linux-kernel - Re: [PATCH 05/12] dma-buf: add explicit buffer pinning

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Message-ID: <20190417190546.GK13337@phenom.ffwll.local>
Date:   Wed, 17 Apr 2019 21:05:46 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>, sumit.semwal@...aro.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org
Subject: Re: [PATCH 05/12] dma-buf: add explicit buffer pinning

On Wed, Apr 17, 2019 at 04:40:11PM +0200, Daniel Vetter wrote:
> 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@....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.

btw just realized that you already have the pin/unpin here. I think even
more reasons to squash the invalidate/pin bits together, since they
don't really work separately.
-Daniel

> > 
> > > 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch