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

[Hiremath, Vaibhav]

> -----Original Message-----
> From: Valkeinen, Tomi
> Sent: Thursday, January 09, 2014 1:52 PM
> To: Hiremath, Vaibhav; Ivaylo Dimitrov
> Cc: Tony Lindgren; linux-omap at vger.kernel.org; linux-arm-
> kernel at lists.infradead.org; linux-fbdev at vger.kernel.org
> Subject: Re: [PATCH 1/2] ARM: omapfb: add coherent dma memory support
> 
> 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.
> 

Will check and update you shortly.

Thanks,
Vaibhav