[PATCH 2/2] DMA: PL08x: fix channel pausing to timeout rather than lockup

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Sun, Jan 30, 2011 at 08:48:27AM -0800, Dan Williams wrote:
> On Thu, Jan 27, 2011 at 4:37 AM, Russell King - ARM Linux
> <linux@arm.linux.org.uk> wrote:
> > If a transfer is initiated from memory to a peripheral, then data is
> > fetched and the channel is marked busy. ?This busy status persists until
> > the HALT bit is set and the queued data has been transfered to the
> > peripheral. ?Waiting indefinitely after setting the HALT bit results in
> > system lockups. ?Timeout this operation, and print an error when this
> > happens.
> >
> > Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
> > ---
> > This happens when a M->P transfer is started, but the peripheral doesn't
> > take the data. ?The DMA controller will mark itself active and fetch data
> > from memory, in anticipation of the peripheral wanting the data. ?This
> > advances the source address.
> >
> > An alternative approach here is to just set the HALT bit and return
> > immediately, without waiting for the primecell to show that it is no
> > longer active. ?In any case, resuming the channel will allow the
> > transfer to continue without data loss.
> >
> 
> Ok, and the other infinite busy wait in pl08x_start_txd() is
> guaranteed to be after a resume operation, or does it also need a
> timeout?

The channel should not be active at that point.  If it is, that means
we're trying to setup a new transfer on a physical channel on top of an
already active transfer, which obviously could result in the previous
transfer being destroyed.

Maybe a BUG_ON(pl08x_phy_channel_busy(phychan)) would be more appropriate
there - but I think that's something to address in future patches.