Re: [PATCH 2/2] dmaengine: bcm2835: Avoid splitting periods into very small chunks

[Matthias Reichl <hias@horus.com>]

On Mon, Jun 13, 2016 at 10:06:49PM -0700, Eric Anholt wrote:
> Matthias Reichl <hias@horus.com> writes:
> 
> > The current cyclic DMA period splitting implementation can generate
> > very small chunks at the end of each period. For example a 65536 byte
> > period will be split into a 65532 byte chunk and a 4 byte chunk on
> > the "lite" DMA channels.
> >
> > This increases pressure on the RAM controller as the DMA controller
> > needs to fetch two control blocks from RAM in quick succession and
> > could potentially cause latency issues if the RAM is tied up by other
> > devices.
> >
> > We can easily avoid these situations by distributing the remaining
> > length evenly between the last-but-one and the last chunk, making
> > sure that split chunks will be at least half the maximum length the
> > DMA controller can handle.
> >
> > This patch checks if the last chunk would be less than half of
> > the maximum DMA length and if yes distributes the max len+4...max_len*1.5
> > bytes evenly between the last 2 chunks. This results in chunk sizes
> > between max_len/2 and max_len*0.75 bytes.
> >
> > Signed-off-by: Matthias Reichl <hias@horus.com>
> > Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
> > Tested-by: Clive Messer <clive.messer@digitaldreamtime.co.uk>
> > ---
> >  drivers/dma/bcm2835-dma.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
> > index 344bcf92..36b998d 100644
> > --- a/drivers/dma/bcm2835-dma.c
> > +++ b/drivers/dma/bcm2835-dma.c
> > @@ -252,6 +252,20 @@ static void bcm2835_dma_create_cb_set_length(
> >  
> >  	/* have we filled in period_length yet? */
> >  	if (*total_len + control_block->length < period_len) {
> > +		/*
> > +		 * If the next control block is the last in the period
> > +		 * and it's length would be less than half of max_len
> > +		 * change it so that both control blocks are (almost)
> > +		 * equally long. This avoids generating very short
> > +		 * control blocks (worst case would be 4 bytes) which
> > +		 * might be problematic. We also have to make sure the
> > +		 * new length is a multiple of 4 bytes.
> > +		 */
> > +		if (*total_len + control_block->length + max_len / 2 >
> > +		    period_len) {
> > +			control_block->length =
> > +				DIV_ROUND_UP(period_len - *total_len, 8) * 4;
> > +		}
> >  		/* update number of bytes in this period so far */
> >  		*total_len += control_block->length;
> >  		return;
> 
> It seems to me like this would all be a lot simpler if we always split
> the last 2 control blocks evenly (other than 4-byte rounding):

Agreed and thanks a lot for the feedback!

I'll do it that way and then send out a v2.

> u32 period_remaining = period_len - *total_len;
> 
> /* Early exit if we aren't finishing this period */
> if (period_remaining >= max_len) {

This has to be > max_len, but the rest seems fine. We want to split
if we have more than max_len but less than max_len*2 bytes.

> 	/*
> 	 * Split the length between the last 2 CBs, to help hide the
> 	 * latency of fetching the CBs.
> 	 */
> 	if (period_remaining < max_len * 2) {
> 		control_block->length =
>                 	DIV_ROUND_UP(period_remaining, 8) * 4;
>         }
> 	/* update number of bytes in this period so far */
> 	*total_len += control_block->length;
> }
> 
> I'm about to go semi-AFK for a couple weeks.  If there's a good reason
> to only do this when the last block is very short, I'm fine with:
> 
> Acked-by: Eric Anholt <eric@anholt.net>