[PATCH v2 08/10] HID: i2c-hid: Support being a panel follower

[Doug Anderson]

Benjamin,

On Mon, Jun 26, 2023 at 3:49 PM Doug Anderson <dianders at chromium.org> wrote:
>
> Benjamin,
>
> On Thu, Jun 8, 2023 at 8:37 AM Benjamin Tissoires
> <benjamin.tissoires at redhat.com> wrote:
> >
> > > +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.
>
> Now that other concerns are addressed, I started trying to write up a
> v3 and I found myself writing this as the description of the Kconfig
> entry:
>
> --
> config I2C_HID_SUPPORT_PANEL_FOLLOWER
> bool "Support i2c-hid devices that must be power sequenced with a panel"
>
> Say Y here if you want support for i2c-hid devices that need to
> coordinate power sequencing with a panel. This is typically important
> when you have a panel and a touchscreen that share power rails or
> reset GPIOs. If you say N here then the kernel will not try to honor
> any shared power sequencing for your hardware. In the best case,
> ignoring power sequencing when it's needed will draw extra power. In
> the worst case this will prevent your hardware from functioning or
> could even damage your hardware.
>
> If unsure, say Y.
>
> --
>
> I can certainly go that way, but I just wanted to truly make sure
> that's what we want. Specifically:
>
> 1. If we put the panel follower code behind a Kconfig then we actually
> have no idea if a touchscreen was intended to be a panel follower.
> Specifically the panel follower API is the one that detects the
> connection between the panel and the i2c-hid device, so without being
> able to call the panel follower API we have no idea that an i2c-hid
> device was supposed to be a panel follower.
>
> 2. It is conceivable that power sequencing a device incorrectly could
> truly cause hardware damage.
>
> Together, those points mean that if you turn off the Kconfig entry and
> then try to boot on a device that needed that Kconfig setting that you
> might damage hardware. I can code it up that way if you want, but it
> worries me...
>
>
> Alternatives that I can think of:
>
> a) I could change the panel follower API so that panel followers are
> in charge of detecting the panel that they follow. Today, that looks
> like:
>
>        panel_np = of_parse_phandle(dev->of_node, "panel", 0);
>        if (panel_np)
>                /* It's a panel follower */
>        of_node_put(panel_np);
>
> ...so we could put that code in each touchscreen driver and then fail
> to probe i2c-hid if we detect that we're supposed to be a panel
> follower but the Kconfig is turned off. The above doesn't seem
> massively ideal since it duplicates code. Also, one reason why I put
> that code in drm_panel_add_follower() is that I think this concept
> will eventually be needed even for non-DT cases. I don't know how to
> write the non-DT code right now, though...
>
>
> b) I could open-code detect the panel follower case but leave the
> actual linking to the panel follower API. AKA add to i2c-hid:
>
>        if (of_property_read_bool(dev->of_node, "panel"))
>                /* It's a panel follower */
>
> ...that's a smaller bit of code, but feels like an abstraction
> violation. It also would need to be updated if/when we added support
> for non-DT panel followers.
>
>
> c) I could add a "static inline" implementation of b) to "drm_panel.h".
>
> That sounds great and I started doing it. ...but then realized that it
> means adding to drm_panel.h:
>
> #include <linux/device.h>
> #include <linux/of.h>
>
> ...because otherwise of_property_read_bool() isn't defined and "struct
> device" can't be dereferenced. That might be OK, but it looks as if
> folks have been working hard to avoid things like this in header
> files. Presumably it would get uglier if/when we added the non-DT
> case, as well. That being said, I can give it a shot...
>
> --
>
> At this point, I'm hoping for some advice. How important is it for you
> to have a Kconfig for "I2C_HID_SUPPORT_PANEL_FOLLOWER"?
>
> NOTE: even if I don't add the Kconfig, I could at least create a
> function for registering the panel follower that would get most of the
> panel follower logic out of the probe function. Would that be enough?

I'd love to send a new version of this patch series, but I'm still
stuck with the above issue. I'm hoping you might have a minute to
provide your thoughts. If I don't hear anything, I'll try a v3 where I
don't have the Kconfig for "I2C_HID_SUPPORT_PANEL_FOLLOWER" but just
try to pull a little more of the code out of the probe function.

Thanks for your time!

-Doug