-buf importer.
> >
> > pin/unpin calls are noops.
> >
> > pin/unpin exist in the exporter, but importer has not provided an
> > invalidate callback:
> >
> > We map at attach time, and we also have to pin, since the importer can't
> > handle the buffer disappearing, at attach time. We unmap/unpin at detach.
>
> For this case we should have a WARN in pin/unpin, to make sure importers
> don't do something stupid. One more thought below on pin/unpin.
>
> > pin/unpin from exporter, invalidate from importer:
> >
> > Full dynamic mapping. We assume the importer will do caching, attach
> > fences as needed, and pin the underlying bo when it needs it it
> > permanently, without attaching fences (i.e. the scanout case).
> >
> > Assuming I'm not terribly off with my understanding, then I think it'd be
> > best to introduce the entire new dma-buf api in the first patch, and flesh
> > it out later. Instead of spread over a few patches. Plus the above (maybe
> > prettier) as a nice kerneldoc overview comment for how dynamic dma-buf is
> > supposed to work really.
> > -Daniel
> >
> > > ---
> > > drivers/dma-buf/dma-buf.c | 39 +++++++++++++++++++++++++++++++++++++++
> > > include/linux/dma-buf.h | 37 +++++++++++++++++++++++++++++++------
> > > 2 files changed, 70 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > > index a3738fab3927..f23ff8355505 100644
> > > --- a/drivers/dma-buf/dma-buf.c
> > > +++ b/drivers/dma-buf/dma-buf.c
> > > @@ -630,6 +630,41 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
> > > }
> > > EXPORT_SYMBOL_GPL(dma_buf_detach);
> > >
> > > +/**
> > > + * dma_buf_pin - Lock down the DMA-buf
> > > + *
> > > + * @dmabuf: [in] DMA-buf to lock down.
> > > + *
> > > + * Returns:
> > > + * 0 on success, negative error code on failure.
> > > + */
> > > +int dma_buf_pin(struct dma_buf *dmabuf)
>
> Hm, I think it'd be better to pin the attachment, not the underlying
> buffer. Attachment is the thin the importer will have to pin, and it's at
> attach/detach time where dma-buf needs to pin for importers who don't
> understand dynamic buffer sharing.
>
> Plus when we put that onto attachments, we can do a
>
> WARN_ON(!attach->invalidate);
>
> sanity check. I think that would be good to have.
Another validation idea: dma-buf.c could track the pin_count on the struct
dma_buf, and if an exporter tries to invalidate while pinned WARN and bail
out. Because that's clearly a driver bug.
All in the interest in making the contract between importers and exporters
as clear as possible.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
next prev parent reply other threads:[~2019-04-17 14:40 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-16 18:38 dynamic DMA-buf sharing between devices Christian König
2019-04-16 18:38 ` [PATCH 01/12] dma-buf: add dynamic caching of sg_table Christian König
2019-04-16 18:38 ` [PATCH 02/12] dma-buf: add dma_buf_(un)map_attachment_locked variants v3 Christian König
2019-05-27 10:56 ` Christian König
2019-04-16 18:38 ` [PATCH 03/12] dma-buf: lock the reservation object during (un)map_dma_buf v3 Christian König
2019-04-17 14:08 ` Daniel Vetter
2019-04-17 14:14 ` Christian König
2019-04-17 14:26 ` Daniel Vetter
2019-04-17 17:10 ` Christian König
2019-04-16 18:38 ` [PATCH 04/12] dma-buf: add optional invalidate_mappings callback v5 Christian König
2019-04-17 14:01 ` Daniel Vetter
2019-04-17 14:33 ` Daniel Vetter
2019-04-17 19:07 ` Daniel Vetter
2019-04-17 19:13 ` Christian König
2019-04-18 8:08 ` Daniel Vetter
2019-04-18 8:28 ` Koenig, Christian
2019-04-18 8:40 ` Daniel Vetter
2019-04-16 18:38 ` [PATCH 05/12] dma-buf: add explicit buffer pinning Christian König
2019-04-17 14:20 ` Daniel Vetter
2019-04-17 14:30 ` Daniel Vetter
2019-04-17 14:40 ` Daniel Vetter [this message]
2019-04-17 19:05 ` Daniel Vetter
2019-04-19 19:05 ` Alex Deucher
2019-04-16 18:38 ` [PATCH 06/12] drm: remove prime sg_table caching Christian König
2019-04-16 18:38 ` [PATCH 07/12] drm/ttm: remove the backing store if no placement is given Christian König
2019-04-16 18:38 ` [PATCH 08/12] drm/ttm: use the parent resv for ghost objects Christian König
2019-04-16 18:38 ` [PATCH 09/12] drm/amdgpu: add independent DMA-buf export v3 Christian König
2019-04-16 18:38 ` [PATCH 10/12] drm/amdgpu: add independent DMA-buf import v4 Christian König
2019-04-16 18:38 ` [PATCH 11/12] drm/amdgpu: add DMA-buf pin/unpin implementation Christian König
2019-04-16 18:38 ` [PATCH 12/12] drm/amdgpu: add DMA-buf invalidation callback v2 Christian König
2019-04-17 13:52 ` dynamic DMA-buf sharing between devices Chunming Zhou
2019-04-17 13:59 ` Christian König
2019-04-17 14:14 ` Chunming Zhou
2019-04-18 9:13 ` Daniel Vetter
2019-04-18 10:57 ` Christian König
2019-04-27 0:01 ` [PATCH 01/12] dma-buf: add dynamic caching of sg_table Liam Mark
2019-05-22 16:17 ` Sumit Semwal
2019-05-22 17:27 ` Christian König
2019-05-22 18:30 ` Daniel Vetter
2019-05-23 11:21 ` Koenig, Christian
2019-05-23 11:30 ` Daniel Vetter
2019-05-23 11:32 ` Daniel Vetter
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