linux-kernel - Re: [PATCH v2 2/5] Documentation: mtd: add a DT property to set the number of dummy cycles

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Date:	Wed, 22 Jul 2015 15:43:54 +0200
From:	Marek Vasut <marex@...x.de>
To:	Cyrille Pitchen <cyrille.pitchen@...el.com>
Cc:	nicolas.ferre@...el.com, broonie@...nel.org,
	linux-spi@...r.kernel.org, dwmw2@...radead.org,
	computersforpeace@...il.com, zajec5@...il.com, beanhuo@...ron.com,
	juhosg@...nwrt.org, shijie.huang@...el.com, ben@...adent.org.uk,
	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, linux-mtd@...ts.infradead.org
Subject: Re: [PATCH v2 2/5] Documentation: mtd: add a DT property to set the number of dummy cycles

On Wednesday, July 22, 2015 at 03:17:07 PM, Cyrille Pitchen wrote:
> Depending on the SPI clock frequency, the Fast Read op code and the
> Single/Dual Data Rate mode, the number of dummy cycles can be tuned to
> improve transfer speed.
> The actual number of dummy cycles is specific for each memory model and is
> provided by the manufacturer thanks to the memory datasheet.
> 
> Signed-off-by: Cyrille Pitchen <cyrille.pitchen@...el.com>
> ---
>  Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
> b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt index
> 2bee68103b01..4387567d8024 100644
> --- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
> +++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
> @@ -19,6 +19,11 @@ Optional properties:
>                     all chips and support for it can not be detected at
> runtime. Refer to your chips' datasheet to check if this is supported by
> your chip.
> +- m25p,num-dummy-cycles : Set the number of dummy cycles for Fast Read
> commands. +                          Depending on the manufacturer
> additional dedicated +                          commands are sent to the
> flash memory so the +                          controller and the memory
> can agree on the number of +                          dummy cycles to use.

Can't you just try negotiating this value at probe time, starting with some
high value and see how low you can get with the negotiations ? This way, you'd 
be able to effectively auto-detect this value at probe-time.

I might be wrong though :)

Best regards,
Marek Vasut
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