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

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Message-ID: <20170206175533.GZ3140@e110455-lin.cambridge.arm.com>
Date:   Mon, 6 Feb 2017 17:55:33 +0000
From:   Liviu Dudau <liviu.dudau@....com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Rob Herring <robh@...nel.org>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel@...ts.freedesktop.org, devicetree@...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>
Subject: Re: [PATCH 3/5] drm: convert drivers to use of_graph_get_remote_node

On Mon, Feb 06, 2017 at 05:34:07PM +0000, Russell King - ARM Linux wrote:
> On Mon, Feb 06, 2017 at 05:23:06PM +0000, Liviu Dudau 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:
> > > > > -	/* 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).
> 
> Not really "due to testing" but if you run a real distro, they tend to
> have a multi-threaded behaviour when loading kernel modules at boot.

Yeah, a lot of times I'm using a toy "distribution" (buildroot) as it boots
faster under ARM models than a "real" (read systemd-based) distro would.

> 
> > 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.
> 
> -EAGAIN doesn't get you that, and in any case, solving that problem is
> exactly why the component API exists - so that DRM only comes up once
> all the necessary components are available.
> 
> -EAGAIN also doesn't get you that from inside a probe function - such
> an error will be reported in the kernel log, and no further action
> will be taken (the device driver probe will be failed, and not
> automatically retried.

I stand corrected on the behaviour of the driver then. That was the original
intent, to generate a re-probe of the driver.

> 
> The only case that we automatically retry is if a driver returns
> -EPROBE_DEFER.  Everything else causes a probe failure.

OK, I will fix the driver if Rob's patch still requires it.

Best regards,
Liviu

> 
> -- 
> RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
> according to speedtest.net.

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