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Date:   Wed, 29 Aug 2018 14:38:43 +0300
From:   Sakari Ailus <sakari.ailus@iki.fi>
To:     Philippe De Muyter <phdm@macq.eu>
Cc:     Hans Verkuil <hverkuil@xs4all.nl>,
        Luca Ceresoli <luca@lucaceresoli.net>,
        linux-media@vger.kernel.org, Leon Luo <leonl@leopardimaging.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        linux-kernel@vger.kernel.org, laurent.pinchart@ideasonboard.com
Subject: Re: [PATCH 3/7] media: imx274: don't hard-code the subdev name to
 DRIVER_NAME
Message-ID: <20180829113843.4v63cxf3clvbzbtd@valkosipuli.retiisi.org.uk>
References: <20180824163525.12694-1-luca@lucaceresoli.net>
 <20180824163525.12694-4-luca@lucaceresoli.net>
 <20180825144915.tq7m5jlikwndndzq@valkosipuli.retiisi.org.uk>
 <799f4d1a-b91d-0404-7ef0-965d123319da@lucaceresoli.net>
 <113d3e05-9331-bd54-0e49-46c5e132339f@xs4all.nl>
 <20180828160255.GA9763@frolo.macqel>
 <20180829110721.zlpqfmusaw4nh7et@valkosipuli.retiisi.org.uk>
 <20180829112936.GA15244@frolo.macqel>
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On Wed, Aug 29, 2018 at 01:29:36PM +0200, Philippe De Muyter wrote:
> Hi Sakari,
> 
> On Wed, Aug 29, 2018 at 02:07:21PM +0300, Sakari Ailus wrote:
> > Hi Philippe,
> > 
> > On Tue, Aug 28, 2018 at 06:02:55PM +0200, Philippe De Muyter wrote:
> > > Hi Hans, Sakari and Luca
> > > 
> > > On Tue, Aug 28, 2018 at 11:22:28AM +0200, Hans Verkuil wrote:
> > > > On 26/08/18 22:41, Luca Ceresoli wrote:
> > > > > Hi Sakari,
> > > > > 
> > > > > On 25/08/2018 16:49, Sakari Ailus wrote:
> > > > >> Hi Luca,
> > > > >>
> > > > >> On Fri, Aug 24, 2018 at 06:35:21PM +0200, Luca Ceresoli wrote:
> > > > >>> Forcibly setting the subdev name to DRIVER_NAME (i.e. "IMX274") makes
> > > > >>> it non-unique and less informative.
> > > > >>>
> > > > >>> Let the driver use the default name from i2c, e.g. "IMX274 2-001a".
> > > > >>>
> > > > >>> Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
> > > > >>> ---
> > > > >>>  drivers/media/i2c/imx274.c | 1 -
> > > > >>>  1 file changed, 1 deletion(-)
> > > > >>>
> > > > >>> diff --git a/drivers/media/i2c/imx274.c b/drivers/media/i2c/imx274.c
> > > > >>> index 9b524de08470..570706695ca7 100644
> > > > >>> --- a/drivers/media/i2c/imx274.c
> > > > >>> +++ b/drivers/media/i2c/imx274.c
> > > > >>> @@ -1885,7 +1885,6 @@ static int imx274_probe(struct i2c_client *client,
> > > > >>>  	imx274->client = client;
> > > > >>>  	sd = &imx274->sd;
> > > > >>>  	v4l2_i2c_subdev_init(sd, client, &imx274_subdev_ops);
> > > > >>> -	strlcpy(sd->name, DRIVER_NAME, sizeof(sd->name));
> > > > >>>  	sd->flags |= V4L2_SUBDEV_FL_HAS_DEVNODE | V4L2_SUBDEV_FL_HAS_EVENTS;
> > > > >>>  
> > > > >>>  	/* initialize subdev media pad */
> > > > >>
> > > > >> This ends up changing the entity as well as the sub-device name which may
> > > > >> well break applications.
> > > > > 
> > > > > Right, unfortunately.
> > > > > 
> > > > >> On the other hand, you currently can't have more
> > > > >> than one of these devices on a media device complex due to the name being
> > > > >> specific to a driver, not the device.
> > > > >>
> > > > >> An option avoiding that would be to let the user choose by e.g. through a
> > > > >> Kconfig option would avoid having to address that, but I really hate adding
> > > > >> such options.
> > > > > 
> > > > > I agree adding a Kconfig option just for this would be very annoying.
> > > > > However I think the issue affects a few other drivers (sr030pc30.c and
> > > > > s5c73m3-core.c apparently), thus maybe one option could serve them all.
> > > > > 
> > > > >> I wonder what others think. If anyone ever needs to add another on a board
> > > > >> so that it ends up being the part of the same media device complex
> > > > >> (likely), then changing the name now rather than later would be the least
> > > > >> pain. In this case I'd be leaning (slightly) towards accepting the patch
> > > > >> and hoping there wouldn't be any fallout... I don't see any board (DT)
> > > > >> containing imx274, at least not in the upstream kernel.
> > > > > 
> > > > > I'll be OK with either decision. Should we keep it as is, then I think a
> > > > > comment before that line would be appropriate to clarify it's not
> > > > > correct but it is kept for backward userspace compatibility. This would
> > > > > help avoid new driver writers doing the same mistake, and prevent other
> > > > > people to send another patch like mine.
> > > > 
> > > > In this end, this is a driver bug. I would just fix this, but add a comment
> > > > that states the old name and why it was changed. No need for a dev_info
> > > > IMHO.
> > > > 
> > > > It would be nice if you can check if the same mistake is made in other drivers,
> > > > and update those as well. It's easier if this is all done at the same time.
> > > > 
> > > 
> > > Then we should probably also apply the following patch I submitted :
> > > 
> > > "media: v4l2-common: v4l2_spi_subdev_init : generate unique name"
> > > 	https://patchwork.kernel.org/patch/10553035/
> > > 
> > > and perhaps
> > > 
> > > "media: v4l2-common: simplify v4l2_i2c_subdev_init name generation"
> > > 	https://patchwork.kernel.org/patch/10553037/
> > 
> > The problem with this patch is that the existing naming scheme is very
> > similar while the new one offers no tangible benefits apart from being in
> > line with the rest of the kernel. That's still not a benefit for uAPI:
> > changing the name is certain to break user space applications.
> 
> I agree with you on the patch for v4l2_i2c_subdev_init (I wrote 'perhaps'),
> but you don't say anything on the one about v4l2_spi_subdev_init :), which
> fixes an actual bug.  I have 2 identical SPI-controlled sensors on the
> same board, and without my patch they get the same subdev name.  Of course,
> I could fix that in the sensor driver itself, but that's not what we want,
> or do we ?

Good point. I missed the naming of the SPI devices ignored any bus
information there. I'm rather inclined towards taking the SPI patch. Hans,
Mauro, Laurent; any opinion on that?

-- 
Sakari Ailus
e-mail: sakari.ailus@iki.fi