Re: [PATCH 3/3] drm: lcdif: Wait for vblank before disabling DMA

[Lucas Stach]

Am Dienstag, dem 31.03.2026 um 00:46 +0200 schrieb Paul Kocialkowski:
> It is necessary to wait for the full frame to finish streaming
> through the DMA engine before we can safely disable it by removing
> the DISP_PARA_DISP_ON bit. Disabling it in-flight can leave the
> hardware confused and unable to resume streaming for the next frame.
> 
> This causes the FIFO underrun and empty status bits to be set and
> a single solid color to be shown on the display, coming from one of
> the pixels of the previous frame. The issue occurs sporadically when
> a new mode is set, which triggers the crtc disable and enable paths.
> 
> Setting the shadow load bit and waiting for it to be cleared by the
> DMA engine allows waiting for completion.
> 
> The NXP BSP driver addresses this issue with a hardcoded 25 ms sleep.
> 
> Fixes: 9db35bb349a0 ("drm: lcdif: Add support for i.MX8MP LCDIF variant")
> Signed-off-by: Paul Kocialkowski <paulk@xxxxxxxxxxx>
> Co-developed-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/mxsfb/lcdif_kms.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mxsfb/lcdif_kms.c b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> index 1aac354041c7..7dce7f48d938 100644
> --- a/drivers/gpu/drm/mxsfb/lcdif_kms.c
> +++ b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> @@ -393,6 +393,22 @@ static void lcdif_disable_controller(struct lcdif_drm_private *lcdif)
>  	if (ret)
>  		drm_err(lcdif->drm, "Failed to disable controller!\n");
>  
You can drop this no-op poll above...

> +	/*
> +	 * It is necessary to wait for the full frame to finish streaming
> +	 * through the DMA engine before we can safely disable it by removing
> +	 * the DISP_PARA_DISP_ON bit. Disabling it in-flight can leave the
> +	 * hardware confused and unable to resume streaming for the next frame.
> +	 */
> +	reg = readl(lcdif->base + LCDC_V8_CTRLDESCL0_5);
> +	reg |= CTRLDESCL0_5_SHADOW_LOAD_EN;
> +	writel(reg, lcdif->base + LCDC_V8_CTRLDESCL0_5);
> +
.. then setting the shadow load enable bit can be merged with the
access clearing the DMA enable bit.

> +	ret = readl_poll_timeout(lcdif->base + LCDC_V8_CTRLDESCL0_5,
> +				 reg, !(reg & CTRLDESCL0_5_SHADOW_LOAD_EN),
> +				 0, 36000);	/* Wait ~2 frame times max */

I know this is just a copy from the existing poll, but I don't think
the busy looping makes a lot of sense. I guess relaxing the poll by a
100us or even 200us wait between checks wouldn't hurt.

> +	if (ret)
> +		drm_err(lcdif->drm, "Failed to disable controller!\n");
> +
>  	reg = readl(lcdif->base + LCDC_V8_DISP_PARA);
>  	reg &= ~DISP_PARA_DISP_ON;
>  	writel(reg, lcdif->base + LCDC_V8_DISP_PARA);