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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Fabrizio,

On Tue, Nov 19, 2019 at 11:17:34AM +0000, Fabrizio Castro wrote:
> On 19 November 2019 00:16 Laurent Pinchart wrote:
> > 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).

How about the following logic ?

	if (match_index("lvds-encoder") == 0 ||
	    match_index("lvds-decoder") == 0)
		return -EINVAL;

?

> 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.

Sure, but that doesn't prevent anyone ignoring the validation.

> 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.

I agree with this argument, it would set a precedent, and is probably
not worth duplicating similar code in all drivers. I wonder if this is
something we could handle with core helpers, but maybe it's overkill.

> > > +	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