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From:   Biju Das <biju.das.jz@bp.renesas.com>
To:     Neil Armstrong <narmstrong@baylibre.com>,
        "robert.foss@linaro.org" <robert.foss@linaro.org>
CC:     "Laurent.pinchart@ideasonboard.com" 
        <Laurent.pinchart@ideasonboard.com>,
        "jonas@kwiboo.se" <jonas@kwiboo.se>,
        "jernej.skrabec@gmail.com" <jernej.skrabec@gmail.com>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kieran.bingham@ideasonboard.com" <kieran.bingham@ideasonboard.com>,
        Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Subject: RE: [PATCH] drm/bridge: dw-hdmi: use safe format when first in bridge
 chain
Thread-Topic: [PATCH] drm/bridge: dw-hdmi: use safe format when first in
 bridge chain
Thread-Index: AQHYC60IbG8qGZ+c5EqsUkUuA5+zGKxnQ/NQ
Date:   Mon, 17 Jan 2022 14:27:39 +0000
Message-ID: <OS0PR01MB592290D81CE917A0125C519E86579@OS0PR01MB5922.jpnprd01.prod.outlook.com>
References: <20220117141750.1182223-1-narmstrong@baylibre.com>
In-Reply-To: <20220117141750.1182223-1-narmstrong@baylibre.com>
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Hi Neil,

Thanks for the patch

> Subject: [PATCH] drm/bridge: dw-hdmi: use safe format when first in bridg=
e
> chain
>=20
> When the dw-hdmi bridge is in first place of the bridge chain, this means
> there is now way to select an input format of the dw-hdmi HW component.
>=20
> Since introduction of display-connector, negociation was broken since the
> dw-hdmi negociation code only worked when the dw-hdmi bridge was in last
> position of the bridge chain or behind another bridge also supporting
> input & output format negociation.
>=20
> Commit 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts
> callbacks") was introduced to make negociation work again by making
> display-connector act as a pass-through concerning input & output format
> negociation.
>=20
> But in the case were the dw-hdmi was single in the bridge chain, for
> example on Renesas SoCs, with the disply-connector bridge the dw-hdmi is
> no more single, breaking output format.
>=20
> Reported-by: Biju Das <biju.das.jz@bp.renesas.com>
> Bisected-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
> Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
> Fixes: 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts
> callbacks").
> Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
> ---
>  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>=20
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> index 54d8fdad395f..9f2e1cac0ae2 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> @@ -2551,8 +2551,9 @@ static u32
> *dw_hdmi_bridge_atomic_get_output_bus_fmts(struct drm_bridge *bridge,
>  	if (!output_fmts)
>  		return NULL;
>=20
> -	/* If dw-hdmi is the only bridge, avoid negociating with ourselves
> */
> -	if (list_is_singular(&bridge->encoder->bridge_chain)) {
> +	/* If dw-hdmi is the first or only bridge, avoid negociating with
> ourselves */
> +	if (list_is_singular(&bridge->encoder->bridge_chain) ||
> +	    list_is_first(&bridge->chain_node,
> +&bridge->encoder->bridge_chain)) {
>  		*num_output_fmts =3D 1;
>  		output_fmts[0] =3D MEDIA_BUS_FMT_FIXED;
>=20
> @@ -2673,6 +2674,10 @@ static u32
> *dw_hdmi_bridge_atomic_get_input_bus_fmts(struct drm_bridge *bridge,
>  	if (!input_fmts)
>  		return NULL;
>=20
> +	/* If dw-hdmi is the first bridge fall-back to safe output format */
> +	if (list_is_first(&bridge->chain_node, &bridge->encoder-
> >bridge_chain))
> +		output_fmt =3D MEDIA_BUS_FMT_FIXED;
> +

Based on my debugging, this looks redundant, as get_output_bus_fmts already=
 sets output_fmt =3D MEDIA_BUS_FMT_FIXED,
And *num_output_fmts =3D 1, so the function parameter output_fmt in input_b=
us_fmts will have MEDIA_BUS_FMT_FIXED
And no need to override output_fmt value.  I may be wrong here.

Regards,
Biju

>  	switch (output_fmt) {
>  	/* If MEDIA_BUS_FMT_FIXED is tested, return default bus format */
>  	case MEDIA_BUS_FMT_FIXED:
> --
> 2.25.1