On 2012-11-21 13:40, Thierry Reding wrote:

On Wed, Nov 21, 2012 at 01:06:03PM +0200, Tomi Valkeinen wrote:

(sorry for bouncing back and forth with my private and my @ti addresses.
I can't find an option in thunderbird to only use one sender address,
and I always forget to change it when responding...)

quoted

My suggestion would be to go forward with an in-driver solution, and
look at the DT based solution later if we are seeing an increasing bloat
in the drivers.

Assuming we go with your approach, what's the plan? We're actually
facing this problem right now for Tegra. Basically we have a DRM driver
that can drive the panel, but we're still missing a way to hook up the
backlight and panel enabling code. So we effectively can't support any
of the LVDS devices out there without this series.

Could you describe the hardware setup you have related to the LCD and
backlight? Is it a public board with public schematics?

I've understood that you don't have anything special in your board, just
an LCD and a backlight, and the power sequences are related to powering
up the LCD and the backlight, without anything board specific. If so,
there's no need for board specific code, but just improving the panel
and backlight drivers to support the models you use.

As I understand it, what you propose is similar to what ASoC does. For a
specific board, you'd have to write a driver, presumably for the new
panel/display framework, that provides code to power the panel on and
off. That means we'll have to have a driver for each panel out there
basically, or we'd need to write generic drivers that can be configured
to some degree (via platform data or DT). This is similar to how ASoC
works, where we have a driver that provides support for a specific codec
connected to the Tegra SoC. For the display framework things could be
done in a similar way I suppose, so that Tegra could have one display
driver to handle all aspects of powering on and off the various panels
for the various boards out there.

I think we should only need the board drivers for very special cases. If
there's just a panel and a backlight, without any special dynamic muxing
or other trickery needed, I don't see a need for a board driver. I
presume this is the case for most of the boards.

Obviously, a lot of the code will be similar for other SoCs, but maybe
that's just the way things are if we choose that approach. There's also
the potential for factoring out large chunks of common code later on
once we start to see common patterns.

One thing that's not very clear is how the backlight subsystem should be
wired up with the display framework. I have a patch on top of the Tegra
DRM driver which adds some ad-hoc display support using this power
sequences series and the pwm-backlight.

I think that's a separate issue: how to associate the lcd device and
backlight device together. I don't have a clear answer to this.

There are many ways the backlight may be handled. In some cases the
panel and the backlight are truly independent, and you can use the other
without using the other (not very practical, though =).

But then with some LCDs the backlight may be controlled by sending
commands to the panel, and in this case the two may be quite linked.
Changing the backlight requires the panel driver to be up and running,
and sometimes the sending the backlight commands may need to be (say,
DSI display, with backlight commands going over the DSI bus).

So my feeling is that the panel driver should know about the related
backlight device. In the first case the panel driver would just call
enable/disable in the backlight device when the panel is turned on.

In the second case of the DSI panel... I'm not sure. I've implemented it
so that the panel driver creates the backlight device, and implements
the backlight callbacks. It then sends the DSI commands from those
callbacks.

From reading the proposal for the panel/display framework, it sounds
like a lot more is planned than just enabling or disabling panels, but
it also seems like a lot of code needs to be written to support things
like DSI, DBI or other control busses.

At least for Tegra, and I think the same holds for a wide variety of
other SoCs, dumb panels would be enough for a start. In the interest of
getting a working solution for those setups, maybe we can start small
and add just enough framework to register dumb panel drivers to along
with code to wire up a backlight to light up the display. Then we could
possibly still make it to have a proper solution to support the various
LVDS panels for Tegra with 3.9.

Yes, we (Laurent and me) both agree that we should start simple.

However, the common panel framework is not strictly needed for this. I'm
not sure of the current architecture for Tegra, but for OMAP we already
have panel drivers (omap specific ones, though). The panel drivers may
support multiple models, (for example,
drivers/video/omap2/displays/panel-generic-dpi.c).

I don't see any problem with adding small Tegra specific panel drivers
for the time being, with the intention of converting to common panel
framework when that's available.

Of course, the DT side is an issue. If you now create DT bindings for a
temporary model, and need to change it again later, you'll have some
headaches trying managing that without breaking the old bindings... This
is why I haven't pushed DT bindings for OMAP, as I know I have to change
them in the near future.

 Tomi


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