[Q] block / zynq: DMA bouncing

Guennadi Liakhovetski g.liakhovetski at gmx.de
Mon Jan 27 10:35:18 EST 2014

Hi Ben,

On Mon, 27 Jan 2014, Ben Dooks wrote:

> 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.

Have you tried enabling CONFIG_DMABOUNCE?

Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer

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