Re: [PATCH v4 1/3] gpio: swnode: Add ability to specify native chip selects for SPI

[Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxx>]

On Tue, Apr 09, 2024 at 09:03:30PM +0300, Andy Shevchenko wrote:
> On Tue, Apr 09, 2024 at 02:21:24PM +0100, Charles Keepax wrote:
> > SPI devices can specify a cs-gpios property to enumerate their
> > chip selects. Under device tree, a zero entry in this property can
> > be used to specify that a particular chip select is using the SPI
> > controllers native chip select, for example:
> > 
> >         cs-gpios = <&gpio1 0 0>, <0>;
> > 
> > Here the second chip select is native. However, when using swnodes
> > there is currently no way to specify a native chip select. The
> > proposal here is to register a swnode_gpio_undefined software node,
> > that can be specified to allow the indication of a native chip
> > select. For example:
> > 
> > static const struct software_node_ref_args device_cs_refs[] = {
> > 	{
> > 		.node  = &device_gpiochip_swnode,
> > 		.nargs = 2,
> > 		.args  = { 0, GPIO_ACTIVE_LOW },
> > 	},
> > 	{
> > 		.node  = &swnode_gpio_undefined,
> > 		.nargs = 0,
> > 	},
> > };
> > 
> > Register the swnode as the gpiolib is initialised and
> > check in swnode_get_gpio_device if the returned node matches
> 
> swnode_get_gpio_device()
> 

sorry yeah will fix that up.

> > swnode_gpio_undefined and return -ENOENT, which matches the behaviour
> > of the device tree system when it encounters a 0 phandle.
> 
> ...
> 
> > +config GPIO_SWNODE_UNDEFINED
> > +	bool "Undefined swnode GPIOs"
> 
> Why is this user visible?
> 

Yeah a good shout no reason for it to be.

> > +	help
> > +	  This adds a special place holder for software nodes to contain an
> > +	  undefined GPIO reference, this is primarily used by SPI to allow a
> > +	  list of GPIO chip selects to mark a certain chip select as being
> > +	  controlled the SPI device's internal chip select mechanism and not
> > +	  a GPIO.
> 
> How are drivers supposed to work in case this is not selected?
> 
> ...

Well they don't :-)

Thanks,
Charles