linux-kernel - Re: [PATCH V7 2/2] drm/panel: Add Sitronix ST7701 panel driver

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Date:   Sat, 12 Jan 2019 13:00:56 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     David Airlie <airlied@...ux.ie>,
        linux-amarula@...rulasolutions.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Michael Trimarchi <michael@...rulasolutions.com>,
        Sean Paul <sean@...rly.run>
Subject: Re: [PATCH V7 2/2] drm/panel: Add Sitronix ST7701 panel driver

> >
> > > > > +
> > > > > +     msleep(st7701->sleep_delay);
> > > > > +
> > > > > +     gpiod_set_value(st7701->reset, 0);
> > > > > +
> > > > > +     gpiod_set_value(st7701->reset, 1);
> > > > > +
> > > > > +     gpiod_set_value(st7701->reset, 0);
> > > > No timing constrains here? In prepare there are sleeps intermixed.
> > >
> > > Delay while doing unprare is not needed I suppose.
> >
> > If the purpose is alone to reset the display then a single write '0'
> > should do it I think
> 
> I even tried this just set 0, since prepare is doing a sequence, it
> good behavior to do the reverse during handoff. ie reason I just
> initiated this sequence.
> 
> > And there is a requirement that it must be low for a minimum of 10 us
> > which would be good to have here.
> 
> Sorry, I didn't get this requirement what is this for?
The st7701 will ignore reset pulse shorter than 10 us - see Trw in
table 9 reset timing (page 54).

But as we just assert reset (set it to 0), this timing constraint can be ignored.

> > I aslo found in chapter 9. (page 163 - second line) this statement:
> > "VDDA and VDDI must be powered down with minimum 120msec."
> 
> Yes unprepare is doing the same, it exit from sleep out mode and wait
> for 120msec and do the reset.
> 
> >
> > This is similar to the unprepare delay to be found in simple-panel.c
> > So an unprepare delay seems in order here.
> 
> Look like simple-panel.c is doing delay after reset initiated and
> regulator disabled.


	Sam