Re: [PATCH RESEND v2 08/12] dt-bindings: add binding for generic eDP panel

(off-list ancestor, not in this archive)

On Thu, Feb 14, 2019 at 12:38 PM Rob Herring [off-list ref] wrote:
On Thu, Feb 14, 2019 at 2:04 PM Vasily Khoruzhick [off-list ref] wrote:
On Tue, Feb 5, 2019 at 2:24 AM Thierry Reding [off-list ref] wrote:
On Tue, Feb 05, 2019 at 09:57:37AM +0100, Daniel Vetter wrote:
On Mon, Feb 04, 2019 at 05:22:58PM +0100, Thierry Reding wrote:
On Mon, Feb 04, 2019 at 04:59:09PM +0100, Daniel Vetter wrote:
On Mon, Feb 04, 2019 at 12:22:18PM +0100, Thierry Reding wrote:
On Mon, Feb 04, 2019 at 10:40:12AM +0100, Daniel Vetter wrote:
On Mon, Feb 04, 2019 at 09:23:59AM +0100, Thierry Reding wrote:
On Mon, Feb 04, 2019 at 12:13:55AM -0800, Vasily Khoruzhick wrote:
On Sun, Feb 3, 2019 at 11:43 PM Thierry Reding [off-list ref] wrote:
On Sun, Feb 03, 2019 at 10:54:57AM -0800, Vasily Khoruzhick wrote:
eDP panels usually have EDID EEPROM, so there's no need to define panel
width/height or any modes/timings in dts. But this panel still may have
regulator and/or backlight.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
---
 .../devicetree/bindings/display/panel/panel-edp.txt        | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/display/panel/panel-edp.txt
Please don't try to make panels look more generic than they really are.
You're going to have to provide a compatible string for your device that
is more specific than "panel-edp". You claim that you don't need any
extra information that is panel specific, but you don't know that now.
We have in the past thought that we didn't need things like prepare
delay, but then we ran into situations where we did need them.

Just do what everybody else does. Provide a specific compatible string
and match on that in the panel-simple driver. Even if you can read all
the video timings from an EDID EEPROM, you can still provide a mode in
the panel descriptor to serve as a fallback if for example the EEPROM
is faulty on some device.
Pinebook used several 768p panels that have slightly different timings
and recent batch uses 1080p panel.

What panel descriptor should I use as fallback?
You don't use panel descriptors as fallback. The simple-panel driver
will bind to a panel device and use the corresponding descriptor. If
your device tree contains the correct information, the descriptor is
correct for the panel you have.

In other words you need to ensure that you have the correct panel in
device tree for the board that you're using. This is exactly the same
thing as for other devices.

One way to to this is to have separate device trees for each variant
of the board that you want to support. Another variant may be to have
a common device tree and then have some early firmware update the DTB
with the correct panel information.
This would defeat the point of edp, which is to standardize the mess of
panels (at least somewhat) and avoid having to change the DT/ACPI
tables/firmware for every board you ship. Also, we do have DP quirking
infrastructure already (using the OUI), I think if there's something that
doesn't work then we should quirk it there.
The problem is that while the attempt may have been to standardize, it
failed. It doesn't take into account any of the details such as timing
between things like powering up the display and enabling the backlight
or similar. I don't know how you'd want to "quirk" those kinds of
requirements because they are highly panel specific.
Hm right, we get these from some firmware tables (and mix them with the
spec one, since some of the firmware values are nonsense). I don't even
know whether we can read the timings over dp aux somehow (you can power up
the panel with some pessimistic values to figure those out, and you only
need dp aux to work, which is much simpler than the entire panel).

What does make sense though imo is if we try not to stuff the edp panel
into panel-simple, because it's anything like a simple dumb panel. There's
also some integration awkwardness since with this panel you need to do dp
aux/i2c transactions to get at the information (edid alone isn't good
enough for edp), and I'm not sure how exactly that's supposed to be
instantiated. Maybe a special function to instantiate an edp panel, which
takes both a DT node and the dp_aux controller would be much better,
instead of trying to auto-match against a DT compatible string and load a
panel driver which is almost all fake.

Or we teach dp_aux to register itself and somehow teach panel-edp how it
can get hold of the dp_aux channel it needs.
We already do that. drm_dp_aux registers as an I2C adapter that can be
used to read EDID EEPROMs using I2C-over-AUX transactions. We already
use that on some platforms.

