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

From: Mark Brown <broonie@kernel.org>
To: "Mahapatra, Amit Kumar" <amit.kumar-mahapatra@amd.com>
Cc: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>,
	"p.yadav@ti.com" <p.yadav@ti.com>,
	"miquel.raynal@bootlin.com" <miquel.raynal@bootlin.com>,
	"richard@nod.at" <richard@nod.at>,
	"vigneshr@ti.com" <vigneshr@ti.com>,
	"git@xilinx.com" <git@xilinx.com>,
	"michal.simek@xilinx.com" <michal.simek@xilinx.com>,
	"linux-spi@vger.kernel.org" <linux-spi@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"michael@walle.cc" <michael@walle.cc>,
	"linux-mtd@lists.infradead.org" <linux-mtd@lists.infradead.org>,
	"git (AMD-Xilinx)" <git@amd.com>
Subject: Re: [RFC PATCH 1/2] spi: Add multiple CS support for a single SPI device
Date: Thu, 23 Jun 2022 13:06:38 +0100	[thread overview]
Message-ID: <YrRXTrYN3BuShbzg@sirena.org.uk> (raw)
In-Reply-To: <DM6PR12MB2809F6C7D80B60556218D627DCB59@DM6PR12MB2809.namprd12.prod.outlook.com>

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On Thu, Jun 23, 2022 at 11:39:19AM +0000, Mahapatra, Amit Kumar wrote:

> > >  	/* Mode (clock phase/polarity/etc.) */
> > >  	if (of_property_read_bool(nc, "spi-cpha"))

> > This is changing the DT binding but doesn't have any updates to the binding
> > document.  The binding code also doesn't validate that we don't have too
> > many chip selects.

> The following updates are done in the binding documents for adding multiple
> CS support:
> In jedec,spi-nor.yaml file " maxItems " of the "reg" DT property has been 
> updated to accommodate two CS per SPI device.  

This is a change to a binding for a specific driver, this is changing
the SPI core.

> > I'm also not seeing anything here that checks that the driver supports
> > multiple chip selects - it seems like something that's going to cause issues
> > and we should probably have something to handle that situation.

> In my approach the chip select member (chip_select) of the spi_device structure 
> is changed to an array (chip_select[2]). This array is used to store the CS values 
> coming from the "reg" DT property. 
> In case of multiple chip selects  spi->chip_slect[0] will hold CS0 value & 
> spi->chip_select[1] wil hold CS1 value.
> In case of single chip select the spi->chip_select[0] will hold the chip select value.

That doesn't address the issue, the issue is checking that the driver
can support multiple chip selects.

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