Re: [PATCH v3 08/10] HID: i2c-hid: Support being a panel follower

[Benjamin Tissoires]

On Jul 25 2023, Douglas Anderson wrote:
> As talked about in the patch ("drm/panel: Add a way for other devices
> to follow panel state"), we really want to keep the power states of a
> touchscreen and the panel it's attached to in sync with each other. In
> that spirit, add support to i2c-hid to be a panel follower. This will
> let the i2c-hid driver get informed when the panel is powered on and
> off. From there we can match the i2c-hid device's power state to that
> of the panel.
> 
> NOTE: this patch specifically _doesn't_ use pm_runtime to keep track
> of / manage the power state of the i2c-hid device, even though my
> first instinct said that would be the way to go. Specific problems
> with using pm_runtime():
> * The initial power up couldn't happen in a runtime resume function
>   since it create sub-devices and, apparently, that's not good to do
>   in your resume function.
> * Managing our power state with pm_runtime meant fighting to make the
>   right thing happen at system suspend to prevent the system from
>   trying to resume us only to suspend us again. While this might be
>   able to be solved, it added complexity.
> Overall the code without pm_runtime() ended up being smaller and
> easier to understand.
> 
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> 
> Changes in v3:
> - Add "depends on DRM || !DRM" to Kconfig to avoid randconfig error.
> - Split more of the panel follower code out of the core.
> 
> Changes in v2:
> - i2c_hid_core_panel_prepared() and ..._unpreparing() are now static.
> 
>  drivers/hid/i2c-hid/Kconfig        |  2 +
>  drivers/hid/i2c-hid/i2c-hid-core.c | 82 +++++++++++++++++++++++++++++-
>  2 files changed, 82 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hid/i2c-hid/Kconfig b/drivers/hid/i2c-hid/Kconfig
> index 3be17109301a..2bdb55203104 100644
> --- a/drivers/hid/i2c-hid/Kconfig
> +++ b/drivers/hid/i2c-hid/Kconfig
> @@ -70,5 +70,7 @@ config I2C_HID_OF_GOODIX
>  
>  config I2C_HID_CORE
>  	tristate
> +	# We need to call into panel code so if DRM=m, this can't be 'y'
> +	depends on DRM || !DRM
>  endif
>  
> diff --git a/drivers/hid/i2c-hid/i2c-hid-core.c b/drivers/hid/i2c-hid/i2c-hid-core.c
> index fa8a1ca43d7f..fa6d1f624342 100644
> --- a/drivers/hid/i2c-hid/i2c-hid-core.c
> +++ b/drivers/hid/i2c-hid/i2c-hid-core.c
> @@ -38,6 +38,8 @@
>  #include <linux/mutex.h>
>  #include <asm/unaligned.h>
>  
> +#include <drm/drm_panel.h>
> +
>  #include "../hid-ids.h"
>  #include "i2c-hid.h"
>  
> @@ -107,6 +109,8 @@ struct i2c_hid {
>  	struct mutex		reset_lock;
>  
>  	struct i2chid_ops	*ops;
> +	struct drm_panel_follower panel_follower;
> +	bool			is_panel_follower;
>  };
>  
>  static const struct i2c_hid_quirks {
> @@ -1058,6 +1062,59 @@ static int i2c_hid_core_initial_power_up(struct i2c_hid *ihid)
>  	return ret;
>  }
>  
> +static int i2c_hid_core_panel_prepared(struct drm_panel_follower *follower)
> +{
> +	struct i2c_hid *ihid = container_of(follower, struct i2c_hid, panel_follower);
> +	struct hid_device *hid = ihid->hid;
> +
> +	/*
> +	 * hid->version is set on the first power up. If it's still zero then
> +	 * this is the first power on so we should perform initial power up
> +	 * steps.
> +	 */
> +	if (!hid->version)
> +		return i2c_hid_core_initial_power_up(ihid);
> +
> +	return i2c_hid_core_resume(ihid);
> +}
> +
> +static int i2c_hid_core_panel_unpreparing(struct drm_panel_follower *follower)
> +{
> +	struct i2c_hid *ihid = container_of(follower, struct i2c_hid, panel_follower);
> +
> +	return i2c_hid_core_suspend(ihid);
> +}
> +
> +static const struct drm_panel_follower_funcs i2c_hid_core_panel_follower_funcs = {
> +	.panel_prepared = i2c_hid_core_panel_prepared,
> +	.panel_unpreparing = i2c_hid_core_panel_unpreparing,
> +};
> +
> +static int i2c_hid_core_register_panel_follower(struct i2c_hid *ihid)
> +{
> +	struct device *dev = &ihid->client->dev;
> +	int ret;
> +
> +	ihid->is_panel_follower = true;
> +	ihid->panel_follower.funcs = &i2c_hid_core_panel_follower_funcs;
> +
> +	/*
> +	 * If we're not in control of our own power up/power down then we can't
> +	 * do the logic to manage wakeups. Give a warning if a user thought
> +	 * that was possible then force the capability off.
> +	 */
> +	if (device_can_wakeup(dev)) {
> +		dev_warn(dev, "Can't wakeup if following panel\n");
> +		device_set_wakeup_capable(dev, false);
> +	}
> +
> +	ret = drm_panel_add_follower(dev, &ihid->panel_follower);
> +	if (ret)
> +		return ret;
> +
> +	return 0;
> +}
> +
>  int i2c_hid_core_probe(struct i2c_client *client, struct i2chid_ops *ops,
>  		       u16 hid_descriptor_address, u32 quirks)
>  {
> @@ -1119,7 +1176,15 @@ int i2c_hid_core_probe(struct i2c_client *client, struct i2chid_ops *ops,
>  	hid->bus = BUS_I2C;
>  	hid->initial_quirks = quirks;
>  
> -	ret = i2c_hid_core_initial_power_up(ihid);
> +	/*
> +	 * If we're a panel follower, we'll register and do our initial power
> +	 * up when the panel turns on; otherwise we do it right away.
> +	 */
> +	if (drm_is_panel_follower(&client->dev))
> +		ret = i2c_hid_core_register_panel_follower(ihid);
> +	else
> +		ret = i2c_hid_core_initial_power_up(ihid);

