Re: [PATCH] drm/panfrost: Use drm_gem_dump_map_offset()

From: Steven Price <steven.price@arm.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>,
	Alyssa Rosenzweig <alyssa@rosenzweig.io>,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	Tomeu Vizoso <tomeu.vizoso@collabora.com>
Subject: Re: [PATCH] drm/panfrost: Use drm_gem_dump_map_offset()
Date: Mon, 13 May 2019 16:14:01 +0100	[thread overview]
Message-ID: <0b7f0b7f-3e14-f5bb-368a-08037c2a8529@arm.com> (raw)
In-Reply-To: <155775884217.2165.11223399053598674062@skylake-alporthouse-com>

On 13/05/2019 15:47, Chris Wilson wrote:
> Quoting Daniel Vetter (2019-05-13 15:39:21)
>> On Mon, May 13, 2019 at 03:32:44PM +0100, Steven Price wrote:
>>> panfrost_ioctl_mmap_bo() contains a reimplementation of
>>> drm_gem_dump_map_offset() but with a bug - it allows mapping imported
>>> objects (without going through the exporter). Fix this by switching to
>>> use the generic drm_gem_dump_map_offset() function instead which has the
>>> bonus of simplifying the code.
>>
>> gem_dumb stuff is for kms drivers, panfrost is a render driver. We're
>> generally trying to separate these two worlds somewhat cleanly.
>>
>> I think it'd be good to have a non-dumb version of this in the core, and
>> use that. Or upgrade the dumb version to be that helper for everyone (and
>> drop the _dumb).

I'm slightly confused by what you think is the best course of action here.

I can create a patch dropping the '_dumb' from drm_gem_dump_map_offset()
if that's the objection. Or do you want a helper function with two
callers (the 'dump' and the 'shmem' versions)?

> More specifically, since panfrost is using the drm_gem_shmem helper and
> vm_ops, it too can provide the wrapper as it is the drm_gem_shmem layer
> that implies that trying to mmap an imported object is an issue as that
> is not a universal problem.
mmap'ing an imported object isn't a problem as such. But rather than
going behind the exporter's back we would need to call dma_buf_mmap() to
ensure that the exporter can do whatever is necessary.

I could add a wrapper in drm_gem_shmem_helper, but I'm not sure what the
benefit of a wrapper here is?

Steve