linux-kernel - Re: [PATCH 6/6] spi: sun4i: add DMA transfers support

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Date:   Wed, 4 Apr 2018 08:27:59 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Sergey Suloev <ssuloev@...altech.com>
Cc:     Mark Brown <broonie@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        linux-spi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] spi: sun4i: add DMA transfers support

On Tue, Apr 03, 2018 at 04:03:32PM +0300, Sergey Suloev wrote:
> On 04/03/2018 11:17 AM, Maxime Ripard wrote:
> > On Thu, Mar 29, 2018 at 09:59:07PM +0300, Sergey Suloev wrote:
> > > +static int sun4i_spi_dma_setup(struct device *dev,
> > > +			       struct resource *res)
> > > +{
> > > +	struct spi_master *master = dev_get_drvdata(dev);
> > > +	struct dma_slave_config dma_sconf;
> > > +	int ret;
> > > +
> > > +	master->dma_tx = dma_request_slave_channel_reason(dev, "tx");
> > > +	if (IS_ERR(master->dma_tx)) {
> > > +		dev_err(dev, "Unable to acquire DMA TX channel\n");
> > > +		ret = PTR_ERR(master->dma_tx);
> > > +		goto out;
> > > +	}
> > > +
> > > +	dma_sconf.direction = DMA_MEM_TO_DEV;
> > > +	dma_sconf.src_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
> > > +	dma_sconf.dst_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
> > I guess that would depend on the size of the transfer, right?
>
> no
> "this is the width in bytes of the source (RX)register where DMA data shall
> be read. If the sourceis memory this may be ignored depending on
> architecture."
> AFAIK is should be 1 byte for SPI side and seems to be ignored for memory
> side, but as soon as I don't know what should be correct value for memory
> side I just put 1 there too.

I meant the number of bits per word, sorry. That width would only
apply if you have 8 bits per word, but that seems to always be the
case. So nevermind.

> > > +	dma_sconf.dst_addr = res->start + SUN4I_TXDATA_REG;
> > > +	dma_sconf.dst_maxburst = 1;
> > > +	dma_sconf.src_maxburst = 1;
> >
> > And a burst of 1 seems sub-optimal here.
>
> I did some tests before with 3/4 FIFO size but it didn't work and I got
> stuck with 1 byte.
> It seems like 1 byte is the correct value because in SPI protocol we can
> only send 1 byte in 1 burst.

That's not about the SPI burst, it's about the DMA burst.

> > 
> > > +	ret = sun4i_spi_dma_setup(&pdev->dev, res);
> > > +	if (ret) {
> > > +		if (ret == -EPROBE_DEFER) {
> > > +			/* wait for the dma driver to load */
> > > +			goto err_free_master;
> > > +		}
> > > +		dev_warn(&pdev->dev, "DMA transfer not supported\n");
> > Saying why it's not supported would be great.
>
> I can put more info in this log
> but there is already a message printed from sun4_spi_dma_setup() if any
> error occurs

Then you don't need both.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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