linux-kernel - Re: [PATCH V3 5/9] spi: export spi core function spi_set

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Message-ID: <20190506044425.GN14916@sirena.org.uk>
Date:   Mon, 6 May 2019 13:44:25 +0900
From:   Mark Brown <broonie@...nel.org>
To:     Sowjanya Komatineni <skomatineni@...dia.com>
Cc:     "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Timo Alho <talho@...dia.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        Krishna Yarlagadda <kyarlagadda@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH V3 5/9] spi: export spi core function spi_set_cs

On Mon, Apr 29, 2019 at 10:02:46PM +0000, Sowjanya Komatineni wrote:

Please fix your mail client to word wrap within paragraphs at something
substantially less than 80 columns.  Doing this makes your messages much
easier to read and reply to.

> > On Mon, Apr 15, 2019 at 02:30:30PM -0700, Sowjanya Komatineni wrote:
> > > This patch exports spi_set_cs of the spi core to allow SPI masters to 
> > > use when gpio based chip select is needed.

> > This isn't really what I meant when I said it'd be good to use the
> > core GPIO code - this function doesn't do a huge amount really and
> > the usage of it in your subsequent patch for the > driver isn't
> > exactly joined up with the little it does (which is mainly swapping
> > in the GPIO chip select instead of the hardware chip select) isn't
> > used in your driver usage of this as far as I can see.  The bulk of
> > the chip select handling code in the core is actually in
> > transfer_one_message() which your driver doesn't use as it's got
> > it's own implementation of that; I've not looked in enough detail to
> > figure out if it could use it.

> In SPI Tegra driver, we wanted to have GPIO based CS control when
> cs-gpios is specified in parallel to HW/SW CS. Having parallel GPIO
> based CS is to mimic some of the timing stuff that's needed for some
> spi devices by not actually using HW CS on platform but only for SPI
> HW design logic inside the chip.

> Tegra spi driver don't use set_cs callback so looking into spi_set_cs
> from spi core implementation when cs-gpios property is used it exactly
> the same that is needed for GPIO control CS. So used this in V3. 

> Can you please provide more details on what you are suggesting?
> Do you prefer not to use SPI core spi_set_cs and gpio_set_values APIs
> and instead implement in tegra SPI driver using GPIO descriptors ?

You're probably best open coding in the driver if there's value in using
the hardware chip select.

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