[Mesa-dev] [PATCH 4/4] gbm: Add map/unmap functions

[Michel Dänzer]

On 26.04.2016 03:51, Rob Herring wrote:
> On Mon, Apr 25, 2016 at 9:25 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>> On 25 April 2016 at 13:46, Daniel Stone <daniel at fooishbar.org> wrote:
>>> On 23 April 2016 at 03:08, Rob Herring <robh at kernel.org> wrote:
>>>> On Fri, Apr 22, 2016 at 6:32 PM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>>>>> Can we take a look at the GBM gralloc as well. One thing that worries
>>>>> me is that (most likely) you are requesting/creating a bo without
>>>>> GBM_BO_USE_WRITE whist using MAP + CPU write UNMAP. If you do set the
>>>>> USE_WRITE flag, you're getting a dumb buffer, which I'm not sure how
>>>>> well is going to work.
>>>>
>>>> I'm not using GBM_BO_USE_WRITE and that is not a condition for mapping
>>>> given that flag is tied to cursors (according to comments) and gives
>>>> dumb buffers. Also of note, if gralloc flags are set for r/w often,
>>>> then I request a linear buffer. Here's the gralloc side:
>>>
>>> Right, I wouldn't take GBM_BO_USE_WRITE to have much of an effect on
>>> mappings, as it pessimises allocation like you say.
>>>
>> Ftr, I'm not objecting as to how things are done. Just saying that
>> things should be blindly obvious as one reads the documentation alone.
>> I'm assuming that most people involved are "tainted" (the know to a
>> level how things are implemented) thus things are clearer for them.
> 
> I'm not sure what to document here other than the use flags have no
> impact or restrictions on mapping. If that's not true, then that is a
> limitation within the gallium drivers of which I have little knowledge
> about and need someone tainted to spell out. I suppose documenting
> that buffers with frequent cpu access should use a linear buffer would
> be universally true?

It's actually stricter than that with the radeonsi driver: It sets the
RADEON_GEM_NO_CPU_ACCESS/AMDGPU_GEM_CREATE_NO_CPU_ACCESS flag when
allocating non-linear buffers, signalling to the kernel driver that CPU
access to the buffer doesn't need to work. At least the amdgpu kernel
driver actually enforces this and returns an error if userspace tries
mapping such a buffer.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer