Re: [PATCH v2] dmaengine: bcm2835: Add slave dma support

[Jonathan Bell]

On 06/05/2015 20:21, Noralf TrÃnnes 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 TrÃnnes <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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