From: Arnd Bergmann <arnd@arndb.de>
To: "Reizer, Eyal" <eyalr@ti.com>
Cc: Kalle Valo <kvalo@codeaurora.org>, Eyal Reizer <eyalreizer@gmail.com>,
        "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-spi@vger.kernel.org" <linux-spi@vger.kernel.org>
Subject: Re: [PATCHv2] wlcore: spi: add wl18xx support
Date: Mon, 18 Apr 2016 16:57:03 +0200
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On Monday 18 April 2016 05:55:51 Reizer, Eyal wrote:
> > >
> > > - all wilink family needs special init command for entering wspi mode.
> > >   extra clock cycles should be sent after the spi init command while the
> > >   cs pin is high.
> > > - switch to controling the cs pin from the spi driver for achieveing the
> > >   above.
> > > - the selected cs gpio is read from the spi device-tree node using the
> > >   cs-gpios field and setup as a gpio.
> > > - See the example below for specifying the cs gpio using the cs-gpios entry
> > > &spi0   {
> > >         ...
> > >         cs-gpios = <&gpio0 5 0>;
> > >         ...
> > >         wlcore: wlcore@0 {
> > >                 compatible = "ti,wl1835";
> > >         ...
> > >         ...
> > >         };
> > > };
> > >
> > > Signed-off-by: Eyal Reizer <eyalr@ti.com>
> > 
> > I don't think this can work in general: not all SPI hosts uses GPIOs for
> > controlling CS, so the logic can't work, and it's also a layering violation for the
> > driver to look at the parent.
> > 
> > I would suggest fixing this using a new API function from the SPI core, if we
> > don't already have a generic way to do it.
> >
> Originally this is what I have done until I was pointed to the generic cs-gpio mechanism 
> in the SPI core. 
> It is a generic mechanism already in the SPI core driver.
> See: Documentation/devicetree/bindings/spi/spi-bus.txt

The cs-gpios property is documented as optional, it defines how you should
list the gpios if CS is implemented using gpio, but not all hardware does
it like this.

> It is also part of the generic spi.h (include/Linux/spi/spi.h), already part of 
> " struct spi_device" So it seemed redundant adding another mechanism for 
> implementing the same.
> Platform that interact with a wilink need to use it, and platforms that don't  
> have this capability will probably not interact with a wilink device using SPI.

The cs_gpio field in spi_device belongs to the spi host controller, no other
slave driver uses it.

I wasn't asking for a duplication of this mechanism, but an interface to
use it properly. Internally, the spi core uses the spi_set_cs() function to
pick a CS. Find a way to use that rather than reimplementing it incorrectly.

	Arnd