[Linaro-mm-sig] Memory region attribute bits and multiple mappings

[Daniel Stone]

Hi,

On Wed, Apr 20, 2011 at 02:52:56PM -0700, Rebecca Schultz Zavin wrote:
> The android team's graphics folks, the their counterparts at intel,
> imagination tech, nvidia, qualcomm and arm have all told me that they need a
> method for mapping buffers uncached to userspace.  The common case for this
> is to write vertexes, textures etc to these buffers once and never touch
> them again.  This may happen several (or even several 10s or more) of times
> per frame.  My experience with cache flushes on ARM architectures matches
> Marek's.  Typically write combine makes streaming writes really really fast,
> and on several SOC's we've found it cheaper to flush the whole cache than to
> flush by line.  Clearly this impacts the rest of system performance, not to
> mention the fact that a couple of large textures and you've totally blown
> your caches for the rest of the system.

This was exactly our experience with OMAP hardware (the Nokia N900,
running an OMAP3430); we made strong use of uncached regions for exactly
this reason, and measured a fairly dramatic performance win in both
artificial and realistic usecases.

(This was with a Cortex-A8, so happy days.)

Cheers,
Daniel