Messages in this thread | | | Subject | Re: [PATCH v2] dmaengine: bcm2835: Add slave dma support | From | Martin Sperl <> | Date | Fri, 1 May 2015 23:07:33 +0200 |
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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@tronnes.org> 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).
Martin
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