Re: [PATCH 5/5] drm/stm: ltdc: add support of ycbcr pixel formats

Hi Yannick,

On Wed, Dec 15, 2021 at 10:48:43PM +0100, Yannick Fertre wrote:

This patch adds the following YCbCr input pixel formats on the latest
LTDC hardware version:

1 plane  (co-planar)  : YUYV, YVYU, UYVY, VYUY
2 planes (semi-planar): NV12, NV21
3 planes (full-planar): YU12=I420=DRM YUV420, YV12=DRM YVU420

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>

<snip>

+static inline void ltdc_set_ycbcr_config(struct drm_plane *plane, u32 drm_pix_fmt)
+{
+	struct ltdc_device *ldev = plane_to_ltdc(plane);
+	struct drm_plane_state *state = plane->state;
+	u32 lofs = plane->index * LAY_OFS;
+	u32 val;
+
+	switch (drm_pix_fmt) {
+	case DRM_FORMAT_YUYV:
+		val = (YCM_I << 4) | LxPCR_YF | LxPCR_CBF;
+		break;
+	case DRM_FORMAT_YVYU:
+		val = (YCM_I << 4) | LxPCR_YF;
+		break;
+	case DRM_FORMAT_UYVY:
+		val = (YCM_I << 4) | LxPCR_CBF;
+		break;
+	case DRM_FORMAT_VYUY:
+		val = (YCM_I << 4);
+		break;
+	case DRM_FORMAT_NV12:
+		val = (YCM_SP << 4) | LxPCR_CBF;
+		break;
+	case DRM_FORMAT_NV21:
+		val = (YCM_SP << 4);
+		break;
+	case DRM_FORMAT_YUV420:
+	case DRM_FORMAT_YVU420:
+		val = (YCM_FP << 4);
+		break;
+	default:
+		/* RGB or not a YCbCr supported format */
+		break;
+	}
+
+	/* Enable limited range */
+	if (state->color_range == DRM_COLOR_YCBCR_LIMITED_RANGE)
+		val |= LxPCR_YREN;
+
+	/* enable ycbcr conversion */
+	val |= LxPCR_YCEN;
+
+	regmap_write(ldev->regmap, LTDC_L1PCR + lofs, val);
+}

This patch as commit 484e72d3146b ("drm/stm: ltdc: add support of ycbcr
pixel formats") in -next introduced the following clang warning:

drivers/gpu/drm/stm/ltdc.c:625:2: warning: variable 'val' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
        default:
        ^~~~~~~
drivers/gpu/drm/stm/ltdc.c:635:2: note: uninitialized use occurs here
        val |= LxPCR_YCEN;
        ^~~
drivers/gpu/drm/stm/ltdc.c:600:9: note: initialize the variable 'val' to silence this warning
        u32 val;
               ^
                = 0
1 warning generated.

Would it be okay to just return in the default case (maybe with a
message about an unsupported format?) or should there be another fix?

Cheers,

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