linux-kernel - Re: [PATCH v4 02/10] drm/ingenic: Add support for JZ4780 and HDMI output

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Message-Id: <17BF1D7A-2057-448B-9FD2-907DE0EFD281@goldelico.com>
Date:   Tue, 28 Sep 2021 14:06:03 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Harry Wentland <harry.wentland@....com>,
        Sam Ravnborg <sam@...nborg.org>,
        Maxime Ripard <maxime@...no.tech>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Paul Boddie <paul@...die.org.uk>, devicetree@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        letux-kernel@...nphoenux.org, Jonas Karlman <jonas@...boo.se>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4 02/10] drm/ingenic: Add support for JZ4780 and HDMI
 output

Hi Paul,

> Am 28.09.2021 um 12:21 schrieb H. Nikolaus Schaller <hns@...delico.com>:
> 
>>> @@ -1492,10 +1555,16 @@ static int ingenic_drm_init(void)
>>> {
>>> 	int err;
>>> +	if (IS_ENABLED(CONFIG_DRM_INGENIC_DW_HDMI)) {
>>> +		err = platform_driver_register(ingenic_dw_hdmi_driver_ptr);
>>> +		if (err)
>>> +			return err;
>>> +	}
>> 
>> I don't see why you need to register the ingenic-dw-hdmi driver here. Just register it in the ingenic-dw-hdmi driver.
> 
> Ok, I never though about this (as the code was not from me). We apparently just followed the IPU code pattern (learning by example).
> 
> It indeed looks not necessary and would also avoid the ingenic_dw_hdmi_driver_ptr dependency.
> 
> But: what is ingenic_ipu_driver_ptr then good for?
> 
> If we can get rid of this as well, we can drop patch 1/10 ("drm/ingenic: Fix drm_init error path if IPU was registered") completely.

A quick test shows that it *is* required. At least if I configure everything as modules.
But like you I can't explain why.

Well, just a very rough idea (may be wrong): the bridge chain is not like an i2c bus and
clients are not automatically loaded/probed if linked in the device tree. Therefore the
consumer (ingenic_drm_drv) must register the "clients" like IPU and HDMI.

BR,
Nikolaus