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

[Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxx>]

On Tue, Dec 10, 2024 at 04:15:18PM -0800, Dmitry Torokhov wrote:
> On Tue, Mar 26, 2024 at 02:11:06PM +0000, 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,
> > 	},
> > };
> 
> I am sorry, I am very late to the party, but wouldn't it all work by
> simply setting ".node" to NULL? As far as I can see we have in
> software_node_get_reference_args():
> 
> 	...
> 
> 	if (index * sizeof(*ref) >= prop->length)
> 		return -ENOENT;
> 
> 	ref_array = prop->pointer;
> 	ref = &ref_array[index];
> 
> 	refnode = software_node_fwnode(ref->node);
> 	if (!refnode)
> 		return -ENOENT;
> 
> if ref->node is NULL then software_node_fwnode(ref->node) will return
> NULL and we'll get -ENOENT which will bubble up to
> gpiod_get_index_optional() in SPI core.
> 

I did definitely try that at the time, and there was definitely
some issue with that approach. I think it might have related to
something around how the GPIO stuff structures things. Alas, it
is long enough ago that the details are escaping me, if I manage
to find a spare minute I will try to refresh my memory and report
back.

Thanks,
Charles