[PATCH v2 00/10] drm/panel and i2c-hid: Allow panels and touchscreens to power sequence together

[Maxime Ripard]

Hi Doug,

On Thu, Jun 08, 2023 at 07:38:58AM -0700, Doug Anderson wrote:
> On Thu, Jun 8, 2023 at 12:17 AM Maxime Ripard <mripard at kernel.org> wrote:
> >
> > Hi Douglas,
> >
> > On Wed, Jun 07, 2023 at 02:49:22PM -0700, Douglas Anderson wrote:
> > >
> > > The big motivation for this patch series is mostly described in the patch
> > > ("drm/panel: Add a way for other devices to follow panel state"), but to
> > > quickly summarize here: for touchscreens that are connected to a panel we
> > > need the ability to power sequence the two device together. This is not a
> > > new need, but so far we've managed to get by through a combination of
> > > inefficiency, added costs, or perhaps just a little bit of brokenness.
> > > It's time to do better. This patch series allows us to do better.
> > >
> > > Assuming that people think this patch series looks OK, we'll have to
> > > figure out the right way to land it. The panel patches and i2c-hid
> > > patches will go through very different trees and so either we'll need
> > > an Ack from one side or the other or someone to create a tag for the
> > > other tree to pull in. This will _probably_ require the true drm-misc
> > > maintainers to get involved, not a lowly committer. ;-)
> > >
> > > Version 2 of this patch series doesn't change too much. At a high level:
> > > * I added all the forgotten "static" to functions.
> > > * I've hopefully made the bindings better.
> > > * I've integrated into fw_devlink.
> > > * I cleaned up a few descriptions / comments.
> > >
> > > This still needs someone to say that the idea looks OK or to suggest
> > > an alternative that solves the problems. ;-)
> >
> > Thanks for working on this.
> >
> > I haven't seen in any of your commit messages how the panels were
> > actually "packaged" together?
> >
> > Do a panel model typically come together with the i2c-hid support, or is
> > it added at manufacture time?
> >
> > If it's the latter, it's indeed a fairly loose connection and we need
> > your work.
> >
> > If it's the former though and we don't expect a given panel reference to
> > always (or never) come with a touchscreen attached,
> 
> Thanks for your reply. Let me see what I can do to bring clarity.
> 
> In at least some of the cases, I believe that the panel and the
> touchscreen _are_ logically distinct components, even if they've been
> glued together at some stage in manufacturing. Even on one of the
> "poster child" boards that I talk about in patch #3, the early
> versions of "homestar", I believe this to be the case. However, even
> if the panel and touchscreen are separate components then they still
> could be connected to the main board in a way that they share power
> and/or reset signals. In my experience, in every case where they do
> the EEs expect that the panel is power sequenced first and then the
> touchscreen is power sequenced second. The EEs look at the power
> sequencing requirements of the panel and touchscreen, see that there
> is a valid power sequence protocol where they can share rails, and
> design the board that way. Even if the touchscreen and panel are
> logically separate, the moment the board designers hook them up to the
> same power rails and/or reset signals they become tied. This is well
> supported by my patch series.
> 
> The case that really motivated my patch series, though, is the case
> that Cong Yang recently has been working on. I think most of the
> discussion is in his original patch series [1]. Cong Yang's patch
> series is largely focused on supporting the "ILI9882T" chip and some
> panels that it's used with. I found a datasheet for that, and the
> title from the first page is illustrative: "In-cell IC Integrates TFT
> LCD Driver and Capacitive Touch Controller into a Two Chip Cascade".
> This is an integrated solution that's designed to handle both the LCD
> and the touchscreen.
>
> [1] https://lore.kernel.org/lkml/20230519032316.3464732-1-yangcong5@huaqin.corp-partner.google.com/

Ok, I think we're on the same page at the hardware level then :)

> > I guess we can have
> > something much simpler with a bunch of helpers that would register a
> > i2c-hid device and would be called by the panel driver itself.
> >
> > And then, since everything is self-contained managing the power state
> > becomes easier as well.
> 
> Can you give me more details about how you think this would work?
> 
> When you say that the panel would register an i2c-hid device itself,
> do you mean that we'd do something like give a phandle to the i2c bus
> to the panel and then the panel would manually instantiate the i2c-hid
> device on it? ...and I guess it would need to be a "subclass" of
> i2c-hid that knew about the connection to the panel code? This
> subclass and the panel code would communicate with each other about
> power sequencing needs through some private API (like MFD devices
> usually do?). Assuming I'm understanding correctly, I think that could
> work.

I guess what I had in mind is to do something similar to what we're
doing with hdmi-codec already for example.

We have several logical components already, in separate drivers, that
still need some cooperation.

If the panel and touchscreen are on the same i2c bus, I think we could
even just get a reference to the panel i2c adapter, get a reference, and
pass that to i2c-hid (with a nice layer of helpers).

What I'm trying to say is: could we just make it work by passing a bunch
of platform_data, 2-3 callbacks and a device registration from the panel
driver directly?

> Is it cleaner than my current approach, though?

"cleaner" is subjective, really, but it's a more "mainstream" approach
that one can follow more easily through function calls.

> I guess, alternatively, we could put the "panel" directly under the
> i2c bus in this case. That would probably work for Cong Yang's current
> needs, but we'd end up in trouble if we ever had a similar situation
> with an eDP panel since eDP panels need to be under the DP-AUX bus.

I don't know DP-AUX very well, what is the issue that you're mentioning?

> I guess overall, though, while I think this approach could solve Cong
> Yang's needs, I still feel like it's worth solving the case where
> board designers have made panel and touchscreens "coupled" by having
> them rely on the same power rails and/or reset signals.

Sure, I definitely want that too :)

Maxime
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