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

[Neil Armstrong]

Hi,

On 02/07/2020 16:15, Daniel Vetter wrote:
> On Thu, Jul 2, 2020 at 3:34 PM Neil Armstrong <narmstrong@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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.

Something like:

 /*
  * Amlogic FBC Scatter Memory layout
  *
  * Indicates 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.
+ *
+ * Due to the nature of the layout, these buffers are not expected to
+ * be accessible by the user-space clients but only accessible by the
+ * hardware producers and consumers.
+ *
+ * The user-space clients should expect a failure while trying to mmap
+ * the DMA-BUF handle returned by the producer.
  */

Thanks,
Neil

> -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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> 
> 
>