| Message ID | 1465472504-10191-3-git-send-email-hias@horus.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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): u32 period_remaining = period_len - *total_len; /* Early exit if we aren't finishing this period */ if (period_remaining >= max_len) { /* * 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>
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>
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;