Also note that simple-panel already supports getting video timings from
EDID. If a DDC link is present in DT, the driver will load the modes
from EDID and use them.
Could we extend this to dp aux somehow? For edp you need the dp aux (which
then gives you the ddc link automatically).
I suppose we could do that. We could introduce a new property that would
allow the panel driver to get at the struct drm_dp_aux that can access
the panel. I'm not sure how much that would buy us. I suppose the driver
could go and use that drm_dp_aux to do I2C-over-AUX and ignore any
ddc-bus property in device tree. A drm_dp_aux object could also be used
to access DPCD if that's helpful.

The driver proposed here doesn't need access to DPCD, so I'm not sure
that would immediately help.
You definitely need dp aux to drive edp. That's where a lot of the really
important stuff is stored, and it sounds like on non-broken panels even
the timings (we've never implemented that on i915 somehow).
I'm not sure I understand what you're saying here. I haven't worked with
eDP panels in a while, but my recollection is that you can use DP AUX to
read video timings over EDID. We provide support for that by exporting a
DP AUX controller as I2C adapter (i.e. register with the I2C subsystem)
and then that I2C adapter can be used to read the EDID. I wasn't aware
that eDP panels additionally stored the video timings somewhere else.

What I meant above was that aside from the I2C-over-AUX for reading the
EDID, this driver doesn't do anything else with DP AUX in order to turn
the panel on. Looking at the eDP support we have on Tegra, there's a
DPCD register (DP_SET_POWER) that needs to be written in order to take
the sink device (i.e. panel) out of the power saving state. We do that
as part of the connector implementation rather than within the panel
driver. There are also additional registers such as DP_LINK_BW_SET that
need to be programmed. I think this is also relevant to regular DP and
detailed in the specification.

Given all the above, I'm beginning to think that Rob's right in that
perhaps we shouldn't be treating eDP panels as panels, but rather to
make them look more like DP monitors and make all this code part of the
connector implementation. I think pretty much the only differences to
regular DP are that we might require some lower-level resources that a
DP monitor would usually have built-in (reset or power GPIOs, power
supplies, backlight, ...).
I spent some time poking drm_connector code and I came to conclusion
that it's not a good idea.
Basically edp-connector driver will duplicate simple-panel code and
will bring extra complexity
into bridge driver for no benefit at all.
I said this on irc, but for everyone's benefit, what's used in the
kernel and the binding don't have to be aligned. I still think
following a connector binding in DT makes sense even if the kernel
implementation is actually a panel driver. Really, there's no
difference in bindings between a connector node and panel node.

Also currently there are no stand-alone connector drivers, they all
are part of display controller
driver.
Probably that is something to be refactored. I think we have lots of cases of:

if (is_connector)
    // call connector func
else
    // call panel func

Or maybe that was panel vs. bridge (then a connector)? It's been a
while since I looked at this. In any case, each device in the chain
shouldn't really have much knowledge as to what it is attached to.

One more thing to add is that I'm not sure that drm_connector is
suitable for managing power
and backlight -  I can't find appropriate callback in
drm_connector_funcs to enable power and/or
backlight. Basically there's nothing similar to enable() or disable()
from drm_bridge_funcs.
Nobody has put the 5V supply on HDMI on a switchable regulator? It may
need to be always on for HPD to work, but I wouldn't expect all board
designers to get that right.

I'm not sure if that's enough for eDP panels, though. For example the
AUO B133HTN01 panel, used by the exynos5800-peach-pi, seems to be an eDP
panel. But the driver also specifies a couple of additional delays which
suggests that either it violates the eDP specification or that the eDP
specification doesn't define any power sequencing delays that would've
been needed. Or perhaps these delays are specified somewhere and the
driver just doesn't use them?
Sigh. We can't foresee any bizarre behavior some hardware may or may
not have.
Bingo! You just summed up why we have specific compatibles.

Anyway,
can you propose something that can handle same hardware with different
edp panels via
single software image (u-boot aka firmware is part of software image)
and is acceptable
upstream?
compatible = "vendor,some-panel", "edp-connector";

Either you have to fixup the first string for the actual panel or have
some testing up front that you don't need to.
Where do I take actual panel compatible from?

The kernel can start
with only using the fallback string and if panel constraints turn out
to be specified sufficiently, we'll never need to use the panel
compatible.
So what panel compatible you want me to use if vendor used at least 3
different panels?
Introduce 2 more dts files (3 in total) even if they all work just
fine with a single dts now?
Add one more dts once laptop vendor decides to change panel vendor?

Rob
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