linux-kernel - Re: [PATCH 0/2] Associate ddc adapters with connectors

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Date:   Tue, 25 Jun 2019 16:07:55 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Emil Velikov <emil.velikov@...labora.com>
Cc:     Andrzej Pietrasiewicz <andrzej.p@...labora.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        linux-samsung-soc@...r.kernel.org,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Inki Dae <inki.dae@...sung.com>,
        David Airlie <airlied@...ux.ie>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>, dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel@...ll.ch>, kernel@...labora.com,
        Sean Paul <sean@...rly.run>,
        linux-arm-kernel@...ts.infradead.org, m.szyprowski@...sung.com
Subject: Re: [PATCH 0/2] Associate ddc adapters with connectors

On Tue, Jun 25, 2019 at 02:36:39PM +0100, Emil Velikov wrote:
> On 2019/06/25, Andrzej Pietrasiewicz wrote:
> > Hi Russell,
> > 
> > W dniu 25.06.2019 o 12:03, Russell King - ARM Linux admin pisze:
> > > On Tue, Jun 25, 2019 at 11:46:34AM +0200, Andrzej Pietrasiewicz wrote:
> > > > It is difficult for a user to know which of the i2c adapters is for which
> > > > drm connector. This series addresses this problem.
> > > > 
> > > > The idea is to have a symbolic link in connector's sysfs directory, e.g.:
> > > > 
> > > > ls -l /sys/class/drm/card0-HDMI-A-1/i2c-2
> > > > lrwxrwxrwx 1 root root 0 Jun 24 10:42 /sys/class/drm/card0-HDMI-A-1/i2c-2 \
> > > > 	-> ../../../../soc/13880000.i2c/i2c-2
> > > 
> > > Don't you want the symlink name to be "i2c" or something fixed, rather
> > > than the name of the i2c adapter?  Otherwise, you seem to be encumbering
> > > userspace with searching the directory to try and find the symlink.
> > > 
> > 
> > Thank you for your comment. So you imagine something on the lines of:
> > 
> > lrwxrwxrwx 1 root root 0 Jun 24 10:42 /sys/class/drm/card0-HDMI-A-1/ddc \
> >  	-> ../../../../soc/13880000.i2c/i2c-2
> > 
> > ?
> > 
> Fwiw my Intel machine lists a number of i2c devices:
> /sys/class/drm/card0-DP-1/i2c-6
> /sys/class/drm/card0-DP-2/i2c-7
> /sys/class/drm/card0-eDP-1/i2c-5
> 
> Note: I haven't looked _if_ they relate to ones you're proposing here.
> 
> One thing worth mentioning is, the ones I've seen are not symlinks to
> another sysfs entries. And there aren't any i2c nodes in /dev ...
> 
> Just a random food for thought :-)

Those are the i2c-over-dp-aux controllers. I think we want to list these
too.

Btw to make this more useful maybe some default implementations for
get_modes which automatically dtrt, as a helper? Probably could use that
to squash quite a bit of boilerplate.

Otherwise I like this. Biggest problem I'm seeing here is rolling this out
everywhere, this is a lot of work. And without widespread adoptions it's
not terribly useful for userspace.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch