[PATCH v2 0/2] pinctrl: remove unused 'pinconf-config' debugfs interface

[Vladimir Zapolskiy]

The main goal of the change is to remove .pin_config_dbg_parse_modify
callback before a driver with its support appears. So far the in-kernel
interface did not attract any users since its introduction 5 years ago.

Originally .pin_config_dbg_parse_modify callback and the associated
'pinconf-config' debugfs file were introduced in commit f07512e615dd
("pinctrl/pinconfig: add debug interface"), a short description of
'pinconf-config' usage for debugging can be expressed this way:

Write to 'pinconf-config' (see pinconf_dbg_config_write() function):

% echo -n modify $map_type $device_name $state_name $pin_name $config > \
	/sys/kernel/debug/pinctrl/$pinctrl/pinconf-config

It supposes to update a global (therefore single!) 'pinconf_dbg_conf'
variable with an alternative setting, the arguments should match
an existing pinconf device and some registered pinctrl mapping 'map':

* $map_type is either 'config_pin' or 'config_group', it should match
  'map->type' value of PIN_MAP_TYPE_CONFIGS_PIN or
   PIN_MAP_TYPE_CONFIGS_GROUP accordingly,
* $device_name should match 'map->dev_name' string value,
* $state_name should match 'map->name' string value,
* $pin_name should match 'map->data.configs.group_or_pin' string value,

If all above has matched, then $config is a new value to be set by calling
pinconfops->pin_config_dbg_parse_modify(pctldev, config, matched_config).

After a successful write into 'pinconf-config' a user can read the file
to get information about that single modified pin configuration.

The fact is .pin_config_dbg_parse_modify callback has never been defined
in 'struct pinconf_ops' of any pinconf driver, thus an actual modification
of a pin or group state on any present pinconf controller does not happen,
and it declares that all related code is no more than dead code.

I discovered the issue while attempting to add .pin_config_dbg_parse_modify
support in some drivers and found that too short 'MAX_NAME_LEN' set by

  drivers/pinctrl/pinconf.c:372:#define MAX_NAME_LEN 15

is practically insufficient to store a regular pinctrl device name,
which are like 'e6060000.pin-controller-sh-pfc' or pin names like
'MX6QDL_PAD_ENET_REF_CLK', thus it is another indicator that the code
is barely usable, insufficiently tested and unprepossessing.

Of course it might be possible to increase MAX_NAME_LEN, and then add
.pin_config_dbg_parse_modify callbacks to the drivers, but the whole
idea of such a limited debug option looks inviable. A more flexible
way to functionally substitute the original approach is to implicitly
or explicitly use pinctrl_select_state() function whenever needed.

The proposal is to remove the changes added by commit f07512e615dd
("pinctrl/pinconfig: add debug interface") for now, as described in
patch 2/2 commit message it may affect only non-vanilla kernels formally.

Note that still it is a debugfs UAPI change, which is supposed to be
unstable, however, since 'pinconf-config' file is plainly unusable
on vanilla, I won't expect any user reports or asks to restore it back,
this my assumption nudges me not to add an RFC tag to the published
change.

Changes from v1 to v2:
* added one patch against cirrus/pinctrl-madera-core.c to avoid a compile
  time warning due to a removed header inclusion

Vladimir Zapolskiy (2):
  pinctrl: remove pinctrl/machine.h inclusion from pinctrl/pinconf.h
  pinctrl: remove unused 'pinconf-config' debugfs interface

 drivers/pinctrl/cirrus/pinctrl-madera-core.c |   1 +
 drivers/pinctrl/pinconf.c                    | 222 -------------------
 include/linux/pinctrl/pinconf.h              |   6 -
 3 files changed, 1 insertion(+), 228 deletions(-)

-- 
2.20.1