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Date:   Wed, 2 Feb 2022 09:54:55 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Yannick Fertre <yannick.fertre@foss.st.com>
Cc:     Philippe Cornu <philippe.cornu@foss.st.com>,
        Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>,
        dri-devel@lists.freedesktop.org,
        linux-stm32@st-md-mailman.stormreply.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 5/5] drm/stm: ltdc: add support of ycbcr pixel formats
Message-ID: <Yfq3XwozrxYaFhgD@dev-arch.archlinux-ax161>
References: <20211215214843.20703-1-yannick.fertre@foss.st.com>
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In-Reply-To: <20211215214843.20703-1-yannick.fertre@foss.st.com>
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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,