[Q] block / zynq: DMA bouncing

On Mon, Jan 27, 2014 at 04:13:56PM +0100, Guennadi Liakhovetski wrote:

Hi all,

I'm working on an MMC driver with a DMA capability. All has been working 
well, until at some point I've got a bus error, when the mmc driver had 
been handed in a buffer at 0x3000 physical RAM address. The reason is, 
that on Zynq arch bus masters cannot access RAM below 0x80000. Therefore 
my question: how shall I configure this in software?

The way I found was to use ARM-specific struct dmabounce_device_info and 
implement its .needs_bounce() method to return true for those addresses. 
Is this the right way or is there a better / more straight-forward one?

To do the above I have to enable CONFIG_DMABOUNCE, which then selects 
CONFIG_ZONE_DMA. Having done just that I suddenly discover, that 0x3000 
buffers aren't used any more, so, I cannot actually verify my 
implementation :) Looking at ZONE_DMA it looks like it is still covering 
the whole RAM range (/proc/zoneinfo shows start_pfn=0 in zone DMA), so, I 
don't see why 0x3000 should be excluded now.

So, is using the .needs_bounce() method the correct way to support DMA on 
this arch or is there a better one?

I have a similar issue with Renesas R8A7790 where there is a bus bridge
that can only deal with transactions to one half of the available RAM.

-- 
Ben Dooks, ben at fluff.org, http://www.fluff.org/ben/

Large Hadron Colada: A large Pina Colada that makes the universe disappear.