Re: [PATCH v2 4/6] drm/bridge: ti-sn65dsi86: Remove the mystery delay

From: Andrzej Hajda
Date: Mon Oct 29 2018 - 05:03:32 EST

On 26.10.2018 00:21, Douglas Anderson wrote:
> Let's solve the mystery of commit bf1178c98930 ("drm/bridge:
> ti-sn65dsi86: Add mystery delay to enable()"). Specifically the
> reason we needed that mystery delay is that we weren't paying
> attention to HPD.
> Looking at the datasheet for the same panel that was tested for the
> original commit, I see there's a timing "t3" that times from power on
> to the aux channel being operational. This time is specced as 0 - 200
> ms. The datasheet says that the aux channel is operational at exactly
> the same time that HPD is asserted.
> Scoping the signals on this board showed that HPD was asserted 84 ms
> after power was asserted. That very closely matches the magic 70 ms
> delay that we had. ...and actually, in my testing the 70 ms wasn't
> quite enough of a delay and some percentage of the time the display
> didn't come up until I bumped it to 100 ms (presumably 84 ms would
> have worked too).
> To solve this, we tried to hook up the HPD signal in the bridge.
> ...but in doing so we found that that the bridge didn't report that
> HPD was asserted until ~280 ms after we powered it (!). This is
> explained by looking at the sn65dsi86 datasheet section " HPD
> (Hot Plug/Unplug Detection)". Reading there we see that the bridge
> isn't even intended to report HPD until 100 ms after it's asserted.
> ...but that would have left us at 184 ms. The extra 100 ms
> (presumably) comes from this part in the datasheet:
>> The HPD state machine operates off an internal ring oscillator. The
>> ring oscillator frequency will vary [ ... ]. The min/max range in
>> the HPD State Diagram refers to the possible times based off
>> variation in the ring oscillator frequency.
> Given that the 280 ms we'll end up delaying if we hook up HPD is
> _slower_ than the 200 ms we could just hardcode, for now we'll solve
> the problem by just hardcoding a 200 ms delay in the panel driver
> using the patch in this series ("drm/panel: simple: Support panels
> with HPD where HPD isn't connected").
> If we later find a panel that needs to use this bridge where we need
> HPD then we'll have to come up with some new code to handle it. Given
> the silly debouncing in the bridge chip, though, it seems unlikely.
> One last note is that I tried to solve this through another way: In
> ti_sn_bridge_enable() I tried to use various combinations of
> dp_dpcd_writeb() and dp_dpcd_readb() to detect when the aux channel
> was up. In theory that would let me detect _exactly_ when I could
> continue and do link training. Unfortunately even if I did an aux
> transfer w/out waiting I couldn't see any errors. Possibly I could
> keep looping over link training until it came back with success, but
> that seemed a little overly hacky to me.
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Reviewed-by: Sean Paul <sean@xxxxxxxxxx>
Reviewed-by: Andrzej Hajda <a.hajda@xxxxxxxxxxx>