[PATCH 1/2] ARM: omapfb: add coherent dma memory support

[Tomi Valkeinen]

On 2014-01-09 07:06, Hiremath, Vaibhav wrote:

> I am seeing underflow issue on AM43x device if I use omapfb_vram argument.
> Did you see this on OMAP?
> 
> I am using "omapfb_vram=10M at 0xA0000000", and I believe it is correct way of usage.

Hmm ok... The AM4x seems to have issues anyway, as we're seeing
underflows easily in other situations also.

Well, there's a small difference in the allocation. The normal dma alloc
uses dma_alloc_attrs() and passes DMA_ATTR_WRITE_COMBINE as a flag,
whereas allocating from the absolute address just uses the piece of
memory. I couldn't find how to set write-combine for the abs memory area.

Then again, that's for CPU caching, so I don't see why it would affect
DSS as such (but that's still something we should measure, cpu
read/write perf for normal and abs allocation).

The only thought I have is that somehow the reserved memory area is
missing some configuration that is done for the rest of the memory. But
that's purely a guess, this is totally out of my area of expertise...

Vaibhav, just to be sure, can you run both with normal dma_alloc and
with the reserve, and verify that the dispc register dumps are the same?
I don't see how they could be different, but just to be sure.

 Tomi


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