linux-kernel - Re: [PATCH 3/4] spi: sunxi: Add Allwinner A31 SPI controller driver

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Date:	Fri, 17 Jan 2014 19:05:16 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Mike Turquette <mturquette@...aro.org>,
	Emilio Lopez <emilio@...pez.com.ar>,
	linux-sunxi@...glegroups.com, linux-spi@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, kevin.z.m.zh@...il.com,
	sunny@...winnertech.com, shuge@...winnertech.com,
	zhuzhenhua@...winnertech.com
Subject: Re: [PATCH 3/4] spi: sunxi: Add Allwinner A31 SPI controller driver

On Thu, Jan 16, 2014 at 10:12:01PM +0100, Maxime Ripard wrote:
> On Thu, Jan 16, 2014 at 07:40:03PM +0000, Mark Brown wrote:

> > > +	if (status & SUN6I_INT_CTL_RF_OVF) {
> > > +		sun6i_spi_drain_fifo(sspi, SUN6I_FIFO_DEPTH);
> > > +		sun6i_spi_write(sspi, SUN6I_INT_STA_REG, SUN6I_INT_CTL_RF_OVF);
> > > +		return IRQ_HANDLED;
> > > +	}

> > This looks like an overflow - a log message would be helpful for users
> > and you should possibly be flagging an error on the current transfer.

> Hmmm, that was an attempt at receiving more bytes than the FIFO can
> handle, but I guess the FIFO full interrupt would be more appropriate
> for this.

If you've got an overflow interrupt that suggests that the data is
already corrupted, assuming the interrupt isn't misnamed.

> > > +	ret = clk_set_rate(sspi->mclk, 100000000);
> > > +	if (ret) {
> > > +		dev_err(&pdev->dev, "Couldn't change module clock rate\n");
> > > +		goto err2;
> > > +	}

> > Does this really need to be fatal (or done at all)?  There seems to be
> > another reasonably flexible divider in the IP and it's more common to
> > either set this per transfer to something that rounds nicely or just use
> > the default and rely on the dividers.

> The default parent of the module clock runs at 24MHz, that means that
> we won't be able to reach a spi clock higher than 12MHz, which seems
> quite low. We can always change the rate in the transfer setup code
> though, if needs be.

12MHz is actually quite a common limit for SPI interfaces (half of a
24MHz master clock like the IP itself has) but yeah, you want to go
higher if you can.  Doing it on transfer setup is going to mean that
you save a little power when you don't need the extra speed too.

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