linux-kernel - Re: OMAPFB: CMA allocation failures

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Date:	Tue, 5 Nov 2013 21:55:51 +0200 (EET)
From:	Ивайло Димитров 
	<freemangordon@....bg>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	minchan@...nel.org, pavel@....cz, sre@...ian.org,
	pali.rohar@...il.com, pc+n900@...f.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: OMAPFB: CMA allocation failures

 >-------- Оригинално писмо --------
 >От:  Tomi Valkeinen 
 >Относно: Re: OMAPFB: CMA allocation failures
 >До: Ивайло Димитров
 >Изпратено на: Сряда, 2013, Октомври 30 14:19:32 EET
 >
 >I really dislike the idea of adding the omap vram allocator back. Then
 >again, if the CMA doesn't work, something has to be done.
 >

If I got Minchan Kim's explanation correctly, CMA simply can't be used
for allocation of framebuffer memory, because it is unreliable.

 >Pre-allocating is possible, but that won't work if there's any need to
 >re-allocating the framebuffers. Except if the omapfb would retain and
 >manage the pre-allocated buffers, but that would just be more or less
 >the old vram allocator again.
 >
 >So, as I see it, the best option would be to have the standard dma_alloc
 >functions get the memory for omapfb from a private pool, which is not
 >used for anything else.
 >
 >I wonder if that's possible already? It sounds quite trivial to me.

dma_alloc functions use either CMA or (iirc) get_pages_exact if CMA is
disabled. Both of those fail easily. AFAIK there are several 
implementations with similar functionality, like CMEM and ION but
(correct me if I am wrong) neither of them is upstreamed. In the 
current kernel I don't see anything that can be used for the purpose 
of reliable allocation of big chunks of contiguous memory.
So, something should be done, but honestly, I can't think of anything
but bringing VRAM allocator back. Not that I like the idea of bringing
back ~700 lines of code, but I see no other option if omapfb driver is
to be actually useful.

Regards,
Ivo
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