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

From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "DRI Development" <dri-devel@lists.freedesktop.org>,
	"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
	"Sumit Semwal" <sumit.semwal@linaro.org>,
	"moderated list:DMA BUFFER SHARING FRAMEWORK"
	<linaro-mm-sig@lists.linaro.org>,
	"David Stevens" <stevensd@chromium.org>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	"Christian König" <christian.koenig@amd.com>,
	"open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>
Subject: Re: [Intel-gfx] [PATCH] drm-buf: Add debug option
Date: Wed, 13 Jan 2021 21:50:11 +0100	[thread overview]
Message-ID: <CAKMK7uEnnEj_YAR5Tm3jpS7MNPkqB43JBhQnY_K0YQ+LE9wL1g@mail.gmail.com> (raw)
In-Reply-To: <161055261490.6195.7986280621869334351@build.alporthouse.com>

On Wed, Jan 13, 2021 at 4:43 PM Chris Wilson <chris@chris-wilson.co.uk> wrote:
>
> Quoting Daniel Vetter (2021-01-13 14:06:04)
> > We have too many people abusing the struct page they can get at but
> > really shouldn't in importers. Aside from that the backing page might
> > simply not exist (for dynamic p2p mappings) looking at it and using it
> > e.g. for mmap can also wreak the page handling of the exporter
> > completely. Importers really must go through the proper interface like
> > dma_buf_mmap for everything.
>
> If the exporter doesn't want to expose the struct page, why are they
> setting it in the exported sg_table?

You need to store it somewhere, otherwise the dma-api doesn't work.
Essentially this achieves clearing/resetting the struct page pointer,
without additional allocations somewhere, or tons of driver changes
(since presumably the driver does keep track of the struct page
somewhere too).

Also as long as we have random importers looking at struct page we
can't just remove it, or crashes everywhere. So it has to be some
debug option you can disable.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch