On 08/07/2015 09:25 AM, Russell King - ARM Linux wrote:

On Fri, Aug 07, 2015 at 03:22:56PM +0200, Sebastian Andrzej Siewior wrote:

quoted

For TX-transfers, I would need to update the start-address so the
transfers begins where it stopped. However based on your concern I
can't really assume that the position reported by the HW is the correct
one.

Exactly - I don't believe that existing OMAP DMA hardware can ever support
pausing a mem-to-device transfer without data loss as you have no idea
how many bytes have been prefetched by the DMA, and therefore you have
no idea how many bytes to unwind the hardware position.  It gets worse
than that when you have to cross into the previous descriptor.  It's
really not nice.

So, disallowing pause for mem-to-device is entirely reasonable given the
data loss implications.

 
Thanks for adding your hard-won knowledge to this discussion, Russell.
This saves us a bunch of wasted effort trying to fix x_char with DMA
(and TCSANOW termios changes and throttling).

Regards,
Peter Hurley