linux-kernel - Re: [PATCH] drm/bridge: ldb: add support for an external bridge

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Message-ID: <2860884.vuYhMxLoTh@fedora>
Date: Tue, 28 Oct 2025 17:01:24 +0100
From: Francesco Valla <francesco@...la.it>
To: Liu Ying <victor.liu@....com>, Marek Vasut <marex@...x.de>,
 Andrzej Hajda <andrzej.hajda@...el.com>,
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Alexander Stein <alexander.stein@...tq-group.com>
Cc: Fabian Pflug <f.pflug@...gutronix.de>, dri-devel@...ts.freedesktop.org,
 imx@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/bridge: ldb: add support for an external bridge

Hi,

On Tuesday, 28 October 2025 at 13:27:37 Alexander Stein <alexander.stein@...tq-group.com> wrote:
> Hi Francesco,
> 
> Am Dienstag, 28. Oktober 2025, 13:12:29 CET schrieb Francesco Valla:
> > One option for the LVDS port of the LDB is to be connected to an
> > additional bridge, such as a LVDS to HDMI converter. Add support for
> > such case, along with the direct connection to a panel.
> > 
> > Signed-off-by: Francesco Valla <francesco@...la.it>
> > ---
> > I was trying to add display support for the i.MX93 FRDM on top of the
> > patch sent some time ago by Fabian Pflug [1], using some of the work
> > already done by Alexander Stein but not yet merged [2], but then I
> > noticed that the support for LVDS-HDMI converter bridges was missing
> > from the LDB driver already present for the i.MX93.
> > 
> > Not a fail of the driver itself, obviously, but I wonder if/how the
> > existing i.MX8MP setups (e.g.: [3]), which use the same driver, work
> > correclty. Unfortunately I don't have the i.MX8MP hardware to test them.
> > 
> > Anyhow, a patch for such setup is attached; it was tested on the i.MX93
> > FRDM using [1] and [2] plus some more devicetree modifications.
> > 
> > [1] https://lore.kernel.org/all/20251022-fpg-nxp-imx93-frdm-v3-1-03ec40a1ccc0@pengutronix.de
> > [2] https://lore.kernel.org/all/20250304154929.1785200-1-alexander.stein@ew.tq-group.com
> > [3] https://elixir.bootlin.com/linux/v6.17.5/source/arch/arm64/boot/dts/freescale/imx8mp-evk-lvds0-imx-dlvds-hdmi-channel0.dtso
> > 
> > Regards,
> > Francesco
> > ---
> >  drivers/gpu/drm/bridge/fsl-ldb.c | 26 +++++++++++++++++---------
> >  1 file changed, 17 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/bridge/fsl-ldb.c b/drivers/gpu/drm/bridge/fsl-ldb.c
> > index 5c3cf37200bcee1db285c97e2b463c9355ee6acb..fad436f2e0bfac8b42096a6fcd0022da0f35284e 100644
> > --- a/drivers/gpu/drm/bridge/fsl-ldb.c
> > +++ b/drivers/gpu/drm/bridge/fsl-ldb.c
> > @@ -294,7 +294,6 @@ static int fsl_ldb_probe(struct platform_device *pdev)
> >  	struct device *dev = &pdev->dev;
> >  	struct device_node *panel_node;
> >  	struct device_node *remote1, *remote2;
> > -	struct drm_panel *panel;
> >  	struct fsl_ldb *fsl_ldb;
> >  	int dual_link;
> >  
> > @@ -335,15 +334,24 @@ static int fsl_ldb_probe(struct platform_device *pdev)
> >  		fsl_ldb_is_dual(fsl_ldb) ? "dual-link mode" :
> >  		fsl_ldb->ch0_enabled ? "channel 0" : "channel 1");
> >  
> > -	panel = of_drm_find_panel(panel_node);
> > -	of_node_put(panel_node);
> > -	if (IS_ERR(panel))
> > -		return PTR_ERR(panel);
> > -
> > -	fsl_ldb->panel_bridge = devm_drm_panel_bridge_add(dev, panel);
> > -	if (IS_ERR(fsl_ldb->panel_bridge))
> > -		return PTR_ERR(fsl_ldb->panel_bridge);
> > +	/* First try to get an additional bridge, if not found go for a panel */
> > +	fsl_ldb->panel_bridge = of_drm_find_bridge(panel_node);
> > +	if (fsl_ldb->panel_bridge) {
> > +		of_node_put(panel_node);
> > +	} else {
> > +		struct drm_panel *panel;
> >  
> > +		panel = of_drm_find_panel(panel_node);
> > +		of_node_put(panel_node);
> > +		if (IS_ERR(panel))
> > +			return dev_err_probe(dev, PTR_ERR(panel),
> > +					     "Failed to find panel");
> > +
> > +		fsl_ldb->panel_bridge = devm_drm_panel_bridge_add(dev, panel);
> > +		if (IS_ERR(fsl_ldb->panel_bridge))
> > +			return dev_err_probe(dev, PTR_ERR(fsl_ldb->panel_bridge),
> > +					     "Failed to add panel bridge");
> > +	}
> 
> Without looking into the details this somehow looks similar to
> drm_of_find_panel_or_bridge(), or drmm_of_get_bridge for the managed variant.
> 
> Best regards,
> Alexander
> 
> >  
> >  	if (fsl_ldb_is_dual(fsl_ldb)) {
> >  		struct device_node *port1, *port2;
> > 
> > ---
> > base-commit: fd57572253bc356330dbe5b233c2e1d8426c66fd
> > change-id: 20251028-imx93_ldb_bridge-3c011e7856dc
> > 
> > Best regards,
> > 
> 
> 
> 

Well, I did not know of devm_drm_of_get_bridge(), and unfortunately the
driver I checked (Samsung DSIM) has not yet been updated to use it.
Sorry for the noise.

I'll wait for more feedback (if any) and then send a v2 - which looks
much cleaner now.

Thank you!

Regards,
Francesco

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