[PATCH v3 7/7] drm/bridge: ti-sn65dsi86: Add mystery delay to enable()

[Sean Paul]

On Tue, Aug 14, 2018 at 04:59:31PM +0530, spanda at codeaurora.org wrote:
> On 2018-08-14 03:00, Sean Paul wrote:
> > From: Sean Paul <seanpaul at chromium.org>
> > 
> > This patch adds a 70ms mystery delay to the bridge driver in enable.
> > By experimentation, it seems like it can go anywhere up until we
> > initiate semi-auto link training. If we don't have the delay, link
> > training fails.
> > 
> > I tried to root cause this as best I could, but neither the datasheet
> > for the panel nor the bridge mention a delay of this magnitude in their
> > timing requirements. So for now, add the mystery delay until someone
> > figures out a better fix.
> > 
> > Changes in v3:
> > - Added to the set
> > 
> > Cc: Sandeep Panda <spanda at codeaurora.org>
> > Signed-off-by: Sean Paul <seanpaul at chromium.org>
> > ---
> >  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > index d3e27e52ea759..f8a931cf3665e 100644
> > --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > @@ -458,6 +458,18 @@ static void ti_sn_bridge_enable(struct drm_bridge
> > *bridge)
> >  	unsigned int val;
> >  	int ret;
> > 
> > +	/*
> > +	 * FIXME:
> > +	 * This 70ms was found necessary by experimentation. If it's not
> > +	 * present, link training fails. It seems like it can go anywhere from
> > +	 * pre_enable() up to semi-auto link training initiation below.
> > +	 *
> > +	 * Neither the datasheet for the bridge nor the panel tested mention a
> > +	 * delay of this magnitude in the timing requirements. So for now, add
> > +	 * the mystery delay until someone figures out a better fix.
> > +	 */
> 
> Is this newly found issue? Specific to any board config?

It's specific to cheza. This was found with swboyd changed the panel regulator
from always-on/boot-on to toggle.

I've pushed the tag "mtp-testing-0813" to dpu-staging so you can play around
with it. Without this delay, link training fails.

Sean

> 
> > +	msleep(70);
> > +
> >  	/* DSI_A lane config */
> >  	val = CHA_DSI_LANES(4 - pdata->dsi->lanes);
> >  	regmap_update_bits(pdata->regmap, SN_DSI_LANES_REG,

-- 
Sean Paul, Software Engineer, Google / Chromium OS