RE: [PATCH v2 7/9] drm: rcar-du: lvds: Add dual-LVDS panels support

[Fabrizio Castro <fabrizio.castro@bp.renesas.com>]

Hello Laurent,

Thank you for your feedback!

> From: linux-renesas-soc-owner@vger.kernel.org <linux-renesas-soc-owner@vger.kernel.org> On Behalf Of Laurent Pinchart
> Sent: 15 August 2019 14:09
> Subject: Re: [PATCH v2 7/9] drm: rcar-du: lvds: Add dual-LVDS panels support
> 
> Hi Fabrizio,
> 
> Thank you for the patch.
> 
> On Thu, Aug 15, 2019 at 12:04:31PM +0100, Fabrizio Castro wrote:
> > This patch adds support for dual-LVDS panels.
> >
> > It's very important that we coordinate the efforts of both the
> > primary encoder and the companion encoder to get the right
> > picture on the panel, therefore this patch adds some code
> > to work out if even and odd pixels need swapping.
> > When the encoders are connected to a LVDS panel, the assumption
> > is that by default the panel expects even pixels (0, 2, 4, etc.)
> > on the first input port, and odd pixels (1, 3, 5, etc.) on the
> > seconds port. When DRM_LINK_DUAL_LVDS_ODD_EVEN is found among the
> > link flags, the display expects odd pixels on the first input
> > port, and even pixels on the second port. As a result, the way
> > the encoders are connected to the panel may trigger pixel (data)
> > swapping.
> >
> > Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
> >
> > ---
> > v1->v2:
> > * new patch, resulting from Laurent's feedback
> >
> >  drivers/gpu/drm/rcar-du/rcar_lvds.c | 121 ++++++++++++++++++++++++------------
> >  1 file changed, 81 insertions(+), 40 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/rcar-du/rcar_lvds.c b/drivers/gpu/drm/rcar-du/rcar_lvds.c
> > index 41d28f4..5c24884 100644
> > --- a/drivers/gpu/drm/rcar-du/rcar_lvds.c
> > +++ b/drivers/gpu/drm/rcar-du/rcar_lvds.c
> > @@ -22,6 +22,8 @@
> >  #include <drm/drm_bridge.h>
> >  #include <drm/drm_panel.h>
> >  #include <drm/drm_probe_helper.h>
> > +#include <drm/drm_timings.h>
> > +#include <drm/drm_of.h>
> 
> Please keep the headers alphabetically sorted.

Ok

> 
> >
> >  #include "rcar_lvds.h"
> >  #include "rcar_lvds_regs.h"
> > @@ -69,6 +71,7 @@ struct rcar_lvds {
> >
> >  	struct drm_bridge *companion;
> >  	bool dual_link;
> > +	bool stripe_swap_data;
> >  };
> >
> >  #define bridge_to_rcar_lvds(b) \
> > @@ -439,12 +442,20 @@ static void rcar_lvds_enable(struct drm_bridge *bridge)
> >  	rcar_lvds_write(lvds, LVDCHCR, lvdhcr);
> >
> >  	if (lvds->info->quirks & RCAR_LVDS_QUIRK_DUAL_LINK) {
> > -		/*
> > -		 * Configure vertical stripe based on the mode of operation of
> > -		 * the connected device.
> > -		 */
> > -		rcar_lvds_write(lvds, LVDSTRIPE,
> > -				lvds->dual_link ? LVDSTRIPE_ST_ON : 0);
> > +		u32 lvdstripe = 0;
> > +
> > +		if (lvds->dual_link)
> > +			/*
> > +			 * Configure vertical stripe based on the mode of
> > +			 * operation of the connected device.
> > +			 *
> > +			 * ST_SWAP from LVD1STRIPE is reserved, do not set
> > +			 * in the companion LVDS
> > +			 */
> > +			lvdstripe = LVDSTRIPE_ST_ON |
> > +				(lvds->companion && lvds->stripe_swap_data ?
> > +				 LVDSTRIPE_ST_SWAP : 0);
> 
> Let's sort out the alignment.
> 
> 			lvdstripe = LVDSTRIPE_ST_ON
> 				  | (lvds->companion && lvds->stripe_swap_data ?
> 				     LVDSTRIPE_ST_SWAP : 0);

