[Mesa-dev] [PATCH 4/5] r600g, radeonsi: Use write-combined persistent GTT mappings

Marek Olšák maraeo at gmail.com
Fri Jul 18 13:52:50 PDT 2014

GL_MAP_READ_BIT is always unconditionally set for glBufferData, which
is the most-used function. However, st/mesa can look at the
immutability flag to distinguish between BufferData and BufferStorage
before pipe_buffer_create is called, and set the read flag if the
caller is BufferStorage and GL_MAP_READ_BIT is flagged, and not set
any flag otherwise.


On Fri, Jul 18, 2014 at 9:50 PM, Grigori Goronzy <greg at chown.ath.cx> wrote:
> On 18.07.2014 13:45, Marek Olšák wrote:
>> If the requirements of GL_MAP_COHERENT_BIT are satisfied, then the
>> patch is okay.
> Apart from correctness, I still wonder how this will affect performance,
> most notably CPU reads. This change unconditionally uses write-combined,
> uncached memory for MAP_COHERENT buffers. Unless I am missing something,
> CPU reads will be slow, even if the buffer storage flags indicate that
> the buffer will be read by the CPU. Maybe it's a good idea to avoid
> write combined memory if the buffer storage flags include MAP_READ_BIT?
> Grigori
>> Marek
>> On Fri, Jul 18, 2014 at 5:19 AM, Michel Dänzer <michel at daenzer.net> wrote:
>>> On 17.07.2014 21:00, Marek Olšák wrote:
>>>> On Thu, Jul 17, 2014 at 12:01 PM, Michel Dänzer <michel at daenzer.net> wrote:
>>>>> From: Michel Dänzer <michel.daenzer at amd.com>
>>>>> This is hopefully safe: The kernel makes sure writes to these mappings
>>>>> finish before the GPU might start reading from them, and the GPU caches
>>>>> are invalidated at the start of a command stream.
>>>> The resource flags actually tell you what you can do. If the COHERENT
>>>> flag is set, the mapping must be cached.
>>> Why is that required? As I explain above, we should satisfy the
>>> requirements of the ARB_buffer_storage extension AFAICT.
>>> As pointed out by you and Grigori in other posts, I should probably just
>>> drop the special treatment of persistent mappings though, so the
>>> placement and flags are derived from the buffer usage.
>>> --
>>> Earthling Michel Dänzer            |                  http://www.amd.com
>>> Libre software enthusiast          |                Mesa and X developer
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev

More information about the mesa-dev mailing list