linux-kernel - Re: [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

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Date: Mon, 22 Apr 2024 08:04:25 -0500
From: Adam Ford <aford173@...il.com>
To: Marek Vasut <marex@...x.de>
Cc: dri-devel@...ts.freedesktop.org, aford@...conembedded.com, 
	Frieder Schrempf <frieder.schrempf@...tron.de>, Inki Dae <inki.dae@...sung.com>, 
	Jagan Teki <jagan@...rulasolutions.com>, Marek Szyprowski <m.szyprowski@...sung.com>, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Marco Felsch <m.felsch@...gutronix.de>, Michael Tretter <m.tretter@...gutronix.de>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

On Mon, Apr 22, 2024 at 8:01 AM Marek Vasut <marex@...x.de> wrote:
>
> On 4/22/24 2:09 PM, Adam Ford wrote:
> > On Sun, Apr 21, 2024 at 9:36 AM Marek Vasut <marex@...x.de> wrote:
> >>
> >> On 2/12/24 12:09 AM, Adam Ford wrote:
> >>> When using video sync pulses, the HFP, HBP, and HSA are divided between
> >>> the available lanes if there is more than one lane.  For certain
> >>> timings and lane configurations, the HFP may not be evenly divisible.
> >>> If the HFP is rounded down, it ends up being too small which can cause
> >>> some monitors to not sync properly. In these instances, adjust htotal
> >>> and hsync to round the HFP up, and recalculate the htotal.
> >>>
> >>> Tested-by: Frieder Schrempf <frieder.schrempf@...tron.de> # Kontron BL i.MX8MM with HDMI monitor
> >>> Signed-off-by: Adam Ford <aford173@...il.com>
> >>> ---
> >>> V2:  No changes
> >>>
> >>> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> >>> index 8476650c477c..52939211fe93 100644
> >>> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> >>> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> >>> @@ -1606,6 +1606,27 @@ static int samsung_dsim_atomic_check(struct drm_bridge *bridge,
> >>>                adjusted_mode->flags |= (DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_PVSYNC);
> >>>        }
> >>>
> >>> +     /*
> >>> +      * When using video sync pulses, the HFP, HBP, and HSA are divided between
> >>> +      * the available lanes if there is more than one lane.  For certain
> >>> +      * timings and lane configurations, the HFP may not be evenly divisible.
> >>> +      * If the HFP is rounded down, it ends up being too small which can cause
> >>> +      * some monitors to not sync properly. In these instances, adjust htotal
> >>> +      * and hsync to round the HFP up, and recalculate the htotal. Through trial
> >>> +      * and error, it appears that the HBP and HSA do not appearto need the same
> >>> +      * correction that HFP does.
> >>> +      */
> >>> +     if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE && dsi->lanes > 1) {
> >>
> >> Does this also apply to mode with sync events (I suspect it does), so
> >> the condition here should likely be if (!...burst mode) , right ?
> >
> > Thanks for the review!
> >
> > I was only able to test it with the DSI->ADV6535 bridge, and I'll
> > admit I don't know a lot about DSI interface since I don't have a copy
> > of the spec to read.
> >
> > Are you proposing this should be:
> >
> >   if (!(dsi->mode_flags & MIPI_DSI_MODE_VIDEO_BURST) && dsi->lanes > 1) {
> >
> > I just want to make sure I understand what you're requesting.
>
> Yes, exactly this.

Do you think it should also include checks for
MIPI_DSI_MODE_VIDEO_NO_HFP, MIPI_DSI_MODE_VIDEO_NO_HBP or
MIPI_DSI_MODE_VIDEO_NO_HSA?

It seems like if any of these are set, we should skip this rounding stuff.

adam