linux-kernel - Re: [PATCH 3/3] drivers: iio: adc: Rename the LTC249x iio device

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Date:   Mon, 29 Aug 2022 17:03:34 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     "Regus, Ciprian" <Ciprian.Regus@...log.com>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        "u.kleine-koenig@...gutronix.de" <u.kleine-koenig@...gutronix.de>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] drivers: iio: adc: Rename the LTC249x iio device

On Mon, 29 Aug 2022 06:13:52 +0000
"Regus, Ciprian" <Ciprian.Regus@...log.com> wrote:

> Hi Lars,
> 
> Thanks for the review. I have one question before going further
> with this patch.
> 
> > On 8/22/22 14:51, Ciprian Regus wrote:  
> > > Set the iio device's name based on the chip used.  
> > 
> > While the change is correct it breaks the ABI. This needs a bit of a
> > better explanation what is being done, why, what are the potential
> > problems, why do we want to do it anyway.  
> 
> The reason for this change is to make it easier to identify the device
> (by having a generic name) when using an IIO client. The current name is
> based on the i2c_client's kobj name, which has the format
> "i2c_instance -i2c_address " (e.g. 1-0076). So, the renaming would allow
> for interacting with the device without knowing which bus it's connected
> to.
> 
> Also, as far as I can see, the IIO ABI states that the device's name is
> "Typically a part number".
> 
> For the reason given, would this change be considered an
> improvement, or is the ABI breaking too much of a drawback?
> (in which case I'll just drop this patch)

Hi Ciprian

There are a set of drivers that do it similarly to this (amongst other
things because I wasn't paying attention a few years back).

Generally we've considered them unfixable unfortunately because it's
common to use name to find the device - so userspace breakage is
very likely.

If we are introducing new device support thought here can't be a platform
already relying on the old naming.  So weird though it seems, can you fix
this just for the newly supported models and leave the old ones 'broken'?
(with a big note on why!)  Perhaps do that by leaving .name = NULL for the
older parts and doing a check on that to select behaviour.

Thanks,

Jonathan

> 
> Thanks,
> Ciprian
> >   
> > >
> > > Signed-off-by: Ciprian Regus <ciprian.regus@...log.com>
> > > ---
> > >   drivers/iio/adc/ltc2496.c      | 1 +
> > >   drivers/iio/adc/ltc2497-core.c | 2 +-
> > >   drivers/iio/adc/ltc2497.c      | 2 ++
> > >   drivers/iio/adc/ltc2497.h      | 1 +
> > >   4 files changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/iio/adc/ltc2496.c b/drivers/iio/adc/ltc2496.c
> > > index 98338104c24a..86470f49e8ca 100644
> > > --- a/drivers/iio/adc/ltc2496.c
> > > +++ b/drivers/iio/adc/ltc2496.c
> > > @@ -89,6 +89,7 @@ static void ltc2496_remove(struct spi_device *spi)
> > >
> > >   static struct chip_info ltc2496_info = {
> > >   	.resolution = 16,
> > > +	.name = "ltc2496"
> > >   };
> > >
> > >   static const struct of_device_id ltc2496_of_match[] = {
> > > diff --git a/drivers/iio/adc/ltc2497-core.c b/drivers/iio/adc/ltc2497-core.c
> > > index b2752399402c..6dd9ab601904 100644
> > > --- a/drivers/iio/adc/ltc2497-core.c
> > > +++ b/drivers/iio/adc/ltc2497-core.c
> > > @@ -169,7 +169,7 @@ int ltc2497core_probe(struct device *dev, struct  
> > iio_dev *indio_dev)  
> > >   	struct ltc2497core_driverdata *ddata = iio_priv(indio_dev);
> > >   	int ret;
> > >
> > > -	indio_dev->name = dev_name(dev);
> > > +	indio_dev->name = ddata->chip_info->name;
> > >   	indio_dev->info = &ltc2497core_info;
> > >   	indio_dev->modes = INDIO_DIRECT_MODE;
> > >   	indio_dev->channels = ltc2497core_channel;
> > > diff --git a/drivers/iio/adc/ltc2497.c b/drivers/iio/adc/ltc2497.c
> > > index bb5e0d4301e2..a0aad71c8130 100644
> > > --- a/drivers/iio/adc/ltc2497.c
> > > +++ b/drivers/iio/adc/ltc2497.c
> > > @@ -99,9 +99,11 @@ static int ltc2497_remove(struct i2c_client *client)
> > >   static struct chip_info ltc2497_info[] = {
> > >   	[TYPE_LTC2497] = {
> > >   		.resolution = 16,
> > > +		.name = "ltc2497"
> > >   	},
> > >   	[TYPE_LTC2499] = {
> > >   		.resolution = 24,
> > > +		.name = "ltc2499"
> > >   	}
> > >   };
> > >
> > > diff --git a/drivers/iio/adc/ltc2497.h b/drivers/iio/adc/ltc2497.h
> > > index f4d939cfd48b..0e86e38248ee 100644
> > > --- a/drivers/iio/adc/ltc2497.h
> > > +++ b/drivers/iio/adc/ltc2497.h
> > > @@ -12,6 +12,7 @@ enum chip_type {
> > >
> > >   struct chip_info {
> > >   	u32 resolution;
> > > +	char *name;
> > >   };
> > >
> > >   struct ltc2497core_driverdata {  
> >   
>