linux-kernel - Re: [PATCH] drm: rcar_lvds: Fix dual link mode operations

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Date:   Thu, 1 Aug 2019 18:11:29 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Jacopo Mondi <jacopo+renesas@...ndi.org>
Cc:     Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        fabrizio.castro@...renesas.com,
        "open list:DRM DRIVERS FOR RENESAS" <dri-devel@...ts.freedesktop.org>,
        "open list:DRM DRIVERS FOR RENESAS" 
        <linux-renesas-soc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: rcar_lvds: Fix dual link mode operations

Hello Jacopo,

Thank you for the patch.

On Tue, Jul 23, 2019 at 06:57:00PM +0200, Jacopo Mondi wrote:
> The R-Car LVDS encoder units support dual-link operations by splitting
> the pixel output between the primary encoder and the companion one.

s/the companion one/its companion/

> 
> In order for the primary encoder to succesfully control the companion's
> operations this should not fail at probe time and register itself its
> associated drm bridge so that the primary one can find it.

This is hard to parse.

> Currently the companion encoder fails at probe time, causing the
> registration of the primary to fail preventing the whole DU unit to be
> registered correctly.
> 
> Fixes: fa440d870358 ("drm: rcar-du: lvds: Add support for dual-link mode")
> Reported-by: Fabrizio Castro <fabrizio.castro@...renesas.com>
> Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> 
> ---
> The "Fixes" tag refers to a patch currently part of the
> renesas-drivers-2019-07-09-v5.2 branch of Geert's renesas-drivers tree.
> 
>  drivers/gpu/drm/rcar-du/rcar_lvds.c | 31 +++++++++++++++++++++--------
>  1 file changed, 23 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/rcar-du/rcar_lvds.c b/drivers/gpu/drm/rcar-du/rcar_lvds.c
> index bada7ee98544..8b015ba95895 100644
> --- a/drivers/gpu/drm/rcar-du/rcar_lvds.c
> +++ b/drivers/gpu/drm/rcar-du/rcar_lvds.c
> @@ -767,14 +767,29 @@ static int rcar_lvds_parse_dt(struct rcar_lvds *lvds)
>  	of_node_put(remote_input);
>  	of_node_put(remote);
> 
> -	/*
> -	 * On D3/E3 the LVDS encoder provides a clock to the DU, which can be
> -	 * used for the DPAD output even when the LVDS output is not connected.
> -	 * Don't fail probe in that case as the DU will need the bridge to
> -	 * control the clock.
> -	 */
> -	if (lvds->info->quirks & RCAR_LVDS_QUIRK_EXT_PLL)
> -		return ret == -ENODEV ? 0 : ret;
> +	switch (ret) {
> +	case -ENODEV:
> +		/*
> +		 * On D3/E3 the LVDS encoder provides a clock to the DU, which
> +		 * can be used for the DPAD output even when the LVDS output is
> +		 * not connected. Don't fail probe in that case as the DU will
> +		 * need the bridge to control the clock.
> +		 */
> +		if (lvds->info->quirks & RCAR_LVDS_QUIRK_EXT_PLL)
> +			ret = 0;
> +		break;
> +	case -ENXIO:
> +		/*
> +		 * When the LVDS output is used in dual link mode, the
> +		 * companion encoder fails at
> +		 * 'rcar_lvds_parse_dt_companion()'. Don't fail probe in
> +		 * that case as the master encoder will need the companion's
> +		 * bridge to control its operations.
> +		 */
> +		if (lvds->info->quirks & RCAR_LVDS_QUIRK_DUAL_LINK)
> +			ret = 0;

As -ENXIO can only be returned by rcar_lvds_parse_dt_companion(), and
rcar_lvds_parse_dt_companion() is only called when the
RCAR_LVDS_QUIRK_DUAL_LINK flag is set, this essentially means that you
always zero the error returned from rcar_lvds_parse_dt_companion(). This
is both complicated and too drastic as the second -ENXIO error case
shouldn't be ignored. It would be better and simpler to return 0 from
rcar_lvds_parse_dt_companion() when the renesas,companion property can't
be found.

> +		break;
> +	}
> 
>  	return ret;
>  }

-- 
Regards,

Laurent Pinchart