linux-kernel - Re: [PATCH v5 2/6] drm: lcdif: Drop unnecessary NULL pointer check on lcdif->bridge

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Message-ID: <05b44685-d6a7-5f6e-0f55-04c96e94a9e1@denx.de>
Date:   Tue, 9 May 2023 02:02:39 +0200
From:   Marek Vasut <marex@...x.de>
To:     Liu Ying <victor.liu@....com>, dri-devel@...ts.freedesktop.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Cc:     stefan@...er.ch, airlied@...il.com, daniel@...ll.ch,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        conor+dt@...nel.org, shawnguo@...nel.org, s.hauer@...gutronix.de,
        kernel@...gutronix.de, festevam@...il.com, linux-imx@....com,
        krzysztof.kozlowski@...aro.org, LW@...o-electronics.de,
        alexander.stein@...tq-group.com
Subject: Re: [PATCH v5 2/6] drm: lcdif: Drop unnecessary NULL pointer check on
 lcdif->bridge

On 5/8/23 07:57, Liu Ying wrote:

Hi,

> diff --git a/drivers/gpu/drm/mxsfb/lcdif_kms.c b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> index 262bc43b1079..e54200a9fcb9 100644
> --- a/drivers/gpu/drm/mxsfb/lcdif_kms.c
> +++ b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> @@ -394,7 +394,7 @@ static void lcdif_crtc_mode_set_nofb(struct lcdif_drm_private *lcdif,
>   	struct drm_display_mode *m = &lcdif->crtc.state->adjusted_mode;
>   	u32 bus_flags = 0;
>   
> -	if (lcdif->bridge && lcdif->bridge->timings)
> +	if (lcdif->bridge->timings)
>   		bus_flags = lcdif->bridge->timings->input_bus_flags;
>   	else if (bridge_state)
>   		bus_flags = bridge_state->input_bus_cfg.flags;
> @@ -463,30 +463,21 @@ static void lcdif_crtc_atomic_enable(struct drm_crtc *crtc,
>   	struct drm_display_mode *m = &lcdif->crtc.state->adjusted_mode;
>   	struct drm_bridge_state *bridge_state = NULL;
>   	struct drm_device *drm = lcdif->drm;
> -	u32 bus_format = 0;
> +	u32 bus_format;
>   	dma_addr_t paddr;
>   
> -	/* If there is a bridge attached to the LCDIF, use its bus format */
> -	if (lcdif->bridge) {
> -		bridge_state =
> -			drm_atomic_get_new_bridge_state(state,
> -							lcdif->bridge);
> -		if (!bridge_state)
> -			bus_format = MEDIA_BUS_FMT_FIXED;
> -		else
> -			bus_format = bridge_state->input_bus_cfg.format;
> -
> -		if (bus_format == MEDIA_BUS_FMT_FIXED) {
> -			dev_warn_once(drm->dev,
> -				      "Bridge does not provide bus format, assuming MEDIA_BUS_FMT_RGB888_1X24.\n"
> -				      "Please fix bridge driver by handling atomic_get_input_bus_fmts.\n");
> -			bus_format = MEDIA_BUS_FMT_RGB888_1X24;
> -		}
> -	}
> +	bridge_state = drm_atomic_get_new_bridge_state(state, lcdif->bridge);
> +	if (!bridge_state)
> +		bus_format = MEDIA_BUS_FMT_FIXED;
> +	else
> +		bus_format = bridge_state->input_bus_cfg.format;

The code below seems to change the logic slightly.

Could it happen that:
- bridge_state is valid (i.e. non-NULL)
- bridge_state->input_bus_cfg.format is set to 0 (i.e. not set) ?
   (note that MEDIA_BUS_FMT_FIXED is defined as 0x0001)

> -	/* If all else fails, default to RGB888_1X24 */
> -	if (!bus_format)
> +	if (bus_format == MEDIA_BUS_FMT_FIXED) {
> +		dev_warn_once(drm->dev,
> +			      "Bridge does not provide bus format, assuming MEDIA_BUS_FMT_RGB888_1X24.\n"
> +			      "Please fix bridge driver by handling atomic_get_input_bus_fmts.\n");
>   		bus_format = MEDIA_BUS_FMT_RGB888_1X24;
> +	}
>   
>   	clk_set_rate(lcdif->clk, m->crtc_clock * 1000);

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