Re: [PATCH 2/3] iio: adc: mcp3422: Checking for error on probe

[Maarten Brock <m.brock@vanmierlo.com>]

On 2017-07-03 13:10, jic23@kernel.org wrote:
> On 03.07.2017 09:42, Maarten Brock wrote:
>> On 2017-07-01 12:07, Jonathan Cameron wrote:
>>> On Wed, 28 Jun 2017 23:53:10 +0200
>>> Angelo Compagnucci <angelo.compagnucci@gmail.com> wrote:
>>> 
>>>> Some part of the configuration are not touched after the probe
>>>> and if something goes wrong on writing the initial one,
>>>> the chip will misbehave.
>>>> Adding an error checking ensures that the inital configuration will
>>>> be written correctly. Moreover ensures that a sensible configuration
>>>> will be saved in driver data and used subsequently as intended.
>>> 
>>> Jonathan
>> 
>> Would this fix mean that loading the driver fails if the update_config
>> fails? And thus if the driver is not a module, would require a reboot
>> of the OS?
> Hmm. This is difficult to handle.  If we were waiting on another 
> resource
> coming up that was reflected by the load of a later driver, we 'could'
> use deferred probing. Is that true here?
> 
> Wolfram, any thoughts - the issue here is that the i2c bus master is
> implemented on an FPGA which hasn't necessarily started by the time 
> this
> driver fires up.

In my case it wasn't the master that was implemented in the FPGA, but 
the
channel from the master to the pins. I guess if the master was 
implemented
in the FPGA and not loaded yet, the master driver would fail to load.

> I'm a little loath to put in a rather mysterious deferral if we don't
> need it.  The slave driver definitely feels like the wrong place to be 
> doing
> this.
> 
> What we should be looking at here I think is the i2c bus not being 
> instantiated
> until the fpga is ready.  That way these slave devices wouldn't come up
> until somewhat later in the process and the driver probe will succeed.

I can envision other use-cases, like the device not yet being powered 
up.

> We would normally only retry i2c transactions if we had either:
> * known flaky hardware - the sort of thing that fails once every 100 
> times.

I would consider every I2C device in this category. Maybe not 1 in 100, 
but not 1
in a million either. With open-drain instead of push-pull drivers and 
thus a
relatively high impedance when signals are rising I would expect some 
disturbance
every once in while. And this is most probably perfectly fine when 
taking
samples. But this fix expects the initialization to always pass when it 
could
easily retry again later on and report an error to the application if it 
still
fails.

One could even argue that at probe time this device needs no write to 
the config
register at all. The driver will select the channel and PGA as necessary 
anyway,
which is a good moment to set the CONTINOUS conversion bit 
unconditionally as
well.

Maarten

> * a known reason the device isn't responding (and not able to use
> clock stretching)
> So device is busy doing a conversion and ignores the bus during that.
> 
> Jonathan
> 
>> Seems like a rather steep requirement for something that can be so
>> easily fixed later on by e.g. caching an invalid config channel.
>> There's not even a single retry. And I don't suppose the I2C driver
>> will auto-retry either.
>> 
>> Maarten
>> 
>>>> ---
>>>>  drivers/iio/adc/mcp3422.c | 4 +++-
>>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>> 
>>>> diff --git a/drivers/iio/adc/mcp3422.c b/drivers/iio/adc/mcp3422.c
>>>> index 6737df8..63de705 100644
>>>> --- a/drivers/iio/adc/mcp3422.c
>>>> +++ b/drivers/iio/adc/mcp3422.c
>>>> @@ -382,7 +382,9 @@ static int mcp3422_probe(struct i2c_client 
>>>> *client,
>>>>  		| MCP3422_CHANNEL_VALUE(0)
>>>>  		| MCP3422_PGA_VALUE(MCP3422_PGA_1)
>>>>  		| MCP3422_SAMPLE_RATE_VALUE(MCP3422_SRATE_240));
>>>> -	mcp3422_update_config(adc, config);
>>>> +	err = mcp3422_update_config(adc, config);
>>>> +	if (err < 0)
>>>> +		return err;
>>>> 
>>>>  	err = devm_iio_device_register(&client->dev, indio_dev);
>>>>  	if (err < 0)
>> 
>> --
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