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Message-ID: <82dce909dd89e1beadb23cd0392dc9f925b61a51.camel@infradead.org>
Subject: Re: [PATCH 1/2] dt-bindings: spi: dw: add cs-override property
From:   David Woodhouse <dwmw2@infradead.org>
To:     Talel Shenhar <talel@amazon.com>, ronenk@amazon.com
Cc:     mark.rutland@arm.com, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, ronenk@amazon.com, barakw@amazon.com
Date:   Wed, 10 Oct 2018 12:29:45 +0100
In-Reply-To: <e4c91e84-34f6-2c3f-4e26-62e6bd25a96f@amazon.com>
References: <1539155293-21750-1-git-send-email-talel@amazon.com>
         <20181010101825.GA6294@sirena.org.uk>
         <e4c91e84-34f6-2c3f-4e26-62e6bd25a96f@amazon.com>
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On Wed, 2018-10-10 at 13:34 +0300, Talel Shenhar wrote:
> > On Wed, Oct 10, 2018 at 10:08:12AM +0300, Talel Shenhar wrote:
> >=20
> > > 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,
> >=20
> > 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?
>=20
> Thank you for your prompt response.
>=20
> 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.

In particular, that would have been an incompatible change and would
render the hardware non-function with existing drivers. By making it
opt-in, old drivers work.... as long as the FIFO doesn't stall in the
middle of a transaction, which no worse than with the "standard" DW
implementation anyway.

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