Re: [PATCH v3 3/3] dma: omap-dma: add support for pause of non-cyclic transfers

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Aug 11, 2015 at 03:02:44PM +0300, Peter Ujfalusi wrote:
> On 08/07/2015 11:00 PM, Sebastian Andrzej Siewior wrote:
> > +	/*
> > +	 * We do not allow DMA_MEM_TO_DEV transfers to be paused.
> > +	 * From the AM572x TRM, 16.1.4.18 Disabling a Channel During Transfer:
> > +	 * "When a channel is disabled during a transfer, the channel undergoes
> > +	 * an abort, unless it is hardware-source-synchronized …".
> > +	 * A source-synchronised channel is one where the fetching of data is
> > +	 * under control of the device. In other words, a device-to-memory
> > +	 * transfer. So, a destination-synchronised channel (which would be a
> > +	 * memory-to-device transfer) undergoes an abort if the the CCR_ENABLE
> > +	 * bit is cleared.
> > +	 * From 16.1.4.20.4.6.2 Abort: "If an abort trigger occurs, the channel
> > +	 * aborts immediately after completion of current read/write
> > +	 * transactions and then the FIFO is cleaned up." The term "cleaned up"
> > +	 * is not defined. TI recommends to check that RD_ACTIVE and WR_ACTIVE
> > +	 * are both clear _before_ disabling the channel, otherwise data loss
> > +	 * will occur.
> > +	 * The problem is that if the channel is active, then device activity
> > +	 * can result in DMA activity starting between reading those as both
> > +	 * clear and the write to DMA_CCR to clear the enable bit hitting the
> > +	 * hardware. If the DMA hardware can't drain the data in its FIFO to the
> > +	 * destination, then data loss "might" occur (say if we write to an UART
> > +	 * and the UART is not accepting any further data).
> 
> I don't know if you have checked it, but probably the TX DMA could be also
> used when the PRZEFETCH is disabled for the channel? Just a guess

The docs aren't very clear on that... and iirc Santosh's reply didn't
suggest that the prefetch bit had any influence on this behaviour.  Given
the wording in the documentation which seems to be quite explicit about
the conditions, and it omits talking about the prefetch bit, I can only
assume that the prefetch bit has no influence over this behaviour.

For example, what happens if the DMA to the device has started - the
device has raised its DMA request line.  The DMA controller has then gone
to memory and has fetched some data and incremented the source address.
Meanwhile, we've cleared the ENABLE bit.  What happens then?  Does the
DMA controller drain the read data to the device, or does it "clean up"
the FIFO by discarding the data?

Given that the conditions under which the FIFO is drained to the
destination are very specific, and which explicitly excludes destination-
synchronised transfers, the only conclusion that's possible without
knowing the implementation intimately is that the FIFO is "cleaned up"
which suggests that it's discarded rather than drained to the destination.

As this DMA controller is in all of the OMAP devices and similar, I
don't think we can rely on the behaviour of any one implementation
either - we don't know what the differences are between the
implementations in different generations of devices without TI providing
more detailed documentation in this area across their various devices.

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