Re: i.MX8MP: Fix HDMI LCDIF FIFO underruns

Marco Felsch
2026-03-20T08:04:31Z

On 26-03-20, Marek Vasut wrote:
> On 3/19/26 12:45 PM, Krzysztof Hałasa wrote:
> > Running (and terminating) Weston on i.MX8MP sometimes produced black
> > or grey screens. The LCDIF3 INT_STATUS_D0 register showed a FIFO underrun:
> >
> > 32FC6024: 1000002
> >
> > The PANIC0_THRES register was set to:
> >
> > 32FC6238: AA0154
> >
> > which apparently meant low = 2736 bytes (0xAA * 16 + 16), high = 5456 bytes
> > (0x154 * 16 + 16).
> >
> > With this patch (based on NXP's driver) the watermarks are only 16 bytes
> > higher (register values increased by 1) and the underruns are gone.
> >
> > Tested on Solidrun Hummingboard Mate with a 1080p60 HDMI display.
> >
> > Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
> >
> > --- a/drivers/gpu/drm/mxsfb/lcdif_kms.c
> > +++ b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> > @@ -353,8 +353,8 @@ static void lcdif_enable_controller(struct lcdif_drm_private *lcdif)
> > u32 reg;
> > /* Set FIFO Panic watermarks, low 1/3, high 2/3 . */
> > - writel(FIELD_PREP(PANIC0_THRES_LOW_MASK, 1 * PANIC0_THRES_MAX / 3) |
> > - FIELD_PREP(PANIC0_THRES_HIGH_MASK, 2 * PANIC0_THRES_MAX / 3),
> > + writel(FIELD_PREP(PANIC0_THRES_LOW_MASK, DIV_ROUND_UP(1 * PANIC0_THRES_MAX, 3)) |
> > + FIELD_PREP(PANIC0_THRES_HIGH_MASK, DIV_ROUND_UP(2 * PANIC0_THRES_MAX, 3)),
>
> I am starting to wonder, whether this should be configurable in DT, because
> it seems there might be users that need to tweak the watermarks one way or
> the other for different video outputs ?

+1 to provide a override mechanism via DT.

Regards,
Marco

>
> +CC Liu
>
>

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