linux-kernel - Re: [PATCH 11/17] spi: dw: Fix native CS being unset

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Message-ID: <20200514143537.GH5127@sirena.org.uk>
Date:   Thu, 14 May 2020 15:35:37 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Serge Semin <fancer.lancer@...il.com>,
        Gregory Clement <gregory.clement@...tlin.com>,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Georgy Vlasov <Georgy.Vlasov@...kalelectronics.ru>,
        Ramil Zaripov <Ramil.Zaripov@...kalelectronics.ru>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Allison Randal <allison@...utok.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Gareth Williams <gareth.williams.jx@...esas.com>,
        Rob Herring <robh+dt@...nel.org>, linux-mips@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Phil Edworthy <phil.edworthy@...esas.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Thor Thayer <thor.thayer@...ux.intel.com>,
        "wuxu.wu" <wuxu.wu@...wei.com>,
        Xinwei Kong <kong.kongxinwei@...ilicon.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        linux-spi <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/17] spi: dw: Fix native CS being unset

On Thu, May 14, 2020 at 02:55:58PM +0300, Serge Semin wrote:
> On Thu, May 14, 2020 at 10:31:13AM +0200, Linus Walleij wrote:

> > We had some related discussion what to do with this case
> > when a controller can support active high CS if and only if
> > it is using a GPIO instead of the native CS. We didn't really
> > figure it out, I suppose ideally we should use two flags in the
> > master but that exercise is for another day.

> Even though it might be painful, but as I see it the best way to generically fix
> this problem would be to change the controller->set_cs() callback
> semantics. SPI core should pass a CS activation flag to the set_cs()
> callback instead of the CS pin logical level (just propagate the enable argument
> passed to the spi_set_cs() SPI core method). So if an SPI controller supports
> the Active-high native CS, during the set_cs() callback invocation it would
> analyze the spi_device flags state to figure out whether the slave needs the

The idea with set_cs() is to support controllers that allow the chip
select to be directly managed.  If the controller is interpreting or
automatically managing chip select at all then set_cs() is not likely to
be a good fit, if the controller does support unfiltered management then
anything else is going to result in there being a bunch of duplicate
code between drivers.

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