linux-kernel - Re: [PATCH v2 2/2] hwmon: iio: Add labels from IIO channels

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Date: Fri, 28 Jun 2024 20:08:34 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: Sean Anderson <sean.anderson@...ux.dev>
Cc: Guenter Roeck <linux@...ck-us.net>, Jean Delvare <jdelvare@...e.com>,
 linux-iio@...r.kernel.org, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>
Subject: Re: [PATCH v2 2/2] hwmon: iio: Add labels from IIO channels

On Thu, 27 Jun 2024 14:37:01 -0400
Sean Anderson <sean.anderson@...ux.dev> wrote:

> On 6/24/24 15:24, Jonathan Cameron wrote:
> > On Mon, 24 Jun 2024 11:47:39 -0700
> > Guenter Roeck <linux@...ck-us.net> wrote:
> >   
> >> On 6/24/24 10:46, Sean Anderson wrote:  
> >> > Add labels from IIO channels to our channels. This allows userspace to
> >> > display more meaningful names instead of "in0" or "temp5".
> >> > 
> >> > Although lm-sensors gracefully handles errors when reading channel
> >> > labels, the ABI says the label attribute
> >> >     
> >> >> Should only be created if the driver has hints about what this voltage
> >> >> channel is being used for, and user-space doesn't.    
> >> > 
> >> > Therefore, we test to see if the channel has a label before
> >> > creating the attribute.
> >> >     
> >> 
> >> FWIW, complaining about an ABI really does not belong into a commit
> >> message. Maybe you and lm-sensors don't care about error returns when
> >> reading a label, but there are other userspace applications which may
> >> expect drivers to follow the ABI. Last time I checked, the basic rule
> >> was still "Don't break userspace", and that doesn't mean "it's ok to
> >> violate / break an ABI as long as no one notices".
> >>   
> >> > Signed-off-by: Sean Anderson <sean.anderson@...ux.dev>
> >> > ---
> >> > 
> >> > Changes in v2:
> >> > - Check if the label exists before creating the attribute
> >> > 
> >> >   drivers/hwmon/iio_hwmon.c | 45 ++++++++++++++++++++++++++++++++++-----
> >> >   1 file changed, 40 insertions(+), 5 deletions(-)
> >> > 
> >> > diff --git a/drivers/hwmon/iio_hwmon.c b/drivers/hwmon/iio_hwmon.c
> >> > index 4c8a80847891..5722cb9d81f9 100644
> >> > --- a/drivers/hwmon/iio_hwmon.c
> >> > +++ b/drivers/hwmon/iio_hwmon.c
> >> > @@ -33,6 +33,17 @@ struct iio_hwmon_state {
> >> >   	struct attribute **attrs;
> >> >   };
> >> >   
> >> > +static ssize_t iio_hwmon_read_label(struct device *dev,
> >> > +				  struct device_attribute *attr,
> >> > +				  char *buf)
> >> > +{
> >> > +	struct sensor_device_attribute *sattr = to_sensor_dev_attr(attr);
> >> > +	struct iio_hwmon_state *state = dev_get_drvdata(dev);
> >> > +	struct iio_channel *chan = &state->channels[sattr->index];
> >> > +
> >> > +	return iio_read_channel_label(chan, buf);
> >> > +}
> >> > +    
> >> 
> >> I personally find it a bit kludgy that an in-kernel API would do a
> >> sysfs write like this and expect a page-aligned buffer as parameter,
> >> but since Jonathan is fine with it:  
> > 
> > That's a good point that I'd not picked up on and it probably makes sense
> > to address that before it bites us on some other subsystem.
> > 
> > It was more reasonable when the only path was to a light wrapper that went
> > directly around the sysfs callback. Now we are wrapping these up for more
> > general use we should avoid that restriction.
> > 
> > Two approaches to that occur to me.
> > 1) Fix up read_label() everywhere to not use sysfs_emit and take a size
> >    of the buffer to print into. There are only 11 implementations so
> >    far so this should be straight forward.  
> 
> This API is the same as the existing iio_read_channel_ext_info. It is
> used for the same purpose: forwarding sysfs reads/writes from one
> device to another (see e.g. iio-mux and iio-rescale). ext_info is used
> by around 85 drivers, so it is not so trivial to change the API. While I
> agree that the current API is unusual, it's not too bad given that we
> get the same guarantees from device_attribute.show.

Fair enough.  Maybe we can clean this up at somepoint but let's not do 
it today. Applied to the togreg branch of iio.git and pushed out as testing for
0-day to poke at it and maybe find something we missed.

Jonathan

> 
> --Sean
> 
> > 2) Add a bounce buffer so we emit into a suitable size for sysfs_emit()
> >   then reprint from there into a buffer provided via this interface with
> >   the appropriate size provided.  This one is clunky and given the relatively
> >   few call sits I think fixing it via option 1 is the better route forwards.  
> 
>