Hi Geert,
> From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Sent: 22 November 2019 08:17
> Subject: Re: [PATCH v4 12/13] [HACK] drm/bridge: lvds-codec: Enforce device specific compatible strings
>
> Hi Fabrizio,
>
> On Thu, Nov 21, 2019 at 5:00 PM Fabrizio Castro
> <fabrizio.castro@xxxxxxxxxxxxxx> wrote:
> > > From: devicetree-owner@xxxxxxxxxxxxxxx <devicetree-owner@xxxxxxxxxxxxxxx> On Behalf Of Laurent Pinchart
> > > Sent: 19 November 2019 21:52
> > > Subject: Re: [PATCH v4 12/13] [HACK] drm/bridge: lvds-codec: Enforce device specific compatible strings
> > >
> > > 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
> > > > > > 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
> > > > > > compatible string we match against and the index of the matching
> > > > > > compatible string in the list.
> > > > > >
> > > > > > Signed-off-by: Fabrizio Castro <fabrizio.castro@xxxxxxxxxxxxxx>
> > > > > >
> > > > > > ---
> > > > > > 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."
>
> > > > > > --- a/drivers/gpu/drm/bridge/lvds-codec.c
> > > > > > +++ b/drivers/gpu/drm/bridge/lvds-codec.c
>
> > > > > > @@ -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;
> > >
> > >
> >
> > Now I see what you mean
> >
> > > > 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.
> >
> > I was hoping others would comment as well, but perhaps this topic is not too exciting.
> >
> > Geert, what do you think about this? Is this something we should enforce
> > in drivers?
>
> So IIUIC, you want to enforce the presence of both specific and generic
> compatible values (in that order) in the driver (except for
> "thine,thc63lvdm83d", as that predates the introduction of the generic
> compatible value)?
Yeah, this is what Laurent would want ideally.
> However, the driver would not really care about the actual hardware-specific
> value, as it would still match against the generic one, and the
> hardware-specific one may not even be listed in the driver's match table?
Exactly.
>
> By definition, you can have one or more compatible values listed in a
> device node, from most-specific to least-specific. Typically the driver
> cannot know if a more specific value is missing, but YAML DT binding
> validation can.
>
> In this case it is a bit special, as there is a generic one involved, so
> you can assume there should be a more specific one, too.
> If you want to handle this in the core, you probably need to add an
> "is_generic" flag to struct of_device_id.
That's actually an interesting way of looking at this.
Laurent?
Thanks,
Fab
>
> Rob/Mark?
>
> > > > > > + 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;
>
> -ENODEV?
> So a "more generic" driver can take over?
>
> > > > > > + }
> > > > > > + }
> > > > > > +
> > > > > > 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,
> > > > > > },
> > > > > > {},
> > > > > > };
>
> Gr{oetje,eeting}s,
>
> Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
> -- Linus Torvalds
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