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Date:   Tue, 26 Mar 2019 20:49:07 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Måns Rullgård <mans@...sr.com>
Cc:     David Airlie <airlied@...ux.ie>, Chen-Yu Tsai <wens@...e.org>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sun4i: hdmi: add support for ddc-i2c-bus property

On Thu, Mar 21, 2019 at 06:00:45PM +0000, Måns Rullgård wrote:
> Maxime Ripard <maxime.ripard@...tlin.com> writes:
>
> > On Tue, Mar 19, 2019 at 12:48:19PM +0000, Måns Rullgård wrote:
> >> Maxime Ripard <maxime.ripard@...tlin.com> writes:
> >>
> >> > On Mon, Mar 18, 2019 at 04:23:56PM +0000, Måns Rullgård wrote:
> >> >> Maxime Ripard <maxime.ripard@...tlin.com> writes:
> >> >>
> >> >> > On Thu, Mar 14, 2019 at 04:09:13PM +0000, Måns Rullgård wrote:
> >> >> >> Maxime Ripard <maxime.ripard@...tlin.com> writes:
> >> >> >>
> >> >> >> > On Mon, Mar 11, 2019 at 04:11:06PM +0000, Måns Rullgård wrote:
> >> >> >> >> Maxime Ripard <maxime.ripard@...tlin.com> writes:
> >> >> >> >>
> >> >> >> >> > Hi!
> >> >> >> >> >
> >> >> >> >> > On Mon, Mar 11, 2019 at 01:47:13PM +0000, Mans Rullgard wrote:
> >> >> >> >> >> Sometimes it is desirabled to use a separate i2c controller for ddc
> >> >> >> >> >> access.  This adds support for the ddc-i2c-bus property of the
> >> >> >> >> >> hdmi-connector node, using the specified controller if provided.
> >> >> >> >> >>
> >> >> >> >> >> Signed-off-by: Mans Rullgard <mans@...sr.com>
> >> >> >> >> >> ---
> >> >> >> >> >>  drivers/gpu/drm/sun4i/sun4i_hdmi.h     |  1 +
> >> >> >> >> >>  drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c | 37 +++++++++++++++++++++++---
> >> >> >> >> >>  2 files changed, 35 insertions(+), 3 deletions(-)
> >> >> >> >> >>
> >> >> >> >> >> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi.h b/drivers/gpu/drm/sun4i/sun4i_hdmi.h
> >> >> >> >> >> index b685ee11623d..b08c4453d47c 100644
> >> >> >> >> >> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi.h
> >> >> >> >> >> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi.h
> >> >> >> >> >> @@ -269,6 +269,7 @@ struct sun4i_hdmi {
> >> >> >> >> >>  	struct clk		*tmds_clk;
> >> >> >> >> >>
> >> >> >> >> >>  	struct i2c_adapter	*i2c;
> >> >> >> >> >> +	struct i2c_adapter	*ddc_i2c;
> >> >> >> >> >>
> >> >> >> >> >>  	/* Regmap fields for I2C adapter */
> >> >> >> >> >>  	struct regmap_field	*field_ddc_en;
> >> >> >> >> >> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> >> >> >> >> >> index 061d2e0d9011..5b2fac79f5d6 100644
> >> >> >> >> >> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> >> >> >> >> >> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> >> >> >> >> >> @@ -212,7 +212,7 @@ static int sun4i_hdmi_get_modes(struct drm_connector *connector)
> >> >> >> >> >>  	struct edid *edid;
> >> >> >> >> >>  	int ret;
> >> >> >> >> >>
> >> >> >> >> >> -	edid = drm_get_edid(connector, hdmi->i2c);
> >> >> >> >> >> +	edid = drm_get_edid(connector, hdmi->ddc_i2c ?: hdmi->i2c);
> >> >> >> >> >
> >> >> >> >> > You can't test whether ddc_i2c is NULL or not...
> >> >> >> >> >
> >> >> >> >> >>  	if (!edid)
> >> >> >> >> >>  		return 0;
> >> >> >> >> >>
> >> >> >> >> >> @@ -228,6 +228,28 @@ static int sun4i_hdmi_get_modes(struct drm_connector *connector)
> >> >> >> >> >>  	return ret;
> >> >> >> >> >>  }
> >> >> >> >> >>
> >> >> >> >> >> +static struct i2c_adapter *sun4i_hdmi_get_ddc(struct device *dev)
> >> >> >> >> >> +{
> >> >> >> >> >> +	struct device_node *phandle, *remote;
> >> >> >> >> >> +	struct i2c_adapter *ddc;
> >> >> >> >> >> +
> >> >> >> >> >> +	remote = of_graph_get_remote_node(dev->of_node, 1, -1);
> >> >> >> >> >> +	if (!remote)
> >> >> >> >> >> +		return ERR_PTR(-EINVAL);
> >> >> >> >> >> +
> >> >> >> >> >> +	phandle = of_parse_phandle(remote, "ddc-i2c-bus", 0);
> >> >> >> >> >> +	of_node_put(remote);
> >> >> >> >> >> +	if (!phandle)
> >> >> >> >> >> +		return NULL;
> >> >> >> >> >> +
> >> >> >> >> >> +	ddc = of_get_i2c_adapter_by_node(phandle);
