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

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Message-ID: <5579B95F.9060307@atmel.com>
Date:	Thu, 11 Jun 2015 18:37:51 +0200
From:	Cyrille Pitchen <cyrille.pitchen@...el.com>
To:	Mark Brown <broonie@...nel.org>
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

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?
>
Hi Mark,

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.

Besides, the FIFO size can not be read or guessed from other registers:
When designing the FIFO feature, no dedicated registers were added to
store this size. Unused spaces in the I/O register range are limited and
better reserved for future usages. Instead, the FIFO size of each
peripheral is documented in the programmer datasheet.

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.

Best Regards,

Cyrille
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