Re: [PATCH 2/2] spi: imx: fix use of native chip-selects with devicetree - Uwe Kleine-König

From: "Uwe Kleine-König" <u.kleine-koenig-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>
To: Greg Ungerer <gerg-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org>,
	Sascha Hauer <s.hauer-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>
Cc: linux-spi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Vladimir Zapolskiy
	<vladimir_zapolskiy-nmGgyN9QBj3QT0dZR+AlfA@public.gmane.org>,
	kernel-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org,
	fabio.estevam-3arQi8VN3Tc@public.gmane.org,
	shawnguo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org,
	linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org
Subject: Re: [PATCH 2/2] spi: imx: fix use of native chip-selects with devicetree
Date: Tue, 21 Mar 2017 20:23:42 +0100	[thread overview]
Message-ID: <20170321192342.4fxlb2zjcas53uow@pengutronix.de> (raw)
In-Reply-To: <20678cef-f667-9915-aa00-8877ad152a8c-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org>

On Tue, Mar 21, 2017 at 11:22:27PM +1000, Greg Ungerer wrote:
> Hi Uwe,
> 
> On 21/03/17 22:11, Uwe Kleine-König wrote:
> > On Tue, Mar 21, 2017 at 09:53:52PM +1000, Greg Ungerer wrote:
> > > On 21/03/17 18:05, Uwe Kleine-König wrote:
> > > > On Tue, Mar 21, 2017 at 12:05:20PM +1000, Greg Ungerer wrote:
> > > > > On 20/03/17 23:22, Vladimir Zapolskiy wrote:
> > > > > > For that type of bindings locally I have a hackish spi-imx driver change,
> > > > > > which supports this option, but I'm unsure if it is universal enough.
> > > > > 
> > > > > Do you mean supporting no cs-gpios tag?
> > > > > That would be nice, but it would seem not many users of this are
> > > > > using native chip selects.
> > > > 
> > > > The reason for this is that the native chip selects are less flexible
> > > > than gpios because you cannot control when they deassert. IIRC they do
> > > > it too much for some chips. So the only reason to stick to them is that
> > > > on some SoCs not all pins have a GPIO function. Not sure if transfer
> > > > speed is another reason, but I would expect that the gain isn't that
> > > > big.
> > > 
> > > For the particular SPI device I am using, a Silicon Labs 32260,
> > > it actually wants the assertion and de-assertion of the chip-select
> > > between each byte. So it is the only way it can work for me.
> > 
> > That should be doable with gpio-cs, too. You just need the right flags
> > in your spi transfer IIRC.
> 
> Do you know which flag(s)  do that?

Looking at the source it's not about flags, but you have to split your
transfer into several messages. AFAICT that's how the spi stuff is
supposed to work. That is, at the start of a message CS is asserted and
(only) at it's end CS is deasserted. So the imx core with native chip
select actually misbehaves by toggling CS between each word.

@broonie: Can you confirm or contradict please?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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