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

From: Greg Ungerer <gerg@linux-m68k.org>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: linux-spi@vger.kernel.org,
	Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>,
	kernel@pengutronix.de, fabio.estevam@nxp.com,
	shawnguo@kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/2] spi: imx: fix use of native chip-selects with devicetree
Date: Tue, 21 Mar 2017 21:53:52 +1000	[thread overview]
Message-ID: <942cfc4b-445b-ca51-1823-2391cea62abf@linux-m68k.org> (raw)
In-Reply-To: <20170321080551.ie4xm5hr3fki365s@pengutronix.de>

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.

Regards
Greg