Re: [PATCH v2 11/14] drm/bridge: ti-sn65dsi86: Power things properly for reading the EDID

Andrzej Hajda
2021-04-01T11:12:55Z

W dniu 31.03.2021 o 16:48, Doug Anderson pisze:
> Hi,
>
> On Wed, Mar 31, 2021 at 4:08 AM Andrzej Hajda <a.hajda@samsung.com> wrote:
>>
>> W dniu 30.03.2021 o 04:53, Douglas Anderson pisze:
>>> eDP panels won't provide their EDID unless they're powered on. Let's
>>> chain a power-on before we read the EDID. This roughly matches what
>>> was done in 'parade-ps8640.c'.
>>>
>>> NOTE: The old code attempted to call pm_runtime_get_sync() before
>>> reading the EDID. While that was enough to power the bridge chip on,
>>> it wasn't enough to talk to the panel for two reasons:
>>> 1. Since we never ran the bridge chip's pre-enable then we never set
>>> the bit to ignore HPD. This meant the bridge chip didn't even _try_
>>> to go out on the bus and communicate with the panel.
>>> 2. Even if we fixed things to ignore HPD, the EDID still wouldn't read
>>> if the panel wasn't on.
>>>
>>> One thing that's a bit odd here is taking advantage of the EDID that
>>> the core might have cached for us. See the patch ("drm/edid: Use the
>>> cached EDID in drm_get_edid() if eDP"). We manage to get at the cache
>>> by:
>>> - Instantly failing aux transfers if we're not powered.
>>> - If the first read of the EDID fails we try again after powering.
>>>
>>> Fixes: 58074b08c04a ("drm/bridge: ti-sn65dsi86: Read EDID blob over DDC")
>>> Signed-off-by: Douglas Anderson <dianders@chromium.org>
>>> ---
>>> Depending on what people think of the other patches in this series,
>>> some of this could change.
>>> - If everyone loves the "runtime PM" in the panel driver then we
>>> could, in theory, put the pre-enable chaining straight in the "aux
>>> transfer" function.
>>> - If everyone hates the EDID cache moving to the core then we can
>>> avoid some of the awkward flow of things and keep the EDID cache in
>>> the sn65dsi86 driver.
>>
>> I wonder if this shouldn't be solved in the core - ie caller of
>> get_modes callback should be responsible for powering up the pipeline,
>> otherwise we need to repeat this stuff in every bridge/panel driver.
>>
>> Any thoughts?
> Yeah, I did look at this a little bit. Presumably it would only make
> sense to do it for eDP connections since:
>
> a) The concept of reading an EDID doesn't make sense for things like MIPI.

I guess you mean MIPI DSI, and yes I agree, more generally it usually(!)
doesn't make sense for any setup with fixed display panel.

On the other hand there are DSI/HDMI or DSI/DP adapters which usually
have EDID reading logic.

And the concept makes sense for most connectors accepting external
displays: HDMI, DP, MHL, VGA...

>
> b) For something with an external connector (DP and HDMI) you don't
> even know they're inserted unless the EDID is ready to read (these
> devices are, essentially, always powered).

Usually there are two elements which are not the same:

1. HotPlug signal/wire.

2. EDID reading logic.

The logic responsible for reading EDID needs to be enabled only for time
required for EDID reading :) So it's power state often must be
controlled explicitly by the bridge driver. So even if in many cases
pre_enable powers on the logic for EDID reading it does not make it the
rule, so I must step back from my claim that it is up to caller :)

>
> So I started off trying to do this in the core for eDP, but then it
> wasn't completely clear how to write this code in a way that was super
> generic. Specifically:
>
> 1. I don't think it's a 100% guarantee that everything is powered on
> in pre-enable and powered off in post-disable. In this bridge chip
> it's true, but maybe not every eDP driver? Would you want me to just
> assume this, or add a flag?

Ok, pre_enable should power on the chip, but for performing
initialization of video transport layer. Assumption it will power on
EDID logic is incorrect, so my claim seems wrong, but also this patch
looks incorrect :)

In general only device containing EDID logic knows how to power it up.

Since I do not know your particular case I can propose few possible ways
to investigate:

- call bridge.next->get_modes - you leave responsibility for powering up
to the downstream device.

- ddc driver on i2c request should power up the panel - seems also correct,

Regards

Andrzej

>
> 2. It wasn't totally clear to me which state to use for telling if the
> bridge had already been pre-enabled so I could avoid double-calling
> it. I could dig more if need be but I spent a bit of time looking and
> was coming up empty. If you have advice I'd appreciate it, though.
>
> 3. It wasn't clear to me if I should be using the atomic version
> (drm_atomic_bridge_chain_pre_enable) if I put this in the core and how
> exactly to do this, though I am a self-admitted DRM noob. I can do
> more digging if need be. Again, advice is appreciated.
>
> 4. Since I got feedback that the EDID caching belongs in the driver,
> not in the core [1] then we might end up powering things up
> pointlessly since the core wouldn't know if the driver was going to
> return the cache or not.
>
> Given that this patch isn't too much code and not too complicated (and
> will be even less complicated if I move the EDID caching back into the
> driver), maybe we can land it and if we see the pattern repeat a bunch
> more times then think about moving it to the core?
>
>
> [1] https://lore.kernel.org/dri-devel/YGMvO3PNDCiBmqov@intel.com/
>
> -Doug
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