linux-kernel - Re: [PATCH v3 2/3] spi: atmel: update DT bindings documentation

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Date:	Mon, 15 Jun 2015 16:49:35 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Cyrille Pitchen <cyrille.pitchen@...el.com>
Cc:	nicolas.ferre@...el.com, linux-spi@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	devicetree@...r.kernel.org, robh+dt@...nel.org, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org
Subject: Re: [PATCH v3 2/3] spi: atmel: update DT bindings documentation

On Thu, Jun 11, 2015 at 06:37:51PM +0200, Cyrille Pitchen wrote:
> Le 09/06/2015 19:25, Mark Brown a écrit :
> > On Tue, Jun 09, 2015 at 01:53:53PM +0200, Cyrille Pitchen wrote:
> >> - add new property "atmel,fifo-size"

> > Why is this a property and not something we know from the IP version?

> Please be aware that the VERSION register can not be used to guess the
> size of FIFOs. Indeed, for a given hardware version, the SPI controller
> can be integrated on Atmel SoCs with different FIFO sizes. Also the
> "atmel,fifo-size" property is optional as older SPI controllers don't
> embed FIFO at all.

...

> Finally, on a given SoC, there can be several instances of the SPI
> controller with different FIFO sizes. This explain why we'd rather use a
> dedicated DT property than use the "compatible" property.

> I hope these pieces of information will help to clarify this point.
> Of course, we are open to other suggestions.

Ugh, what a wonderfully consistent hardware design :(  Please make it
clear in the documentation what is going on here, this looks like an
obvious bug in the DT binding - a very common pattern for bugs is to do
version quirks like this as properties.

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