linux-kernel - Re: [PATCH v8 1/6] drm/fourcc: Add modifier definitions for describing Amlogic Video Framebuffer Compression

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Date:   Thu, 2 Jul 2020 16:15:44 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     Simon Ser <contact@...rsion.fr>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "jianxin.pan@...ogic.com" <jianxin.pan@...ogic.com>,
        Kevin Hilman <khilman@...libre.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-amlogic@...ts.infradead.org" 
        <linux-amlogic@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v8 1/6] drm/fourcc: Add modifier definitions for
 describing Amlogic Video Framebuffer Compression

On Thu, Jul 2, 2020 at 3:34 PM Neil Armstrong <narmstrong@...libre.com> wrote:
>
> On 02/07/2020 15:18, Daniel Vetter wrote:
> > On Thu, Jul 02, 2020 at 09:23:11AM +0000, Simon Ser wrote:
> >> On Thursday, July 2, 2020 9:47 AM, Neil Armstrong <narmstrong@...libre.com> wrote:
> >>
> >>> Finally is also adds the Scatter Memory layout, meaning 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.
> >>
> >> Still not sure how to handle this one, since this breaks fundamental
> >> assumptions about modifiers.
> >
> > I wonder whether we should require special allocations for these, and then
> > just outright reject mmap on these buffers. mmap on dma-buf isn't a
> > required feature.
>
> Yes, it's the plan to reject mmap on these buffers, but it can't be explained
> in the modifiers description and it's a requirement of the producer, not the
> consumer.

Hm I think worth to add that as a note to the modifier. Just to make
sure. And avoids questions like the one from Simon.
-Daniel

>
> >
> > That would make sure that userspace cannot look at them.
> >
> > Also I'm kinda suspecting that there's not unlimited amounts of this magic
> > invisible storage available anyway.
> > -Daniel
> >
>
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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