Hi, On Fri, Mar 25, 2022 at 8:54 AM Sankeerth Billakanti (QUIC) <[email protected]> wrote: > > > -----Original Message----- > > From: Doug Anderson <[email protected]> > > Sent: Saturday, March 19, 2022 5:26 AM > > To: Stephen Boyd <[email protected]> > > Cc: Sankeerth Billakanti (QUIC) <[email protected]>; open list:OPEN > > FIRMWARE AND FLATTENED DEVICE TREE BINDINGS > > <[email protected]>; dri-devel <[email protected]>; > > freedreno <[email protected]>; linux-arm-msm <linux-arm- > > [email protected]>; LKML <[email protected]>; Rob Clark > > <[email protected]>; Sean Paul <[email protected]>; > > quic_kalyant <[email protected]>; Abhinav Kumar (QUIC) > > <[email protected]>; Kuogee Hsieh (QUIC) > > <[email protected]>; Andy Gross <[email protected]>; > > [email protected]; Rob Herring <[email protected]>; > > [email protected]; Sean Paul <[email protected]>; David Airlie > > <[email protected]>; Daniel Vetter <[email protected]>; Thierry Reding > > <[email protected]>; Sam Ravnborg <[email protected]>; > > [email protected]; quic_vproddut <[email protected]> > > Subject: Re: [PATCH v5 6/9] drm/msm/dp: wait for hpd high before any sink > > interaction > > > > Hi, > > > > On Fri, Mar 18, 2022 at 4:27 PM Stephen Boyd <[email protected]> > > wrote: > > > > > > > > Pushing hpd state checking into aux transactions looks like the > > > > > wrong direction. Also, as I said up above I am concerned that even > > > > > checking the GPIO won't work and we need some way to ask the > > > > > bridge if HPD is asserted or not and then fallback to the GPIO > > > > > method if the display phy/controller doesn't have support to check > > > > > HPD internally. Something on top of DRM_BRIDGE_OP_HPD? > > > > > > > > If we could somehow get the HPD status from the bridge in the panel > > > > driver it definitely would be convenient. It does feel like that's > > > > an improvement that could be done later, though. We've already > > > > landed a few instances of doing what's done here, like for > > > > parade-ps8640 and analogix_dp. I suspect designing a new mechanism > > > > might not be the most trivial. > > > > > > What is done in the bridge drivers is to wait for a fixed timeout and > > > assume aux is ready? Or is it something else? If there's just a fixed > > > timeout for the eDP case it sounds OK to do that for now and we can > > > fine tune it later to actually check HPD status register before the > > > panel tries to read EDID. > > > > Right. For the parade chip (which is only used for eDP as far as I > > know--never > > DP) waits for up to 200 ms. See ps8640_ensure_hpd(). > > > > So I guess tl;dr to Sankeerth that it's OK for his patch to have the wait > > in the > > aux transfer function, but only for eDP. Other discussions here are about > > how we could make it better in future patches. > > > > > > The aux transactions for external DP are initiated by the dp_display driver > only after the > display is hot plugged to the connector. The phy_init is necessary for the > aux transactions > to take place. So, for the DP case, like Doug mentioned below, this patch is > introducing > an overhead of three register reads to detect hpd_high before performing aux > transactions. > So, we felt this was okay to do for DP.
Personally I'm not that upset about the 3 register reads. The problem Stephen pointed out is bigger. It's possible that a DP cable is unplugged _just_ as we started an AUX transaction. In that case we'll have a big delay here when we don't actually need one. > On the other hand, for eDP, it is necessary to wait for panel ready through > this hpd connect status. > Currently there is no way to know which type of connector it is in the dp_aux > sub-module. > > However, as the discussion suggested, to have the wait only for eDP, I am > thinking to pass the > connector_type information to aux sub-module and register different > aux_transfer functions > for eDP and DP. The eDP transfer function will wait for hpd_high and the DP > transfer function > will be same as the one before this patch. Personally I wouldn't register two separate functions. You could just store a boolean in your structure and only wait for HPD if this is eDP. One extra "if" test doesn't seem like it justifies splitting off into two functions... -Doug
