Re: [PATCH v2 1/1] gpu/drm: bridge: tc358768: Add delay after PLL setup

[Svyatoslav Ryhel]

ср, 19 серп. 2026 р. о 12:53 Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx> пише:
>
> Hello Svyatoslav,
>
> On Thu Jun 25, 2026 at 10:25 AM CEST, Svyatoslav Ryhel wrote:
> > After tc358768_setup_pll() enables PLL_CKEN and the lock indicator comes
> > up, the DSI register sequence runs near immediately. On TF700T this
>
> Do you refer to the Asus Transformer Pad TF700T here? Please add the full
> name of the device.
>

Yes, I am referring to Asus Transformer Pad TF700T. Sure, my bad.


> > results in the bridge claiming PLL lock but producing no DSI video output:
> > the panel powers up, the backlight comes on, but the framebuffer is never
> > scanned out.
> >
> > Insert a small settling delay between PLL setup and the rest of the
> > attach.
> >
> > Suggested-by: Michele Brocco <brocco@xxxxxxxxxxxx>
> > Tested-by: Michele Brocco <brocco@xxxxxxxxxxxx> # TF700T
> > Signed-off-by: Svyatoslav Ryhel <clamor95@xxxxxxxxx>
> > ---
> >  drivers/gpu/drm/bridge/tc358768.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
> > index 0f2820b37302..ea358c8da0e9 100644
> > --- a/drivers/gpu/drm/bridge/tc358768.c
> > +++ b/drivers/gpu/drm/bridge/tc358768.c
> > @@ -824,6 +824,9 @@ static void tc358768_bridge_atomic_pre_enable(struct drm_bridge *bridge,
> >               return;
> >       }
> >
> > +     /* Allow the PLL to settle */
> > +     mdelay(20);
>
> Why 20 ms, and not another amount? Is it backed by some datasheet? Or just
> "it's what fixed my usecase"?
>
> The extra delay is a panel requirement maybe, and definitely specific to
> the specific hardware pipeline of the TF700T. Would it make sense to add a
> DT property so only hardware needing a delay can add it, and add as much as
> needed?
>

This is a fair point. Thank you Luca. I will experiment a bit more to
see how this can be arranged differently.

> Luca
>
> --
> Luca Ceresoli, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com