linux-kernel - Re: [PATCH] v4l: async: make v4l2 coexists with devicetree nodes in a dt overlay

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Message-ID: <20161125082121.GB16630@valkosipuli.retiisi.org.uk>
Date:   Fri, 25 Nov 2016 10:21:21 +0200
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Javi Merino <javi.merino@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Javier Martinez Canillas <javier@....samsung.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        laurent.pinchart@...asonboard.com
Subject: Re: [PATCH] v4l: async: make v4l2 coexists with devicetree nodes in
 a dt overlay

Hi Javi,

On Wed, Nov 23, 2016 at 04:15:11PM +0000, Javi Merino wrote:
> On Wed, Nov 23, 2016 at 05:10:42PM +0200, Sakari Ailus wrote:
> > Hi Javi,
> 
> Hi Sakari,
> 
> > On Wed, Nov 23, 2016 at 10:09:57AM +0000, Javi Merino wrote:
> > > In asd's configured with V4L2_ASYNC_MATCH_OF, if the v4l2 subdev is in
> > > a devicetree overlay, its of_node pointer will be different each time
> > > the overlay is applied.  We are not interested in matching the
> > > pointer, what we want to match is that the path is the one we are
> > > expecting.  Change to use of_node_cmp() so that we continue matching
> > > after the overlay has been removed and reapplied.
> > > 
> > > Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> > > Cc: Javier Martinez Canillas <javier@....samsung.com>
> > > Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > > Signed-off-by: Javi Merino <javi.merino@...nel.org>
> > > ---
> > > Hi,
> > > 
> > > I feel it is a bit of a hack, but I couldn't think of anything better.
> > > I'm ccing devicetree@ and Pantelis because there may be a simpler
> > > solution.
> > 
> > First I have to admit that I'm not an expert when it comes to DT overlays.
> > 
> > That said, my understanding is that the sub-device and the async sub-device
> > are supposed to point to the exactly same DT node. I wonder if there's
> > actually anything wrong in the current code.
> > 
> > If the overlay has changed between probing the driver for the async notifier
> > and the async sub-device, there should be no match here, should there? The
> > two nodes actually point to a node in a different overlay in that case.
> 
> Overlays are parts of the devicetree that can be added and removed.
> When the overlay is applied, the camera driver is probed and does
> v4l2_async_register_subdev().  However, v4l2_async_belongs() fails.
> The problem is with comparing pointers.  I haven't looked at the
> implementation of overlays in detail, but what I see is that the
> of_node pointer changes when you remove and reapply an overlay (I
> guess it's dynamically allocated and when you remove the overlay, it's
> freed).

The concern here which we were discussing was whether the overlay should be
relied on having compliant configuration compared to the part which was not
part of the overlay.

As external components are involved, quite possibly also the ISP DT node
will require changes, not just the image source (TV tuner, camera sensor
etc.). This could be because of number of CSI-2 lanes or parallel bus width,
for instance.

I'd also be interested in having an actual driver implement support for
removing and adding a DT overlay first so we'd see how this would actually
work. We need both in order to be able to actually remove and add DT
overlays _without_ unbinding the ISP driver. Otherwise it should already
work in the current codebase.

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@....fi	XMPP: sailus@...iisi.org.uk