Messages in this thread | | | Subject | Re: [PATCH v2] dmaengine: bcm2835: Add slave dma support | From | Martin Sperl <> | Date | Mon, 11 May 2015 07:11:43 +0200 |
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> On 08.05.2015, at 13:20, Jonathan Bell <jonathan@raspberrypi.org> wrote: >> > 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.
I have now submitted a patch for spi-bcm2835 to make use of dma, so there is one candidate for this kind of behavior. So please go forward with the merge.
Also note that with the spi-HW dma support of the bcm2835 it is necessary to do a RX transfer even if the data is not used (similar for TX).
Right now we have to allocate a dummy buffer to run these kind of “one-way” transfers where we need 2 DMA channels.
The bcm2835 dma hw supports such dummy-transfer modes via BCM2835_DMA_S_IGNORE and BCM2835_DMA_D_IGNORE.
So maybe we can add a “flag” to the dmaengine_prep_slave_sg that will allow such kind of behavior to get implemented?
That is not a necessity, but would be a welcome improvement.
Tested-by: Martin Sperl <kernel@martin.sperl.org>
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