Re: [PATCH v2 2/2] drm: lcdif: Wait for vblank before disabling DMA

Lucas Stach
2026-04-07T08:28:39Z

Am Donnerstag, dem 02.04.2026 um 20:33 +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@sys-base.io>
> Co-developed-by: Lucas Stach <l.stach@pengutronix.de>

Looks good to me. Since the co-developed tag requires a sign-off from
me, I'll add that instead of a review tag. And you might want to move
your sign-off down, as you are the one submitting the patch.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

> ---
> drivers/gpu/drm/mxsfb/lcdif_kms.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/mxsfb/lcdif_kms.c b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> index a00c4f6d63f4..0d04a0028671 100644
> --- a/drivers/gpu/drm/mxsfb/lcdif_kms.c
> +++ b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> @@ -375,14 +375,23 @@ static void lcdif_disable_controller(struct lcdif_drm_private *lcdif)
> int ret;
>
> reg = readl(lcdif->base + LCDC_V8_CTRLDESCL0_5);
> + /* Disable the layer for DMA. */
> reg &= ~CTRLDESCL0_5_EN;
> + /*
> + * 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 |= CTRLDESCL0_5_SHADOW_LOAD_EN;
> writel(reg, lcdif->base + LCDC_V8_CTRLDESCL0_5);
>
> + /* Wait for the frame to finish or timeout after 50 ms. */
> ret = readl_poll_timeout(lcdif->base + LCDC_V8_CTRLDESCL0_5,
> - reg, !(reg & CTRLDESCL0_5_EN),
> - 0, 36000); /* Wait ~2 frame times max */
> + reg, !(reg & CTRLDESCL0_5_SHADOW_LOAD_EN),
> + 200, 50000);
> if (ret)
> - drm_err(lcdif->drm, "Failed to disable controller!\n");
> + drm_err(lcdif->drm, "Timed out waiting for final vblank!\n");
>
> reg = readl(lcdif->base + LCDC_V8_DISP_PARA);
> reg &= ~DISP_PARA_DISP_ON;