linux-kernel - Re: [PATCH 2/2] gpiolib: add support for fetching descriptors from static properties

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Date:   Tue, 30 Jul 2019 14:30:33 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        linux-input@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH 2/2] gpiolib: add support for fetching descriptors from
 static properties

Hi Dmitry,

On Sat, Jul 13, 2019 at 12:52:59AM -0700, Dmitry Torokhov wrote:
> Now that static device properties understand notion of child nodes, let's
> teach gpiolib to tie such children and machine GPIO descriptor tables.
> We will continue using a single table for entire device, but instead of
> using connection ID as a lookup key in the GPIO descriptor table directly,
> we will perform additional translation: fwnode_get_named_gpiod() when
> dealing with property_set-backed fwnodes will try parsing string property
> with name matching connection ID and use result of the lookup as the key in
> the table:
> 
> static const struct property_entry dev_child1_props[] __initconst = {
> 	...
> 	PROPERTY_ENTRY_STRING("gpios",		"child-1-gpios"),
> 	{ }
> };
> 
> static struct gpiod_lookup_table dev_gpiod_table = {
> 	.dev_id = "some-device",
> 	.table = {
> 		...
> 		GPIO_LOOKUP_IDX("B", 1, "child-1-gpios", 1, GPIO_ACTIVE_LOW),
> 		...
> 	},
> };

We don't need struct gpiod_lookup_table anymore. We can mimic DT with
the software nodes now that we have those "reference properties". A
gpio reference with the software nodes would look something like this:

        enum {
                GPIO_CONTROLLER,
                MY_DEVICE
        };

        static const struct software_node nodes[];

        static const struct software_node_ref_args reset_gpio_ref = {
                .node = &nodes[GPIO_CONTROLLER],
                .nargs = 2,
                .args = {
                        14,                     /* line number */
                        GPIO_ACTIVE_HIGH        /* flags */
                }
        };

        static const struct software_node_reference my_refs[] = {
                { "reset-gpios", 1, &reset_gpio_ref }
        };

        /* Optionally, we could support gpiochip finding by name... */
        static const struct property_entry my_props[] = {
                PROPERTY_ENTRY_STRING("gpio-controller", "name_of_the_controller")
        };

        static const struct software_node nodes[] = {
                [GPIO_CONTROLLER]       = { "gpio_controller" },
                [MY_DEVICE]             = { "my_device", NULL, my_props, my_refs }
        };

        void my_init(void)
        {
                ...
                ret = software_node_register_nodes(nodes);
                ...
        }

In gpiolib we should now be able to access that reference with
fwnode_property_get_references_args():

        static int gpiochip_match_fwnode(struct gpio_chip *chip, void *fwnode)
        {
                /* The fwnode member needs to be added to struct gpio_chip */
                return chip->fwnode == fwnode;
        }

        static struct gpio_desc *gpiod_find(struct device *dev,
                                            const char *con_id,
                                            unsigned int idx,
                                            unsigned long flags)
        {
                struct fwnode_reference_args args;
                struct gpio_chip *chip;
                struct gpio_desc *desc;
                const char *name;
                int ret;

                ret = fwnode_property_get_refernce_args(dev_fwnode(dev), con_id,
                                                        NULL, idx, &args);
                ...

                /* Let's find the gpiochip */
                chip = gpiochip_find(args.fwnode, gpiochip_match_fwnode);
                ...

                /* Or optionally with find_chip_by_name() */
                //ret = device_property_read_string(dev, "gpio-controller", &name);
                ...
                //chip = find_chip_by_name(name);
                ...

                /* I'm assuming hwnum is the same as line number? */
                desc = gpiochip_get_desc(chip, args.args[0]);
                *flags = args.args[1];

                return desc;
        }

The above is just an example, but I'm pretty sure that something like
it (with a little bit of tuning) is all that we need.

thanks,

-- 
heikki