On 06/05/2015 20:21, Noralf Trønnes wrote:
Den 01.05.2015 23:07, skrev Martin Sperl:
Tests with the initial (and incomplete) version of the spi-bcm2835
driver
with DMA transfer support show that the dma-engine works as expected
with
this patch.
There is one one observation:
On 18.04.2015, at 13:06, Noralf Trønnes <noralf@xxxxxxxxxxx> wrote:
+static struct dma_async_tx_descriptor *
+bcm2835_dma_prep_slave_sg(struct dma_chan *chan,
+ struct scatterlist *sgl,
+ unsigned int sg_len,
+ enum dma_transfer_direction direction,
+ unsigned long flags, void *context)
+{
...
+ /* Enable */
+ if (i == sg_len - 1 && len - j <= max_size)
+ control_block->info |= BCM2835_DMA_INT_EN;
The observation is that an interrupt is always triggered - even in
the case
where flags does NOT have DMA_PREP_INTERRUPT set.
This may not be necessary and avoid interrupts.
So maybe the above if clause should get extended by:
&& (flags & DMA_PREP_INTERRUPT)
to only trigger an interrupt when really requested.
I am not sure if there are any side-effects because of this besides
having the
requirement on the client to run dmaengine_terminate_all() on that
specific dma
channel without interrupts when the transfer is finished.
In the case of SPI we have TX feed the fifo - which finishes early -
, but we
only need to the interrupt when RX finishes reading the fifo, which
indicates
that the SPI-transfer is fully finished.
So having an interrupt on TX is not necessary for the process.
The same observations may also apply to bcm2835_dma_prep_dma_cyclic
(which is
outside of this patch provided by Noralf).
Jonathan, can you comment on this?
Noralf.
I agree that the interrupt generated would be spurious - in the case
where it is not required.
However if you do && (flags & DMA_PREP_INTERRUPT) then all users of this
driver need to explicitly set interrupt flags when doing a
scatter-gather transfer. As I understand it, currently the only upstream
client of this driver is the I2S driver which only uses cyclic anyway.
Not requiring an interrupt on completion is a bit of an edge case - the
default among other dmaengine drivers appears to be to enable interrupts
unconditionally.
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