> >> >> >> >> >> +	of_node_put(phandle);
> >> >> >> >> >> +	if (!ddc)
> >> >> >> >> >> +		return ERR_PTR(-EPROBE_DEFER);
> >> >> >> >> >> +
> >> >> >> >> >> +	return ddc;
> >> >> >> >> >
> >> >> >> >> > ... Since even in (most) error cases you're returning a !NULL pointer.
> >> >> >> >> >
> >> >> >> >> >> +}
> >> >> >> >> >> +
> >> >> >> >> >>  static const struct drm_connector_helper_funcs sun4i_hdmi_connector_helper_funcs = {
> >> >> >> >> >>  	.get_modes	= sun4i_hdmi_get_modes,
> >> >> >> >> >>  };
> >> >> >> >> >> @@ -575,6 +597,12 @@ static int sun4i_hdmi_bind(struct device *dev, struct device *master,
> >> >> >> >> >>  		goto err_disable_mod_clk;
> >> >> >> >> >>  	}
> >> >> >> >> >>
> >> >> >> >> >> +	hdmi->ddc_i2c = sun4i_hdmi_get_ddc(dev);
> >> >> >> >> >> +	if (IS_ERR(hdmi->ddc_i2c)) {
> >> >> >> >>
> >> >> >> >> ... which is checked here.
> >> >> >> >>
> >> >> >> >> The property is optional, so the idea was to return null in that case
> >> >> >> >> and use the built-in controller.  If the property exists but some error
> >> >> >> >> occurs, we want to abort rather than proceed with the fallback which
> >> >> >> >> almost certainly won't work.
> >> >> >> >>
> >> >> >> >> Maybe I got something wrong in that logic.
> >> >> >> >
> >> >> >> > Indeed, I just got confused. I guess returning ENODEV in such a case,
> >> >> >> > and testing for that, would make things more obvious.
> >> >> >>
> >> >> >> There's also a case I hadn't thought of: property exists but isn't a
> >> >> >> valid phandle.  What do you think is the correct action in that case?
> >> >> >
> >> >> > I think we would have that one covered. of_parse_phandle will return
> >> >> > !NULL, but then of_get_i2c_adapter_by_node will return NULL since we
> >> >> > wouldn't have an associated i2c adapter to the bogus phandle, and you
> >> >> > are checking for that already.
> >> >>
> >> >> of_parse_phandle() doesn't differentiate between a missing property and
> >> >> an existing non-phandle value.  The following cases are possible with
> >> >> this patch:
> >> >>
> >> >> - ddc-i2c-bus points to valid i2c controller node: use this for ddc
> >> >> - no ddc-i2c-bus property: return NULL, use internal i2c
> >> >> - ddc-i2c-bus exists but isn't a phandle: likewise
> >> >> - ddc-i2c-bus points to a non-i2c-controller node: EPROBE_DEFER
> >> >>
> >> >> The last two cases obviously mean the devicetree is invalid, so perhaps
> >> >> it doesn't matter much what happens then.  I don't think it's possible
> >> >> to distinguish between a well-formed phandle pointing to some bogus node
> >> >> and a good one where the i2c driver hasn't been probed yet.
> >> >
> >> > Ah, I see what you mean now. Yeah, there's not much we can do against
> >> > a wrong / corrupted DT. The DT validation would help prevent the third
> >> > case, and possibly the fourth, but that's basically the only thing we
> >> > can do.
> >>
> >> We need to return -EPROBE_DEFER in the case that everything is fine but
> >> the i2c driver hasn't been probed yet.  From here, that is
> >> indistinguishable from of_parse_phandle() returning a completely bogus
> >> node.
> >
> > That's unfortunate, but if we start to not trust the DT content, we
> > have far worse to deal with.
> >
> >> If the ddc-i2c-bus property doesn't contain a phandle at all, we could
> >> either return an error or fall back to the internal i2c.  The patch does
> >> the latter because that's less code.  I don't think that's any worse
> >> than aborting entirely in terms of user experience.
> >
> > I'm totally fine with the latter behaviour as well. And like I said,
> > the DT validation can help us prevent that case from happening
> > entirely at compilation time.
>
> Well, do you want any changes to the patch or not?

Yeah, return an error pointer with ENODEV instead of NULL, and test
for that.

Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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