linux-kernel - Re: [PATCH] spi: cadence: Correct handling of native chipselect

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Message-ID: <20191127115419.GA10451@ediswmail.ad.cirrus.com>
Date:   Wed, 27 Nov 2019 11:54:19 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     Gregory Clement <gregory.clement@...tlin.com>,
        Mark Brown <broonie@...nel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] spi: cadence: Correct handling of native chipselect

On Wed, Nov 27, 2019 at 11:42:47AM +0100, Linus Walleij wrote:
> On Tue, Nov 26, 2019 at 5:41 PM Charles Keepax
> <ckeepax@...nsource.cirrus.com> wrote:
> The original patch
> f3186dd87669 ("spi: Optionally use GPIO descriptors for CS GPIOs")
> came with the assumption that native chip select handler needed
> was to be converted to always expect a true (1) value to their
> ->set_cs() callbacks for asserting chip select, and this was one of
> the drivers augmented to expect that.
> 

Which is fine, I am not greatly invested in either symantics
of the set_cs callback although if we were changing that we
should have probably updated the kerneldoc comments for it.

Although I do have a question if that is that case what is the
expected way to handle the polarity of the chip select? Because
it seems to me you would end up with each driver checking the
SPI_CS_HIGH flag in set_cs and doing the invert locally, whereas
with the pass the logic level system the core can centralise that
inversion.

> As
> 3e5ec1db8bfe ("spi: Fix SPI_CS_HIGH setting when using native and GPIO CS")
> essentially undo that semantic change and switches back to
> the old semantic, all the drivers that were converted to expect
> a high input to their ->set_cs() callbacks for asserting CS need
> to be reverted back as well, but that didn't happen.
> 
> So we need to fix not just cadence but also any other driver setting
> ->use_gpio_descriptors() and also supplying their own
> ->set_cs() callback and expecting this behaviour, or the fix
> will have fixed broken a bunch of drivers.
> 
> But we are lucky: there aren't many of them.
> In addition to spi-cadence.c this seems to affect only spi-dw.c
> and I suppose that is what Gregory was using? Or
> something else?
> 

I will go do some digging and see what I can find.

Thanks,
Charles