linux-kernel - Re: [PATCH 1/2] dt-bindings: spi: dw: add cs-override property

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Date:   Wed, 10 Oct 2018 12:27:53 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Talel Shenhar <talel@...zon.com>
Cc:     linux-spi@...r.kernel.org, robh+dt@...nel.org,
        mark.rutland@....com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, ronenk@...zon.com, barakw@...zon.com,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH 1/2] dt-bindings: spi: dw: add cs-override property

On Wed, Oct 10, 2018 at 02:23:40PM +0300, Talel Shenhar wrote:
> On 10/10/2018 01:18 PM, Mark Brown wrote:
> > On Wed, Oct 10, 2018 at 10:08:12AM +0300, Talel Shenhar wrote:

> >> The dw spi controller has an auto-deselect of Chip-Select, in case there is
> >> no data inside the Tx FIFO. While working on platforms with Alpine chips,

> > Why would we ever want to use this behaviour?  It will be broken for any
> > non-trivial SPI message such as those made with multiple transfers
> > anyway.  Why not just unconditionally control it manually?

> This behavior (auto-deselect of Chip-Select) is the default behavior of dw spi controller hw.
> On Alpine chip there is additional behavior added to the dw spi controller hw that allows the sw to disable this behavior.
> This patch allows the dw driver to enable this hw workaround and add the needed sw manual control for it.

If this is a modified IP with additional features then it should be
given a new compatible string rather than having a property - it's not
just configuration of the existing IP, it's a new thing and we may find
there are other quirks that have to be taken care of for it.

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