linux-kernel - Re: [RFC PATCH 1/5] spi: introduce flag for memory mapped read

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Message-ID: <20150805124412.GN20873@sirena.org.uk>
Date:	Wed, 5 Aug 2015 13:44:12 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	"R, Vignesh" <vigneshr@...com>,
	devicetree <devicetree@...r.kernel.org>,
	Brian Norris <computersforpeace@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-spi <linux-spi@...r.kernel.org>,
	Huang Shijie <b32955@...escale.com>,
	MTD Maling List <linux-mtd@...ts.infradead.org>,
	linux-omap@...r.kernel.org, David Woodhouse <dwmw2@...radead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 1/5] spi: introduce flag for memory mapped read

On Wed, Aug 05, 2015 at 02:40:01PM +0200, Michal Suchanek wrote:
> On 5 August 2015 at 13:50, Mark Brown <broonie@...nel.org> wrote:

> > As far as I can tell you want to set a per spi_message flag saying that
> > the message is a flash read command?  If that's what this is trying to
> > do then why do you need to set the flag at all?  If the message is in a
> > clearly defined format and it's more efficient to use this mmap mode
> > then surely the driver can just recognise that the format is approprate
> > and switch into mmap mode without being explicitly told - I'm not clear
> > what the flag adds here.

> ehm, the read command is just one byte.

> I don't think sending 03 or other random byte as the first byte of a
> SPI transfer can be used as reliable detection that we are talking to
> a SPI flash memory.

Why care - if something is physically in the same format as a flash read
command how would a device be able to tell that it wasn't actually a
flash read command?  The signals sent on the bus are going to be
identical anyway.

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