linux-kernel - Re: [PATCH v1] spi: fix client driver breakages when using GPIO descriptors

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Date:   Thu, 12 Nov 2020 11:41:36 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Sven Van Asbroeck <thesven73@...il.com>
CC:     Mark Brown <broonie@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        Lukas Wunner <lukas@...ner.de>,
        linux-spi <linux-spi@...r.kernel.org>,
        Simon Han <z.han@...bus.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] spi: fix client driver breakages when using GPIO
 descriptors

On Wed, Nov 11, 2020 at 11:24:14AM -0500, Sven Van Asbroeck wrote:
> On Wed, Nov 11, 2020 at 10:48 AM Mark Brown <broonie@...nel.org> wrote:
> >
> > Applied to
> >
> >    https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-next
> 
> Thank you !
> 
> Now that our minds are still focused on this subject, should
> commit 138c9c32f090 ("spi: spidev: Fix CS polarity if GPIO descriptors
> are used")
> be reverted?
> 
> This fixed spidev to deal with SPI_CS_HIGH on gpiod.
> But after our fix, its behaviour will probably be broken again.
> 
> Another candidate for revert is
> commit ada9e3fcc175 ("spi: dw: Correct handling of native chipselect")
> although I don't understand that code well enough to be sure.
> 
> Adding Charles Keepax.

Looks like the code has changed a fair amount since my patch. The
important detail from it was trying to clarify the semantics of the
controller->set_cs callback. That function is called with a boolean
argument and that argument could have two possible meanings:

1) True means apply a high logic level to the chip select line.
2) True mean apply chip select.

Under interpretation 2) the chip select line would be set to a
different logic level depending on if the device is active high or
active low.

If I remember correctly at the point of my patch the core had just
changed between the two a couple of times but now consistently did 1)
(and looks like it still does), my patch intended to updated the
spi-dw driver to match that, as my SPI had stopped working. I think
it then turned out, my patch broke some other use-cases and that
the bit in the IP basically had 2) semantics in hardware. Which is
what this patch fixed:

commit 9aea644ca17b ("spi: dw: Fix native CS being unset")

After that patch my patch is mostly replaced so I don't think it
would make any sense to revert my patch at this point, and I
don't think your patch will break the spi-dw driver. I don't
have easy access to the hardware right now to test, but I will
give it is quick run when that option becomes available to me
again.

Your fix looks good to me, but I suspect you do need to fix the
spidev stuff although I have haven't looked at that in detail.

Thanks,
Charles