Ok

> 
> > +		rcar_lvds_write(lvds, LVDSTRIPE, lvdstripe);
> >  	}
> >
> >  	/*
> > @@ -706,13 +717,31 @@ static int rcar_lvds_parse_dt_companion(struct rcar_lvds *lvds)
> >  	return ret;
> >  }
> >
> > +static int rcar_lvds_get_remote_port_reg(struct device_node *np)
> > +{
> > +	struct device_node *endpoint_node, *remote_endpoint;
> > +	struct of_endpoint endpoint;
> > +
> > +	endpoint_node = of_graph_get_endpoint_by_regs(np, 1, 0);
> > +	if (!endpoint_node)
> > +		return -ENODEV;
> > +	remote_endpoint = of_graph_get_remote_endpoint(endpoint_node);
> > +	if (!remote_endpoint) {
> > +		of_node_put(endpoint_node);
> > +		return -ENODEV;
> > +	}
> > +	of_graph_parse_endpoint(remote_endpoint, &endpoint);
> > +	of_node_put(endpoint_node);
> > +	of_node_put(remote_endpoint);
> > +
> > +	return endpoint.port;
> > +}
> > +
> >  static int rcar_lvds_parse_dt(struct rcar_lvds *lvds)
> >  {
> >  	struct device_node *local_output = NULL;
> > -	struct device_node *remote_input = NULL;
> >  	struct device_node *remote = NULL;
> > -	struct device_node *node;
> > -	bool is_bridge = false;
> > +	const struct drm_timings *timings = NULL;
> >  	int ret = 0;
> >
> >  	local_output = of_graph_get_endpoint_by_regs(lvds->dev->of_node, 1, 0);
> > @@ -740,45 +769,57 @@ static int rcar_lvds_parse_dt(struct rcar_lvds *lvds)
> >  		goto done;
> >  	}
> >
> > -	remote_input = of_graph_get_remote_endpoint(local_output);
> > +	ret = drm_of_find_panel_or_bridge(lvds->dev->of_node, 1, 0,
> > +					  &lvds->panel, &lvds->next_bridge);
> > +	if (ret) {
> > +		ret = -EPROBE_DEFER;
> > +		goto done;
> > +	}
> > +	if (lvds->info->quirks & RCAR_LVDS_QUIRK_DUAL_LINK) {
> > +		if (lvds->next_bridge)
> > +			timings = lvds->next_bridge->timings;
> > +		else
> > +			timings = lvds->panel->timings;
> 
> I wonder if we should use devm_drm_panel_bridge_add() (or
> drm_panel_bridge_add()) and use the bridge API only. It would require a
> small change in the drm_panel_bridge to copy the timings pointer, but
> apart from that I think it should be fine. If it creates too much churn
> due to connector handling then we can skip it for now and I'll handle it
> later (but I'd appreciate if you could copy the timings pointer in
> drm_panel_bridge already).

Will look into this.

