Hi,

On Thu, Jun 8, 2023 at 8:37 AM Benjamin Tissoires
[off-list ref] wrote:

On Jun 07 2023, Douglas Anderson wrote:

quoted

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.

Generally speaking, I'm not that happy when we need to coordinate with
other subsystems for bringing up resources...

Yeah, I'd agree that it's not amazingly elegant. Unfortunately, I
couldn't find any existing clean frameworks that would do what was
needed, which is (presumably) why this problem hasn't been solved
before. I could try to come up with a grand abstraction / new
framework, but that doesn't seem like a great choice either unless we
expect more users...

Anyway, a remark inlined (at least):

quoted

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2:
- i2c_hid_core_panel_prepared() and ..._unpreparing() are now static.

 drivers/hid/i2c-hid/i2c-hid-core.c | 82 +++++++++++++++++++++++++++++-
 1 file changed, 81 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/i2c-hid/i2c-hid-core.c b/drivers/hid/i2c-hid/i2c-hid-core.c
index fa8a1ca43d7f..368db3ae612f 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 +1062,34 @@ 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,
+};

Can we make that above block at least behind a Kconfig?

i2c-hid is often used for touchpads, and the notion of drm panel has
nothing to do with them. So I'd be more confident if we could disable
that code if not required.

Happy to put it behind a Kconfig. I'll plan on that for v3. I'll stub
the functions out if there is no Kconfig, but plan to still leave
structure members just to avoid uglifying the sources too much.

Actually, I'd be even more happier if it were in a different compilation
unit. Not necessary a different module, but at least a different file.

I suspect that it's not worth it, but I'll do this if you feel
strongly about it.

I guess the simplest way I can think of to move this to its own file
would be to put the whole private data structure (struct i2c_hid) in a
private header file and then add prototypes for i2c_hid_core_resume()
and i2c_hid_core_suspend() there. Then I could add something like
i2c_hid_core_handle_panel_follower() that would have all the
registration logic. I'd still need special cases in the core
suspend/resume/remove code unless I add a level of abstraction. While
the level of abstraction is more "pure", it also would make the code
harder to follow.

Unless I hear a strong opinion (or if this series changes
significantly), I'll plan to keep things in the same file and just use
a Kconfig.