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

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Date:   Fri, 31 Aug 2018 16:38:56 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Jan Kundrát <jan.kundrat@...net.cz>
Cc:     Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Orson Zhai <orsonzhai@...il.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Lanqing Liu <lanqing.liu@...eadtrum.com>,
        DTML <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] spi: Introduce one new field to set word delay

On 29 August 2018 at 19:33, Jan Kundrát <jan.kundrat@...net.cz> 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.
>
>
> I need a similar functionality for talking to a SPI device from userspace
> -- see my attempt for implementing this in spi-orion.c at [1]. The device's
> datasheet says that I should wait, e.g., 3µs between each two words. I
> therefore like this patch :).
>
> 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.

Sorry for confusing. Since our SPI word delay unit is clock cycle of
the SPI clock, so my intention of the word_delay's unit is SPI clock
cycles according to our previous discussion.

>
> 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?
>
> 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.
>
> 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?
>
> With kind regards,
> Jan
>
> [1] https://patchwork.kernel.org/patch/10221397/



-- 
Baolin Wang
Best Regards