[PATCH] drm: rcar-du: Add setting to PnALPHAR register on Gen3

[Biju Das]

Hi Laurent,

Thanks for the feedback.

> Subject: Re: [PATCH] drm: rcar-du: Add setting to PnALPHAR register on
> Gen3
> 
> Hi Biju,
> 
> Thank you for the patch.
> 
> On Sat, Apr 23, 2022 at 08:37:28AM +0100, Biju Das wrote:
> > From: LUU HOAI <hoai.luu.ub at renesas.com>
> >
> > In Gen3, when Alpha blend is enabled in the PnMR register, depending
> > on the initial value of the PnALPHAR register, either channel of DU
> > might be black screen.
> > Therefore, this patch prevents the black screen by setting the
> > PnALPHAR register to all 0.
> >
> > In addition, PnALPHAR register will be released in the R-Car Gen3
> > Hardware Manual Rev 2.4 (Sep. 2021).
> >
> > Signed-off-by: LUU HOAI <hoai.luu.ub at renesas.com>
> > Signed-off-by: Biju Das <biju.das.jz at bp.renesas.com>
> > ---
> > This patch is based on [1]
> > [1]
> >
> > Not sure this patches has to go with Fixes tag for stable??
> >
> > Tested the changes on RZ/G2M board
> >
> > root at hihope-rzg2m:/cip-test-scripts#  modetest -M rcar-du -w
> > 54:alpha:55555 root at hihope-rzg2m:/cip-test-scripts# modetest -M rcar-du
> -s "93 at 90:1024x768 at AR24" -d -P "54 at 90:400x300+200+200 at XR24"
> > setting mode 1024x768-75Hz at AR24 on connectors 93, crtc 90 testing
> > 400x300 at XR24 overlay plane 54
> > ---
> >  drivers/gpu/drm/rcar-du/rcar_du_plane.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/rcar-du/rcar_du_plane.c
> > b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
> > index 5c1c7bb04f3f..aff39b9253f8 100644
> > --- a/drivers/gpu/drm/rcar-du/rcar_du_plane.c
> > +++ b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
> > @@ -510,6 +510,12 @@ static void
> > rcar_du_plane_setup_format_gen3(struct rcar_du_group *rgrp,
> >
> >  	rcar_du_plane_write(rgrp, index, PnDDCR4,
> >  			    state->format->edf | PnDDCR4_CODE);
> > +
> > +	/* In Gen3, PnALPHAR register need to be set to 0
> > +	 * to avoid black screen issue when alpha blend is enable
> > +	 * on DU module
> > +	 */
> 
> Comments should start with /* on a line of its own, and you can also
> reflow the text to 80 columns:

OK.

> 
> 	/*
> 	 * In Gen3, PnALPHAR register need to be set to 0 to avoid black
> screen
> 	 * issue when alpha blend is enable on DU module.
> 	 */
> 
> It would however be nicer to document the exact behaviour, but the latest
> version of the documentation I have access to is rev 2.3 and it lists
> PnALPHAR as not available on Gen3.

I don't have access to rev 2.4, but I got access to 
"R-Car-Gen3_Common_OPC_Customer_Notifications_V30.1.pdf" 
where it is mentioned about this issue and solution for fix
which is inline with the patch from R-Car BSP.

"The reason is that a register is not initialized by reset.
This could lead to output wrong image data of other plane or 
wrong color set from BPOR (Background plane output register)."

> 
> Furthermore, is this really the right fix, shouldn't we instead avoid
> enabling alpha-blending in PnMR on Gen3 ?

Avoid enabling alpha-blending in PnMR on Gen3, will it help here?

Here the issue they mentioned as "register is not initialized by reset"

Cheers,
Biju

> 
> > +	rcar_du_plane_write(rgrp, index, PnALPHAR, 0x00000000);
> >  }
> >
> >  static void rcar_du_plane_setup_format(struct rcar_du_group *rgrp,
> 
> --
> Regards,
> 
> Laurent Pinchart