linux-kernel - Re: [RFC PATCH 1/2] spi: Add multiple CS support for a single SPI device

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Date:   Fri, 15 Jul 2022 17:03:31 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Mahapatra, Amit Kumar" <amit.kumar-mahapatra@....com>
Cc:     Michal Simek <michal.simek@...inx.com>,
        Amit Kumar Mahapatra <amit.kumar-mahapatra@...inx.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        "p.yadav@...com" <p.yadav@...com>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        "richard@....at" <richard@....at>,
        "vigneshr@...com" <vigneshr@...com>,
        "git@...inx.com" <git@...inx.com>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "michael@...le.cc" <michael@...le.cc>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "git (AMD-Xilinx)" <git@....com>
Subject: Re: [RFC PATCH 1/2] spi: Add multiple CS support for a single SPI
 device

On Fri, Jul 15, 2022 at 03:36:23PM +0000, Mahapatra, Amit Kumar wrote:

> Thank you for the clarification.
> As per my understanding we would not be needing a new DT 
> property for expressing multi chip select support in the driver.
> We can use "num-cs" property of the spi controller for 
> broadcasting multi chip select support of the driver.
> If "num-cs" value is greater than 1 then the driver can support 
> multiple chip selects.

That's true in some sense but not in the sense of being actually able to
use more than one chip select for a single device.  If the controller
hardware manages the chip selects it may not be physically possible for
it to control more than one chip select.

While it's not an issue for the bindings even hardware that can
physically do it is going to need a driver update (in the core for GPIO
stuff) to do so.

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