Hi,

On Tue, Mar 16, 2021 at 5:44 PM Doug Anderson [off-list ref] wrote:

Hi,

On Tue, Mar 16, 2021 at 2:46 PM Laurent Pinchart
[off-list ref] wrote:

quoted

Hi Doug,

On Mon, Mar 15, 2021 at 09:25:37AM -0700, Doug Anderson wrote:

quoted

On Sat, Mar 13, 2021 at 1:17 PM Laurent Pinchart wrote:

quoted

On Thu, Mar 04, 2021 at 03:52:01PM -0800, Douglas Anderson wrote:

quoted

In commit 58074b08c04a ("drm/bridge: ti-sn65dsi86: Read EDID blob over
DDC") we attempted to make the ti-sn65dsi86 bridge properly read the
EDID from the panel. That commit kinda worked but it had some serious
problems.

The problems all stem from the fact that userspace wants to be able to
read the EDID before it explicitly enables the panel. For eDP panels,
though, we don't actually power the panel up until the pre-enable
stage and the pre-enable call happens right before the enable call
with no way to interject in-between. For eDP panels, you can't read
the EDID until you power the panel. The result was that
ti_sn_bridge_connector_get_modes() was always failing to read the EDID
(falling back to what drm_panel_get_modes() returned) until _after_
the EDID was needed.

To make it concrete, on my system I saw this happen:
1. We'd attach the bridge.
2. Userspace would ask for the EDID (several times). We'd try but fail
   to read the EDID over and over again and fall back to the hardcoded
   modes.
3. Userspace would decide on a mode based only on the hardcoded modes.
4. Userspace would ask to turn the panel on.
5. Userspace would (eventually) check the modes again (in Chrome OS
   this happens on the handoff from the boot splash screen to the
   browser). Now we'd read them properly and, if they were different,
   userspace would request to change the mode.

The fact that userspace would always end up using the hardcoded modes
at first significantly decreases the benefit of the EDID
reading. Also: if the modes were even a tiny bit different we'd end up
doing a wasteful modeset and at boot.

s/and at/at/ ?

Sure, I can correct if/when I respin or it can be corrected when landed.

quoted

quoted

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
index 491c9c4f32d1..af3fb4657af6 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c

@@ -16,6 +16,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/regmap.h>
 #include <linux/regulator/consumer.h>
+#include <linux/workqueue.h>

 #include <asm/unaligned.h>

@@ -130,6 +131,12 @@
  * @ln_assign:    Value to program to the LN_ASSIGN register.
  * @ln_polrs:     Value for the 4-bit LN_POLRS field of SN_ENH_FRAME_REG.
  *
+ * @pre_enabled_early: If true we did an early pre_enable at attach.
+ * @pre_enable_timeout_work: Delayed work to undo the pre_enable from attach
+ *                           if a normal pre_enable never came.

Could we simplify this by using the runtime PM autosuspend feature ? The
configuration of the bridge would be moved from pre_enable to the PM
runtime resume handler, the clk_disable_unprepare() call moved from
post_disable to the runtime suspend handler, and the work queue replaced
by usage of pm_runtime_put_autosuspend().

It's an interesting idea but I don't think I can make it work, at
least not in a generic enough way. Specifically we can also use this
bridge chip as a generic GPIO provider in Linux. When someone asks us
to read a GPIO then we have to power the bridge on
(pm_runtime_get_sync()) and when someone asks us to configure a GPIO
as an output then we actually leave the bridge powered until they stop
requesting it as an output. At the moment the only user of this
functionality (that I know of) is for the HPD pin on trogdor boards
(long story about why we don't use the dedicated HPD) but the API
supports using these GPIOs for anything and I've tested that it works.
It wouldn't be great to have to keep the panel on in order to access
the GPIOs.

The issue you're trying to fix doesn't seem specific to this bridge, so
handling it in the bridge driver bothers me :-S Is there any way we
could handle this in the DRM core ? I don't want to see similar
implementations duplicated in all HDMI/DP bridges.

Yes, it is true that this problem could affect other drivers.  ...and
in full disclosure I think there are other similar workarounds already
present. I haven't personally worked on those chips, but in
ps8640_bridge_get_edid() there is a somewhat similar workaround to
chain a pre-enable (though maybe it's not quite as optimized?). I'm
told that maybe something had to be handled for anx7625 (in
anx7625_get_edid()?) but that definitely doesn't look at all like it's
doing a pre-enable, so maybe things for that bridge just work
differently.

One thing that makes me hesitant about trying to moving this to the
core is that even in sn65dsi86 there is a case where it won't work. As
I mentioned in the patch I'm not aware of anyone using it in
production, but if someone was using the MIPI clock as input to the
bridge chip instead of a fixed "refclk" then trying to get the EDID
after just "pre-enable" falls over.  Said another way: I can say that
with this particular bridge chip, if you're using a fixed refclk, you
can read the EDID after the pre-enable. I don't know if that's always
true with all other bridge chips.

So I guess in summary: I think I could put my code in the core, but I
don't _think_ I can just make it automatically enabled.

* In sn65dsi I'd have to only enable it if we have a fixed refclk.

* Maybe in ps8640 I could just always enable it and replace the
existing code? I'd have to find someone to test.

* In anx7625 things look totally different.

Can you give me any advice on how you'd like me to proceed?

OK, I've got something that maybe looks better. You can tell me what
you think [1]. I did manage to use PM Runtime to avoid some of the
complexity and I put that usage in simple-panel. We'll see if I get
yelled at for adding more to simple-panel. ;-P

[1] https://lore.kernel.org/dri-devel/20210330025345.3980086-1-dianders@chromium.org/ (local)