[Mesa-dev] [PATCH 2/2] gallium: expose ARB_map_buffer_alignment on Radeon

[Marek Olšák]

On Mon, Oct 29, 2012 at 6:29 PM, Ian Romanick <idr at freedesktop.org> wrote:
> On 10/28/2012 09:55 AM, Marek Olšák wrote:
>
> How does Gallium handle GL_WRITE_ONLY mappings of busy buffers?  In the
> past, this would cause drivers to allocate a temporary buffer (of some sort)
> and return that to the application.  In at least some cases, that temporary
> buffer came from malloc... which might not meet the alignment requirement.

It's up to Gallium drivers how they handle it. They can use
align_malloc from u_memory.h. (r300g uses malloced indexed and vertex
buffers on chipsets without TCL, a newer version of this patch adds
align_malloc to several places in that driver)

I think it's only legal to allocate a temporary buffer if one of the
DISCARD(=INVALIDATE) flags is used.

The plan for radeons is to allocate a temporary buffer in GTT (which
is always aligned to the page size). On unmap, we will copy the
temporary buffer to the busy one on the GPU. It's not clear to me how
a malloced buffer can be used to implement a wait-free transfer (I can
only imagine the malloced buffer is copied into the GPU command stream
and the GPU will read it and copy it to the busy buffer).

Marek