linux-kernel - Re: [RFC PATCH v2 3/9] drm/panel: nv3052c: Sleep for 150ms after reset

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Message-ID: <4d2079d66249a7052acded0abf30169a4e95d151.camel@crapouillou.net>
Date:   Mon, 18 Sep 2023 23:01:15 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     John Watts <contact@...kia.org>,
        Jessica Zhang <quic_jesszhan@...cinc.com>
Cc:     dri-devel@...ts.freedesktop.org,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Chris Morgan <macromorgan@...mail.com>,
        Jagan Teki <jagan@...eble.ai>,
        Christophe Branchereau <cbranchereau@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 3/9] drm/panel: nv3052c: Sleep for 150ms after
 reset

Hi John,

Le mardi 19 septembre 2023 à 06:52 +1000, John Watts a écrit :
> On Mon, Sep 18, 2023 at 01:19:03PM -0700, Jessica Zhang wrote:
> > Hi John,
> > 
> > Just wondering, is there some context to this change? I.e., was
> > this made to
> > fix a specific issue?
> > 
> > This seems like a pretty significant increase in wait time so, if
> > it's not a
> > fix, I'm not sure if this would be an improvement on the current
> > behavior.
> > 
> > Thanks,
> > 
> > Jessica Zhang
> 
> Hi Jessica,
> 
> Thank you for the feedback.
> 
> This patch here is required by the data sheet if the screen was
> already running
> and was reset. This is necessary if for example the bootloader set up
> and had
> the screen running. However I have not tested this, it's possible the
> specific
> panels have shorter tolerances for resets. This is purely
> precautionary at
> this stage based on what the data sheet says.
> 
> That said I will be investigating this specific use case with this
> panel over
> the next few months. I am okay separating out this patch until I have
> proof it's
> needed for my particular display. I don't know anything about the ltk
> display.
> 
> The second sleep patch can probably be omitted as I don't think the
> panel being
> prepared then unprepared in rapid succession is a realistic
> situation, but I 
> figured I might as well propose it to see if it's the right thing to
> do.
> 
> Thanks for your time and review,
> John.

The datasheet does say a 5ms sleep time is necesary after a reset. I
assume the 120ms delay you quote is when a *software* reset is
performed in Sleep-out mode. The code here does a hard-reset.

Cheers,
-Paul