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[104.188.17.28]) by smtp.gmail.com with ESMTPSA id t73sm5897854pfc.78.2020.07.16.16.39.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Jul 2020 16:39:14 -0700 (PDT) Date: Thu, 16 Jul 2020 16:39:12 -0700 From: Bjorn Andersson To: Douglas Anderson Cc: Thierry Reding , Sam Ravnborg , robdclark@chromium.org, steev@kali.org, linux-arm-msm@vger.kernel.org, Daniel Vetter , David Airlie , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] drm: panel: simple: Delay HPD checking on boe_nv133fhm_n61 for 15 ms Message-ID: <20200716233912.GA61202@yoga> References: <20200716132120.1.I01e738cd469b61fc9b28b3ef1c6541a4f48b11bf@changeid> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200716132120.1.I01e738cd469b61fc9b28b3ef1c6541a4f48b11bf@changeid> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 16 Jul 13:21 PDT 2020, Douglas Anderson wrote: > On boe_nv133fhm_n62 (and presumably on boe_nv133fhm_n61) a scope shows > a small spike on the HPD line right when you power the panel on. The > picture looks something like this: > > +-------------------------------------- > | > | > | > Power ---+ > +--- > | > ++ | > +----+| | > HPD -----+ +---------------------------+ > > So right when power is applied there's a little bump in HPD and then > there's small spike right before it goes low. The total time of the > little bump plus the spike was measured on one panel as being 8 ms > long. The total time for the HPD to go high on the same panel was > 51.2 ms, though the datasheet only promises it is < 200 ms. > > When asked about this glitch, BOE indicated that it was expected and > persisted until the TCON has been initialized. > > If this was a real hotpluggable DP panel then this wouldn't matter a > whole lot. We'd debounce the HPD signal for a really long time and so > the little blip wouldn't hurt. However, this is not a hotpluggable DP > panel and the the debouncing logic isn't needed and just shows down > the time needed to get the display working. This is why the code in > panel_simple_prepare() doesn't do debouncing and just waits for HPD to > go high once. Unfortunately if we get unlucky and happen to poll the > HPD line right at the spike we can try talking to the panel before > it's ready. > > Let's handle this situation by putting in a 15 ms prepare delay and > decreasing the "hpd absent delay" by 15 ms. That means: > * If you don't have HPD hooked up at all you've still got the > hardcoded 200 ms delay. > * If you've got HPD hooked up you will always wait at least 15 ms > before checking HPD. The only case where this could be bad is if > the panel is sharing a voltage rail with something else in the > system and was already turned on long before the panel came up. In > such a case we'll be delaying 15 ms for no reason, but it's not a > huge delay and I don't see any other good solution to handle that > case. > > Even though the delay was measured as 8 ms, 15 ms was chosen to give a > bit of margin. > Reviewed-by: Bjorn Andersson > Signed-off-by: Douglas Anderson > --- > I don't actually have a device in front of me that is exhibiting these > problems. I believe that it is only some devices and some of the > time. Still, this patch seems safe and seems likely to fix the issue > given the scope shots. > > drivers/gpu/drm/panel/panel-simple.c | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c > index 88493538a147..046a06b55800 100644 > --- a/drivers/gpu/drm/panel/panel-simple.c > +++ b/drivers/gpu/drm/panel/panel-simple.c > @@ -1260,7 +1260,21 @@ static const struct panel_desc boe_nv133fhm_n61 = { > .height = 165, > }, > .delay = { > - .hpd_absent_delay = 200, > + /* > + * When power is first given to the panel there's a short > + * spike on the HPD line. It was explained that this spike > + * was until the TCON data download was complete. On > + * one system this was measured at 8 ms. We'll put 15 ms > + * in the prepare delay just to be safe and take it away > + * from the hpd_absent_delay (which would otherwise be 200 ms) > + * to handle this. That means: > + * - If HPD isn't hooked up you still have 200 ms delay. > + * - If HPD is hooked up we won't try to look at it for the > + * first 15 ms. > + */ > + .prepare = 15, > + .hpd_absent_delay = 185, > + > .unprepare = 500, > }, > .bus_format = MEDIA_BUS_FMT_RGB888_1X24, > -- > 2.28.0.rc0.105.gf9edc3c819-goog >