Re: i.MX8MP: Fix HDMI LCDIF FIFO underruns

[Maxime Ripard]

On Fri, Mar 20, 2026 at 09:04:17AM +0100, Marco Felsch wrote:
> 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@xxxxxxx>
> > > 
> > > --- 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.

If this is related to the output resolution, the DT is the last place
you should deal with this.

Maxime

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