RE: [PATCH v4 12/13] [HACK] drm/bridge: lvds-codec: Enforce device specific compatible strings

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

Hi Laurent,

Thank you for your feedback!

> From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Sent: 19 November 2019 00:16
> Subject: Re: [PATCH v4 12/13] [HACK] drm/bridge: lvds-codec: Enforce device specific compatible strings
> 
> Hi Fabrizio,
> 
> Thank you for the patch.
> 
> On Wed, Nov 13, 2019 at 03:51:31PM +0000, Fabrizio Castro wrote:
> > The lvds-codec driver is a generic stub for transparent LVDS
> > encoders and decoders.
> > It's good practice to list a device specific compatible string
> 
> s/good practice/mandatory/

Will fix

> 
> > before the generic fallback (if any) in the DT node for the relevant
> > LVDS encoder/decoder, and it's also required by the dt-bindings.
> > A notable exception to the generic fallback mechanism is the case
> > of "thine,thc63lvdm83d", as documented in:
> > Documentation/devicetree/bindings/display/bridge/thine,thc63lvdm83d.txt
> > This patch enforces the adoption of a device specific compatible
> > string (as fist string in the list), by using markers for the
> 
> s/fist/first/

Well spotted

> 
> > compatible string we match against and the index of the matching
> > compatible string in the list.
> >
> > Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
> >
> > ---
> > Hi Laurent,
> >
> > I don't think we need to do anything in the driver to address your
> > comment, as we can "enforce" this with the bindings (please see the
> > next patch, as it would help with the "enforcing" of the compatible
> > string for the thine device).
> > I am sending this patch only so that you can see what a possible
> > solution in the driver could look like.
> >
> > v3->v4:
> > * New patch addressing the below comment from Laurent:
> > "I think the lvds-decoder driver should error out at probe time if only
> > one compatible string is listed."
> > ---
> >  drivers/gpu/drm/bridge/lvds-codec.c | 55 +++++++++++++++++++++++++++++++++----
> >  1 file changed, 49 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/lvds-codec.c b/drivers/gpu/drm/bridge/lvds-codec.c
> > index 784bbd3..145c25d 100644
> > --- a/drivers/gpu/drm/bridge/lvds-codec.c
> > +++ b/drivers/gpu/drm/bridge/lvds-codec.c
> > @@ -14,11 +14,16 @@
> >  #include <drm/drm_bridge.h>
> >  #include <drm/drm_panel.h>
> >
> > +struct lvds_codec_data {
> > +	u32 connector_type;
> > +	bool device_specific;
> > +};
> > +
> >  struct lvds_codec {
> >  	struct drm_bridge bridge;
> >  	struct drm_bridge *panel_bridge;
> >  	struct gpio_desc *powerdown_gpio;
> > -	u32 connector_type;
> > +	const struct lvds_codec_data *data;
> >  };
> >
> >  static int lvds_codec_attach(struct drm_bridge *bridge)
> > @@ -65,7 +70,30 @@ static int lvds_codec_probe(struct platform_device *pdev)
> >  	if (!lvds_codec)
> >  		return -ENOMEM;
> >
> > -	lvds_codec->connector_type = (u32)of_device_get_match_data(&pdev->dev);
> > +	lvds_codec->data = of_device_get_match_data(&pdev->dev);
> > +	if (!lvds_codec->data)
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * If we haven't matched a device specific compatible string, we need
> > +	 * to work out if the generic compatible string we matched against was
> > +	 * listed first in the compatible property.
> > +	 */
> 
> Can't we do this unconditionally, and thus drop the lvds_codec_data
> structure ?

I don't think so, and the reason for this is that we have a corner case for
thine,thc63lvdm83d. Here is what's allowed (according to the documentation)
from what's supported upstream (+ this series):
"ti,ds90c185", "lvds-encoder"
"ti,ds90c187", "lvds-encoder"
"ti,sn75lvds83", "lvds-encoder"
"ti,ds90cf384a", "lvds-decoder"
"thine,thc63lvdm83d"

As you can see from the examples above, in most cases it's enough to say it's
all good when we match a compatible string with index > 0, but for the thine
device you _have_ to match the string with index 0 as that's what's currently
documented (please see thine,thc63lvdm83d.txt) and that's what's supported
by device trees already (please see arch/arm/boot/dts/r8a7779-marzen.dts).

