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

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Date:   Wed, 10 Oct 2018 12:29:45 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Talel Shenhar <talel@...zon.com>, ronenk@...zon.com
Cc:     mark.rutland@....com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, ronenk@...zon.com, barakw@...zon.com
Subject: Re: [PATCH 1/2] dt-bindings: spi: dw: add cs-override property

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:
> > 
> > > 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?
> 
> Thank you for your prompt response.
> 
> 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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