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Subject: Re: [PATCH v8 1/6] drm/fourcc: Add modifier definitions for
 describing Amlogic Video Framebuffer Compression
To:     Simon Ser <contact@emersion.fr>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "jianxin.pan@amlogic.com" <jianxin.pan@amlogic.com>,
        Kevin Hilman <khilman@baylibre.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-amlogic@lists.infradead.org" 
        <linux-amlogic@lists.infradead.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
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From:   Neil Armstrong <narmstrong@baylibre.com>
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Organization: Baylibre
Message-ID: <044964ad-b927-57d7-9361-beda5c8d99a8@baylibre.com>
Date:   Thu, 2 Jul 2020 15:34:40 +0200
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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@baylibre.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.

> 
> 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
>