linux-kernel - Re: [PATCH 3/5] drm: convert drivers to use of_graph_get_remote

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Date:   Mon, 6 Feb 2017 11:42:36 -0600
From:   Rob Herring <robh@...nel.org>
To:     Liviu Dudau <liviu.dudau@....com>
Cc:     David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Archit Taneja <architt@...eaurora.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Inki Dae <inki.dae@...sung.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Javier Martinez Canillas <javier@....samsung.com>,
        Stefan Agner <stefan@...er.ch>,
        Alison Wang <alison.wang@...escale.com>,
        Xinliang Liu <z.liuxinliang@...ilicon.com>,
        Rongrong Zou <zourongrong@...il.com>,
        Xinwei Kong <kong.kongxinwei@...ilicon.com>,
        Chen Feng <puck.chen@...ilicon.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        CK Hu <ck.hu@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Marek Vasut <marex@...x.de>,
        Mark Yao <mark.yao@...k-chips.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Mali DP Maintainers <malidp@...s.arm.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Carlo Caione <carlo@...one.org>,
        Kevin Hilman <khilman@...libre.com>,
        Rob Clark <robdclark@...il.com>, Jyri Sarha <jsarha@...com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Eric Anholt <eric@...olt.net>,
        Russell King <rmk+kernel@...linux.org.uk>
Subject: Re: [PATCH 3/5] drm: convert drivers to use of_graph_get_remote_node

On Mon, Feb 6, 2017 at 11:23 AM, Liviu Dudau <liviu.dudau@....com> wrote:
> On Mon, Feb 06, 2017 at 11:09:49AM -0600, Rob Herring wrote:
>> On Mon, Feb 06, 2017 at 10:29:33AM +0000, Liviu Dudau wrote:
>> > On Fri, Feb 03, 2017 at 09:36:33PM -0600, Rob Herring wrote:
>> > > Convert drivers to use the new of_graph_get_remote_node() helper
>> > > instead of parsing the endpoint node and then getting the remote device
>> > > node. Now drivers can just specify the device node and which
>> > > port/endpoint and get back the connected remote device node. The details
>> > > of the graph binding are nicely abstracted into the core OF graph code.
>> > >
>> > > This changes some error messages to debug messages (in the graph core).
>> > > Graph connections are often "no connects" depending on the particular
>> > > board, so we want to avoid spurious messages. Plus the kernel is not a
>> > > DT validator.

[...]

>> > > - /* add the remote encoder port as component */
>> > > - port = of_graph_get_remote_port_parent(ep);
>> > > - of_node_put(ep);
>> > > - if (!port || !of_device_is_available(port)) {
>> > > -         of_node_put(port);
>> > > -         return -EAGAIN;
>> >
>> > The HDLCD change looks reasonable except for this -EAGAIN business. I'll have to
>> > test your changes on my setup to see how this affects having the encoder as a module.
>>
>> What are you expecting to happen with -EAGAIN? This one was a bit of an
>> oddball.
>
> When both the HDLCD and the TDA998x drivers are compiled as modules, the order in which
> they are inserted can be somewhat random (due to testing). It is at that time when you
> want the probe of HDLCD to be retried on the insmod-ing of the tda998x.ko rather than
> fail entirely.
>
>>
>> This condition would only change if you had an overlay. That's a use
>> case that needs to be handled in a common way ('cause I don't want to
>> clean-up every driver doing overlays in their own way latter). Just
>> having "status" changing at runtime would have all sorts of implications
>> in the kernel.
>
> Hmm, not sure what you mean here with overlays. Are you thinking that the
> remote port is initially disabled and then re-enabled by an overlay? That is
> not the only way of_device_is_available() can fail, see above regarding modules.

Russell pretty much answered most of this, but specifically for
of_device_is_available, the only way of_device_is_available() can
change is a DT change with "status" changing. The only way
of_graph_get_remote_port_parent changes is also from a DT change.

Rob