Re: [PATCH v4 7/8] drm/fourcc: amlogic: Add modifier definitions for the Scatter layout

From: Pekka Paalanen <ppaalanen@gmail.com>
To: Neil Armstrong <narmstrong@baylibre.com>
Cc: "mjourdan@baylibre.com" <mjourdan@baylibre.com>,
	Simon Ser <contact@emersion.fr>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	Kevin Hilman <khilman@baylibre.com>,
	"linux-amlogic@lists.infradead.org"
	<linux-amlogic@lists.infradead.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v4 7/8] drm/fourcc: amlogic: Add modifier definitions for the Scatter layout
Date: Wed, 25 Mar 2020 15:49:21 +0200	[thread overview]
Message-ID: <20200325154921.2a87930c@eldfell.localdomain> (raw)
In-Reply-To: <b1386ef5-c3e3-c07b-5982-e3f02441b431@baylibre.com>

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On Wed, 25 Mar 2020 11:24:15 +0100
Neil Armstrong <narmstrong@baylibre.com> wrote:

> Hi,
> 
> On 25/03/2020 10:04, Simon Ser wrote:
> > On Wednesday, March 25, 2020 9:50 AM, Neil Armstrong <narmstrong@baylibre.com> wrote:
> >   
> >> Amlogic uses a proprietary lossless image compression protocol and format
> >> for their hardware video codec accelerators, either video decoders or
> >> video input encoders.
> >>
> >> This introduces the Scatter Memory layout, means the header contains IOMMU
> >> references to the compressed frames content to optimize memory access
> >> and layout.
> >>
> >> In this mode, only the header memory address is needed, thus the content
> >> memory organization is tied to the current producer execution and cannot
> >> be saved/dumped neither transferrable between Amlogic SoCs supporting this
> >> modifier.  
> > 
> > I don't think this is suitable for modifiers. User-space relies on
> > being able to copy a buffer from one machine to another over the
> > network. It would be pretty annoying for user-space to have a blacklist
> > of modifiers that don't work this way.
> > 
> > Example of such user-space:
> > https://gitlab.freedesktop.org/mstoeckl/waypipe/
> >   
> 
> I really understand your point, but this is one of the use-cases we need solve.
> This is why I split the fourcc patch and added an explicit comment.
> 
> Please point me a way to display such buffer, the HW exists, works like that and
> it's a fact and can't change.
> 
> It will be the same for secure zero-copy buffers we can't map from userspace, but
> only the HW decoder can read/write and HW display can read.

The comparison to secure buffers is a good one.

Are buffers with the DRM_FORMAT_MOD_AMLOGIC_FBC_LAYOUT_SCATTER modifier
meaningfully mmappable to CPU always / sometimes / never /
varies-and-cannot-know?

Maybe this type should be handled similar to secure buffers, with the
exception that they are not actually secured but only mostly
inaccessible. Then again, I haven't looked at any of the secure buffer
proposals.

Thanks,
pq

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