nitpicks, but I'm not sure I'm a big fan of having
"if (drm_is_panel_follower(&client->dev))" sprinkled everywhere in the
generic probe/resume/suspend code.

Would it be OK to define a `static int __do_i2c_hid_core_initial_power_up(struct i2c_hid *ihid)`
that would do the actual powering up, and have
i2c_hid_core_initial_power_up() doing the test if we are a panel
follower?

The i2c_hid_core_panel_* will need to be updated to use the `__do_`
prefixed functions.

> +
>  	if (ret)
>  		goto err_mem_free;
>  
> @@ -1143,7 +1208,14 @@ void i2c_hid_core_remove(struct i2c_client *client)
>  	struct i2c_hid *ihid = i2c_get_clientdata(client);
>  	struct hid_device *hid;
>  
> -	i2c_hid_core_power_down(ihid);
> +	/*
> +	 * If we're a follower, the act of unfollowing will cause us to be
> +	 * powered down. Otherwise we need to manually do it.
> +	 */
> +	if (ihid->is_panel_follower)
> +		drm_panel_remove_follower(&ihid->panel_follower);

That part is concerning, as we are now calling hid_drv->suspend() when removing
the device. It might or not have an impact (I'm not sure of it), but we
are effectively changing the path of commands sent to the device.

hid-multitouch might call a feature in ->suspend, but the remove makes
that the physical is actually disconnected, so the function will fail,
and I'm not sure what is happening then.

> +	else
> +		i2c_hid_core_power_down(ihid);

Same here, I *think* it would be best to have the `if (ihid->is_panel_follower)`
test in i2c_hid_core_power_down()  (and have a separate
_do_i2c_hid_core_power_down()).

>  
>  	hid = ihid->hid;
>  	hid_destroy_device(hid);
> @@ -1171,6 +1243,9 @@ static int i2c_hid_core_pm_suspend(struct device *dev)
>  	struct i2c_client *client = to_i2c_client(dev);
>  	struct i2c_hid *ihid = i2c_get_clientdata(client);
>  
> +	if (ihid->is_panel_follower)
> +		return 0;

Not sure we need to split that one with _do_ prefix, it's already split
:)

> +
>  	return i2c_hid_core_suspend(ihid);
>  }
>  
> @@ -1179,6 +1254,9 @@ static int i2c_hid_core_pm_resume(struct device *dev)
>  	struct i2c_client *client = to_i2c_client(dev);
>  	struct i2c_hid *ihid = i2c_get_clientdata(client);
>  
> +	if (ihid->is_panel_follower)
> +		return 0;

Same here, no need to split.

> +
>  	return i2c_hid_core_resume(ihid);
>  }
>  
> -- 
> 2.41.0.487.g6d72f3e995-goog
> 

Cheers,
Benjamin