This patch causes pcs_parse_pinconf() to return -ENOTSUPP when no
pinctrl_map is added.  The current behavior is to return 0 when
!PCS_HAS_PINCONF or !nconfs.  Thus pcs_parse_one_pinctrl_entry()
incorrectly assumes that a map was added and sets num_maps = 2.

Analysis:
=========
The function pcs_parse_one_pinctrl_entry() calls pcs_parse_pinconf()
if PCS_HAS_PINCONF is enabled.  The function pcs_parse_pinconf()
returns 0 to indicate there was no error and num_maps is then set to 2:

 980 static int pcs_parse_one_pinctrl_entry(struct pcs_device *pcs,
 981                                                 struct device_node *np,
 982                                                 struct pinctrl_map **map,
 983                                                 unsigned *num_maps,
 984                                                 const char **pgnames)
 985 {
<snip>
1053         (*map)->type = PIN_MAP_TYPE_MUX_GROUP;
1054         (*map)->data.mux.group = np->name;
1055         (*map)->data.mux.function = np->name;
1056
1057         if (PCS_HAS_PINCONF && function) {
1058                 res = pcs_parse_pinconf(pcs, np, function, map);
1059                 if (res)
1060                         goto free_pingroups;
1061                 *num_maps = 2;
1062         } else {
1063                 *num_maps = 1;
1064         }

However, pcs_parse_pinconf() will also return 0 if !PCS_HAS_PINCONF or
!nconfs.  I believe these conditions should indicate that no map was
added by returning -ENOTSUPP. Otherwise pcs_parse_one_pinctrl_entry()
will set num_maps = 2 even though no maps were successfully added, as
it does not reach "m++" on line 940:

 895 static int pcs_parse_pinconf(struct pcs_device *pcs, struct device_node *np,
 896                              struct pcs_function *func,
 897                              struct pinctrl_map **map)
 898
 899 {
 900         struct pinctrl_map *m = *map;
<snip>
 917         /* If pinconf isn't supported, don't parse properties in below. */
 918         if (!PCS_HAS_PINCONF)
 919                 return 0;
 920
 921         /* cacluate how much properties are supported in current node */
 922         for (i = 0; i < ARRAY_SIZE(prop2); i++) {
 923                 if (of_find_property(np, prop2[i].name, NULL))
 924                         nconfs++;
 925         }
 926         for (i = 0; i < ARRAY_SIZE(prop4); i++) {
 927                 if (of_find_property(np, prop4[i].name, NULL))
 928                         nconfs++;
 929         }
 930         if (!nconfs)
 919                 return 0;
 932
 933         func->conf = devm_kcalloc(pcs->dev,
 934                                   nconfs, sizeof(struct pcs_conf_vals),
 935                                   GFP_KERNEL);
 936         if (!func->conf)
 937                 return -ENOMEM;
 938         func->nconfs = nconfs;
 939         conf = &(func->conf[0]);
 940         m++;

This situtation will cause a boot failure [0] on the BeagleBone Black
(AM3358) when am33xx_pinmux node in arch/arm/boot/dts/am33xx-l4.dtsi
has compatible = "pinconf-single" instead of "pinctrl-single".

The patch fixes this issue by returning -ENOSUPP when !PCS_HAS_PINCONF
or !nconfs, so that pcs_parse_one_pinctrl_entry() will know that no
map was added.

Logic is also added to pcs_parse_one_pinctrl_entry() to distinguish
between -ENOSUPP and other errors.  In the case of -ENOSUPP, num_maps
is set to 1 as it is valid for pinconf to be enabled and a given pin
group to not any pinconf properties.

[0] https://lore.kernel.org/linux-omap/20200529175544.GA3766151@x1/

Fixes: 9dddb4df ("pinctrl: single: support generic pinconf")
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20200608125143.GA2789203@x1Signed-off-by: Linus Walleij <linus.walleij@linaro.org>