linux-kernel - Re: [LINUX RFC v2 1/4] spi: add support of two chip selects & data stripe

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Date:	Fri, 4 Sep 2015 14:35:52 +0200
From:	Martin Sperl <martin@...rl.org>
To:	Mark Brown <broonie@...nel.org>
Cc:	Ranjit Waghmode <ranjit.waghmode@...inx.com>, dwmw2@...radead.org,
	computersforpeace@...il.com, michal.simek@...inx.com,
	soren.brinkmann@...inx.com, zajec5@...il.com, ben@...adent.org.uk,
	marex@...x.de, b32955@...escale.com, knut.wohlrab@...bosch.com,
	juhosg@...nwrt.org, beanhuo@...ron.com,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-spi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	harinik@...inx.com, punnaia@...inx.com
Subject: Re: [LINUX RFC v2 1/4] spi: add support of two chip selects & data stripe


> On 03.09.2015, at 14:12, Mark Brown <broonie@...nel.org> wrote:
> 
> On Wed, Aug 26, 2015 at 11:56:04AM +0530, Ranjit Waghmode wrote:
> 
>> To support dual parallel mode operation of ZynqMP GQSPI controller
>> following API's are added inside the core:
> 
> As covered in SubmittingPatches please try to make each patch a single
> change rather than having multiple separate changes in one commit.
> 
>> +	/* Controller may support more than one chip.
>> +	 * This flag will enable that feature.
>> +	 */
>> +#define SPI_MASTER_BOTH_CS		BIT(8)		/* enable both chips */
> 
> This isn't saying that the controller supports more than one chip, it's
> saying that the controller supports asserting more than one chip select
> at once which isn't the same thing.  I'm also not entirely sure that
> this makes sense as a separate feature to the data striping one - I'm
> struggling to think of a way to use this sensibly separately to that.

Well - there is one use-case that I can think of:
fbtft has the requirement for some devices to control a GPIO to
differentiate between command and data getting transferred
- sort of 9 bit.

Right now it is done outside of spi in the fbtft driver itself wrapping
spi_sync().

Similarly a “hold” line on an eeprom or similar could get (de)asserted
without requiring holding a spi-bus-lock.

But then the current patch would not allow this kind of “generic”
use-case.

Martin


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