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

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Message-ID: <554A69D5.90301@tronnes.org>
Date:	Wed, 06 May 2015 21:21:57 +0200
From:	Noralf Trønnes <noralf@...nnes.org>
To:	Martin Sperl <kernel@...tin.sperl.org>,
	Stephen Warren <swarren@...dotorg.org>,
	jonathan@...pberrypi.org
CC:	dmaengine@...r.kernel.org, vinod.koul@...el.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-rpi-kernel <linux-rpi-kernel@...ts.infradead.org>,
	dan.j.williams@...el.com, linux-spi <linux-spi@...r.kernel.org>
Subject: Re: [PATCH v2] dmaengine: bcm2835: Add slave dma support


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@...nnes.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).

Jonathan, can you comment on this?


Noralf.


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