This patch "classifies" compatible strings, and it considers a good match
device specific compatible strings, or generic compatible strings as long
as they are not listed first.

These days you can leverage the yaml files to validate the device trees,
therefore we should be focusing on writing yaml files in such a way we only
pass the checks we mean to, and by checks I mean:
make dtbs_check

or more specifically, for this series:
make dtbs_check  DT_SCHEMA_FILES=Documentation/devicetree/bindings/display/bridge/lvds-codec.yaml

and that's of course on top of make dt_binding_check.

It's a very common requirement to have a part number specific compatible
string first followed by a generic (fallback) compatible string in the device trees,
most drivers for Renesas SoCs have similar requirements.

If we start doing this here, we'll end up doing it elsewhere as well, and I really
think we shouldn't, but others may see things differently, so I'll wait for others
(and yourself with further comments) to jump in before doing any more work
on this patch.

Thanks,
Fab

> 
> > +	if (!lvds_codec->data->device_specific) {
> > +		const struct of_device_id *match;
> > +		int compatible_index;
> > +
> > +		match = of_match_node(dev->driver->of_match_table,
> > +				      dev->of_node);
> > +		compatible_index = of_property_match_string(dev->of_node,
> > +							    "compatible",
> > +							    match->compatible);
> > +		if (compatible_index == 0) {
> > +			dev_err(dev, "Device specific compatible needed\n");
> > +			return -EINVAL;
> > +		}
> > +	}
> > +
> >  	lvds_codec->powerdown_gpio = devm_gpiod_get_optional(dev, "powerdown",
> >  							     GPIOD_OUT_HIGH);
> >  	if (IS_ERR(lvds_codec->powerdown_gpio)) {
> > @@ -92,7 +120,7 @@ static int lvds_codec_probe(struct platform_device *pdev)
> >
> >  	lvds_codec->panel_bridge =
> >  		devm_drm_panel_bridge_add_typed(dev, panel,
> > -						lvds_codec->connector_type);
> > +					lvds_codec->data->connector_type);
> >  	if (IS_ERR(lvds_codec->panel_bridge))
> >  		return PTR_ERR(lvds_codec->panel_bridge);
> >
> > @@ -119,18 +147,33 @@ static int lvds_codec_remove(struct platform_device *pdev)
> >  	return 0;
> >  }
> >
> > +static const struct lvds_codec_data lvds_codec_decoder_data = {
> > +	.connector_type = DRM_MODE_CONNECTOR_DPI,
> > +	.device_specific = false,
> > +};
> > +
> > +static const struct lvds_codec_data lvds_codec_encoder_data = {
> > +	.connector_type = DRM_MODE_CONNECTOR_LVDS,
> > +	.device_specific = false,
> > +};
> > +
> > +static const struct lvds_codec_data lvds_codec_thc63lvdm83d_data = {
> > +	.connector_type = DRM_MODE_CONNECTOR_LVDS,
> > +	.device_specific = true,
> > +};
> > +
> >  static const struct of_device_id lvds_codec_match[] = {
> >  	{
> >  		.compatible = "lvds-decoder",
> > -		.data = (void *)DRM_MODE_CONNECTOR_DPI,
> > +		.data = &lvds_codec_decoder_data,
> >  	},
> >  	{
> >  		.compatible = "lvds-encoder",
> > -		.data = (void *)DRM_MODE_CONNECTOR_LVDS,
> > +		.data = &lvds_codec_encoder_data,
> >  	},
> >  	{
> >  		.compatible = "thine,thc63lvdm83d",
> > -		.data = (void *)DRM_MODE_CONNECTOR_LVDS,
> > +		.data = &lvds_codec_thc63lvdm83d_data,
> >  	},
> >  	{},
> >  };
> 
> --
> Regards,
> 
> Laurent Pinchart