Re: [Intel-gfx] [PATCH] drm-buf: Add debug option

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
	John Stultz <john.stultz@linaro.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	DMA BUFFER SHARING FRAMEWORK <linux-media@vger.kernel.org>
Subject: Re: [Intel-gfx] [PATCH] drm-buf: Add debug option
Date: Fri, 15 Jan 2021 08:25:46 +0000	[thread overview]
Message-ID: <161069914606.19482.1823329746575120144@build.alporthouse.com> (raw)
In-Reply-To: <YAATPMc+1jmhxnsF@phenom.ffwll.local>

Quoting Daniel Vetter (2021-01-14 09:47:40)
> On Thu, Jan 14, 2021 at 09:45:37AM +0000, Chris Wilson wrote:
> > Quoting Daniel Vetter (2021-01-14 09:30:32)
> > > On Thu, Jan 14, 2021 at 10:23 AM Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > > > The only other problem I see with the implementation is that there's
> > > > nothing that says that each dmabuf->ops->map_dma_buf() returns a new
> > > > sg_table, so we may end up undoing the xor. Or should each dma-buf
> > > > return a fresh dma-mapping for iommu isolation?
> > > 
> > > Maybe I screwed it up, but that's why I extracted the little helpers:
> > > We scramble when we get the sgtable from exporter, and unscramble
> > > before we pass it back. dma-buf.c does some caching and will hand back
> > > the same sgtable, but for that case we don't re-scramble.
> > 
> > The attachment is only mapped once, but there can be more than one
> > attachment, and the backend could return the same sg_table for each
> > mapping. Conceivably, it could return its own private sg_table where it
> > wants to maintain the struct page. Seems like just adding a sentence to
> > @map_dma_buf to clarify that each call should return a new sg_table will
> > suffice.
> 
> Ah yes good point, will augment (once CI stops being angry at me).

Fwiw, with a quick explanation of "don't do this" in the docs,
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris