Re: [PATCH v2 1/3] spi: Introduce one new field to set word delay

From: Mark Brown <broonie@kernel.org>
To: "Jan Kundrát" <jan.kundrat@cesnet.cz>
Cc: Baolin Wang <baolin.wang@linaro.org>,
	linux-spi@vger.kernel.org, robh+dt@kernel.org,
	mark.rutland@arm.com, orsonzhai@gmail.com, zhang.lyra@gmail.com,
	lanqing.liu@spreadtrum.com, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] spi: Introduce one new field to set word delay
Date: Wed, 29 Aug 2018 12:47:05 +0100	[thread overview]
Message-ID: <20180829114705.GC27808@sirena.org.uk> (raw)
In-Reply-To: <01c9b188-5b67-40c0-b08d-99a0543ddf54@cesnet.cz>

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On Wed, Aug 29, 2018 at 01:33:24PM +0200, Jan Kundrát wrote:
> On čtvrtek 16. srpna 2018 14:54:49 CEST, Baolin Wang wrote:
> > + * @word_delay: clock cycles to inter word delay after each word size
> > + *	(set by bits_per_word) transmission.

> The description can be improved because it left me wondering what "clock
> this is about. I suppose it's about the SPI clock cycles and not CPU clock
> cycles, right? I'll be hapy to patch this once Baolin confirms that that is
> the intended meaning.

That's certainly how I read it.

> It seems that this is only implemented in one newly added driver. I'm
> interested in supporting this in spi-orion.c, but that sounds like
> driver-specific work for something which is pretty generic. How should this
> be implemented? Given that drivers for SPI masters can implement a function
> which transfers several words at once, there are not that many better
> possibilities than adding udelay()s, though. Thoughts?

Yeah, you'd need to split the transfer into words and then add a delay
between which would be rather expensive but it's about as good as we can
get I think.

> What is your plan to do with drivers which do not implement this (yet)? If a
> spi_transfer gets queued which asks for a word_delay delay, it is silently
> ignored now, AFAIU.

Yes.  A generic handler would be best.

> What about userspace support, spidev and spi_ioc_transfer (that's my target,
> actually)? Is it OK to s/pad/word_delay/ in the spidev code and pass that to
> the generated struct spi_transfer? In my opinion, once we support specifying
> this from userspace, one has to definitely check that the SPI controller is
> ready to honor this request. Do we want a new bit in spi_controller.flags
> for this?

Not seeing pad in the spidev code?  A feature flag would make sense along
with a generic implementation.

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