linux-kernel - RE: [PATCHv2] wlcore: spi: add wl18xx support

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Date:	Tue, 19 Apr 2016 09:05:45 +0000
From:	"Reizer, Eyal" <eyalr@...com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Kalle Valo <kvalo@...eaurora.org>,
	Eyal Reizer <eyalreizer@...il.com>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>
Subject: RE: [PATCHv2] wlcore: spi: add wl18xx support

Arnd,

> > > >
> > > > - 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@...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.
> 

Understood. As this special CS manipulation is unique to wspi (wilink spi)  I think the 
best option is to move this gpio allocation into wlcore_spi as a new device tree entry
used only by this driver.
If you agree I will submit a v3.

Best Regards,
Eyal