Re: [PATCH] dma: sun4i: expose block size and wait cycle configuration to DMA users

From: Boris Brezillon
Date: Fri Mar 11 2016 - 04:46:10 EST

On Fri, 11 Mar 2016 11:56:07 +0530
Vinod Koul <vinod.koul@xxxxxxxxx> wrote:

> On Wed, Mar 09, 2016 at 12:06:27PM +0100, Boris Brezillon wrote:
> > On Tue, 8 Mar 2016 08:25:47 +0530
> > Vinod Koul <vinod.koul@xxxxxxxxx> wrote:
> > >
> > > Why does dmaengine need to wait? Can you explain that
> >
> > I don't have an answer for that one, but when I set WAIT_CYCLES to 1
> > for the NAND use case it does not work. So I guess it is somehow
> > related to how the DRQ line is controlled on the device side...
> Is the WAIT cycle different for different usages or same for all
> usages/channels?

In Allwinner BSP they adapt it on a per slave device basis, but since
DMA channels are dynamically allocated, you can't know in advance which
physical channel will be attached to a specific device.

Another option I considered was adding a new cell to the sun4i DT
binding to encode these WAIT_CYCLES and BLOCK_SIZE information. But I'm
not sure adding that to the DT is a good idea (not to mention that it
would break DT ABI again, and given the last discussions on this topic,
I'm not sure it's a good idea :-/).

Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering