Re: [PATCH v5] i2c: imx: make bus recovery through pinctrl optional

[Stefan Agner]

On 2016-09-08 16:57, Leo Li wrote:
> On Thu, Sep 8, 2016 at 5:39 PM, Stefan Agner <stefan@xxxxxxxx> wrote:
>> On 2016-09-06 15:40, Leo Li wrote:
>>> On Tue, Sep 6, 2016 at 4:51 PM, Stefan Agner <stefan@xxxxxxxx> wrote:
>>>> On 2016-09-06 13:06, Leo Li wrote:
>>>>> On Tue, Sep 6, 2016 at 1:58 PM, Uwe Kleine-KÃnig
>>>>> <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
>>>>>> On Fri, Aug 19, 2016 at 05:05:22PM -0500, Li Yang wrote:
>> <snip>
>>>>>>> @@ -1081,8 +1090,11 @@ static int i2c_imx_probe(struct platform_device *pdev)
>>>>>>>               return ret;
>>>>>>>       }
>>>>>>>
>>>>>>> +     /* optional bus recovery feature through pinctrl */
>>>>>>>       i2c_imx->pinctrl = devm_pinctrl_get(&pdev->dev);
>>>>>>> -     if (IS_ERR(i2c_imx->pinctrl)) {
>>>>>>> +     /* bailout on -ENOMEM or -EPROBE_DEFER, continue for other errors */
>>>>>>> +     if (PTR_ERR(i2c_imx->pinctrl) == -ENOMEM ||
>>>>>>> +                     PTR_ERR(i2c_imx->pinctrl) == -EPROBE_DEFER) {
>>>>>>>               ret = PTR_ERR(i2c_imx->pinctrl);
>>>>>>>               goto clk_disable;
>>>>>>>       }
>>>>>>
>>>>>> devm_pinctrl_get might return the following error-valued pointers:
>>>>>>  - -EINVAL
>>>>>>  - -ENOMEM
>>>>>>  - -ENODEV
>>>>>>  - -EPROBE_DEFER
>>>>>>
>>>>>> There are several error paths returning -EINVAL, one is when an invalid
>>>>>> phandle is used. Do you really want to ignore that?
>>>>>>
>>>>>> IMO error handling is better done with inverse logic, that is continue
>>>>>> on some explicit error, bail out on all unknown stuff. This tends to be
>>>>>> more robust. Also the comment should be improved to not explain that for
>>>>>> -ENOMEM and -EPROBE_DEFER we bail out (which should be obvious for
>>>>>> anyone who can read C) but to explain why.
>>>>>
>>>>> What you said is true for normal error handling, but in this scenario
>>>>> it is intentional to ignore all pinctrl related errors except critical
>>>>> ones because failing to have pinctrl for an optional feature shouldn't
>>>>> impact the function of normal i2c.  We choose to catch -ENOMEM because
>>>>> the error could also cause problem for i2c probe, and -EPROBE_DEFER
>>>>> because it's possible that the pinctrl will be ready later and we want
>>>>> to give it a chance.  The i2c driver really don't care why the pinctrl
>>>>> was not usable.  I thought I added comment before the
>>>>
>>>> I don't agree. E.g. -EINVAL would appear if you pass devm_pinctrl_get an
>>>> invalid device. Currently you would silently ignore that, which is not
>>>> what you want.
>>>
>>> It is not silently ignored, there will be a message printed out saying
>>> pinctrl is not available and bus recovery is not supported.  On the
>>> contrary, without this change the entire i2c driver fails to work
>>> silently if pinctrl is somehow not working.  And if the system is so
>>> broken that the pointer to the i2c device is NULL, the probe of i2c
>>> would have already failed before this point.  We shouldn't count on an
>>> optional function of the driver to catch fundamental issues like this.
>>>
>>>>
>>>> You want to get the pinctrl in any case expect there isn't one. And that
>>>> is how you should formulate your if statement.
>>>>
>>>> /*
>>>>  * It is ok if no pinctrl device is available. We'll not be able to use
>>>> the
>>>>  * bus recovery feature, but otherwise the driver works fine...
>>>>  */
>>>> if (PTR_ERR(i2c_imx->pinctrl) != -ENODEV)
>>>
>>> I agree that there could be other possibilities that the pinctrl
>>> failed to work beside the reason I described in the commit
>>> message(platform doesn't support pinctrl at all).  But I don't think
>>> any of them other than the -ENOMEM and -EPROBE_DEFER deserves a bail
>>> out for the entire i2c driver.
>>
>> FWIW, I disagree. If there is pinctrl defined, you want be sure that it
>> gets applied properly, no matter what. E.g. when devm_pinctrl_get return
>> EINVAL (Uwe's example) the driver will continue and likely fail in
>> mysterious ways later on because the pins have not been muxed properly.
>> The driver should not load in that situation so that the developer is
>> forced to fix his mistakes. The only reason to bail out here is if there
>> is no pin controller (ENODEV). And it seems that Uwe also tends to that
>> solution.
> 
> With this patch the i2c bus recovery feature will be disabled if the
> devm_pinctrl_get() fails.  The pin mux setting will not be changed in
> either i2c probe stage or at runtime.  I don't think it can cause any
> trouble to normal I2C operation.  IMO, it is not good to *force*

If you have a pin controller, and you make a typo in your device tree
which leads to a wrong phandle and devm_pinctrl_get returning -EINVAL,
the system won't mux the pins... And that will certainly affect normal
I2C operation!

> people fix problem that they don't really care by deliberately enlarge
> the problem.  That's why we don't panic() on any error we found.  For
> those who do care about the bus recovery, they can get the information
> from the console.

IMHO, it is just stupid to ignore errors and then let the developer
later on trace back what the initial issue was. Error out early is a
common sense software design principle...

I am not asking for a panic(), I am just suggesting to only ignore
pinctrl if it returns -ENODEV, the case you care are about.

--
Stefan