> 
> > +		if (timings)
> > +			lvds->dual_link = timings->dual_link;
> > +	}
> >
> > -	for_each_endpoint_of_node(remote, node) {
> > -		if (node != remote_input) {
> > +	if (lvds->dual_link) {
> > +		ret = rcar_lvds_parse_dt_companion(lvds);
> > +		if (lvds->companion && timings) {
> > +			int our_port, comp_port;
> > +			bool odd_even_flag = timings->link_flags &
> > +						DRM_LINK_DUAL_LVDS_ODD_EVEN;
> > +			our_port = rcar_lvds_get_remote_port_reg(
> > +						lvds->dev->of_node);
> > +			if (our_port < 0) {
> > +				ret = our_port;
> > +				goto done;
> > +			}
> > +			comp_port = rcar_lvds_get_remote_port_reg(
> > +						lvds->companion->of_node);
> > +			if (comp_port < 0) {
> > +				ret = comp_port;
> > +				goto done;
> > +			}
> >  			/*
> > -			 * We've found one endpoint other than the input, this
> > -			 * must be a bridge.
> > +			 * We need to match the port where we generate even
> > +			 * pixels (0, 2, 4, etc.) to the port where the sink
> > +			 * expects to receive even pixels, same thing for the
> > +			 * odd pixels. Swap the generation of even and odd
> > +			 * pixels if the connection requires it.
> > +			 * By default (when DRM_LINK_DUAL_LVDS_ODD_EVEN is not
> > +			 * specified) the sink expects even pixels on the
> > +			 * first input port, and odd pixels on the second port.
> 
> I see what you're trying to do, but I'm not sure I like it much :-S
> 
> Peeking into the remove DT node like that creates a dependency between
> this driver and the DT bindings of all possible remote nodes. For this
> to work, you would need to ensure that the odd/even mapping to ports is
> common to all dual-link devices, and thus document that globally in the
> DT bindings. I'm not sure if there's a way around it as hardware
> connections could indeed switch the two lanes, so we need to model that
> somehow. It could be modelled with a swap property in DT, but that would
> still require a standard mapping of odd-even pixels to ports, so maybe
> the easiest option is to document globally that port 0 on the sink is
> for even pixels, and port 1 for odd pixels, and remove the
> DRM_LINK_DUAL_LVDS_ODD_EVEN flag completely. But what will then happen
> if you panel has more than two ports (for audio for instance, or for
> other types of video links) ? It may not be possible to always use port
> 0 and 1 for the LVDS even and odd pixels in DT bindings of a particular
> panel or bridge.

This is the stickiest point of the whole series. The implementation within this
series allows for any number of ports on the sink, the LVDS ports don't have
to be port 0 and port 1, it's enough that the port for the even pixels comes
before the port of the odd pixels (but the logic can be inverted by means of
DRM_LINK_DUAL_LVDS_ODD_EVEN), and if you swap the lvds0 and lvds1
OF graph connections around, the pixels will be swapped automatically.
But of course, there is the dependency between the driver and dt-bindings
you were mentioning, and of top of that every driver would need to work
things out independently at this point in time.

> 
> A creative solution is needed here.

I may have an idea. What if we marked both ends of each OF graph link
with either "even-pixels;" or "odd-pixels;", and exported a function that
given the of_node of two endpoints returned if the link requires swapping?
There'd be no need for the flag at that point, the numbering of the ports
would not matter, and the DT would be comprehensive and very easy to read.

Let me please know your thoughts.

Thanks you for the patience
Fab

> 
> >  			 */
> > -			is_bridge = true;
> > -			of_node_put(node);
> > -			break;
> > +			if (((comp_port - our_port > 0) &&  odd_even_flag) ||
> > +			    ((comp_port - our_port < 0) && !odd_even_flag))
> > +				lvds->stripe_swap_data = true;
> >  		}
> >  	}
> >
> > -	if (is_bridge) {
> > -		lvds->next_bridge = of_drm_find_bridge(remote);
> > -		if (!lvds->next_bridge) {
> > -			ret = -EPROBE_DEFER;
> > -			goto done;
> > -		}
> > -
> > -		if (lvds->info->quirks & RCAR_LVDS_QUIRK_DUAL_LINK)
> > -			lvds->dual_link = lvds->next_bridge->timings
> > -					? lvds->next_bridge->timings->dual_link
> > -					: false;
> > -	} else {
> > -		lvds->panel = of_drm_find_panel(remote);
> > -		if (IS_ERR(lvds->panel)) {
> > -			ret = PTR_ERR(lvds->panel);
> > -			goto done;
> > -		}
> > -	}
> > -
> > -	if (lvds->dual_link)
> > -		ret = rcar_lvds_parse_dt_companion(lvds);
> > -
> >  done:
> >  	of_node_put(local_output);
> > -	of_node_put(remote_input);
> >  	of_node_put(remote);
> >
> >  	/*
> 
> --
> Regards,
> 
> Laurent Pinchart