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Date: Tue, 16 Apr 2024 12:08:50 -0500
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Subject: Re: [PATCH 00/31] Remove use of i2c_match_id in HWMON
To: Guenter Roeck <linux@roeck-us.net>, Rob Herring <robh@kernel.org>
CC: Jean Delvare <jdelvare@suse.com>, Juerg Haefliger <juergh@proton.me>,
        "Riku Voipio" <riku.voipio@iki.fi>, <linux-hwmon@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
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From: Andrew Davis <afd@ti.com>
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On 4/16/24 9:16 AM, Guenter Roeck wrote:
> On Mon, Apr 08, 2024 at 04:49:43AM -0700, Guenter Roeck wrote:
>> On Wed, Apr 03, 2024 at 05:06:43PM -0500, Andrew Davis wrote:
>>> On 4/3/24 4:30 PM, Guenter Roeck wrote:
>>>> On Wed, Apr 03, 2024 at 03:36:02PM -0500, Andrew Davis wrote:
>>>>> Hello all,
>>>>>
>>>>> Goal here is to remove the i2c_match_id() function from all drivers.
>>>>> Using i2c_get_match_data() can simplify code and has some other
>>>>> benefits described in the patches.
>>>>>
>>>>
>>>> The return value from i2c_match_id() is typically an integer (chip ID)
>>>> starting with 0. Previously it has been claimed that this would be
>>>> unacceptable for i2c_get_match_data(), and chip IDs were changed to start
>>>> with 1. Commit ac0c26bae662 ("hwmon: (lm25066) Use i2c_get_match_data()")
>>>> is an example. Either this series is wrong, or the previous claim that
>>>> chip IDs (i.e., the content of .driver_data or .data) must not be 0 was
>>>> wrong. Which one is it ? I find it very confusing that the chip type for
>>>> some drivers now starts with 1 and for others with 0. Given that, I am not
>>>> inclined to accept this series unless it is explained in detail why the
>>>> chip type enum in, for example, drivers/hwmon/pmbus/lm25066.c has to start
>>>> with one but is ok to start with 0 for all drivers affected by this
>>>> series. Quite frankly, even if there is some kind of explanation, I am not
>>>> sure if I am going to accept it because future driver developers won't
>>>> know if they have to start chip types with 0 or 1.
>>>>
>>>
>>> i2c_get_match_data() has no issue with returning 0 when the driver_data
>>> for the match is also 0 (as it will be when the chip type is 0 here).
>>>
>>> The confusion might be that returning 0 is also considered a failure code.
>>> This is a problem in general with returning errors in-band with data, and
>>> that is nothing new as i2c_match_id() does the same thing.
>>>
>>> Actually, i2c_match_id() is worse as most of these drivers take the result
>>> from that and immediately dereference it. Meaning if i2c_match_id() ever did
>>> failed to find a match, they would crash before this series. Luckily i2c_match_id()
>>> can't fail to find a match as far as I can tell, and so for the same reason
>>> neither can i2c_get_match_data(), which means if 0 is returned it is always
>>> because the chip ID was actually 0.
>>>
>>> At some point we should switch all the *_get_match_data() functions to
>>> return an error code and put the match if found as a argument pointer.
>>> Forcing everyone to changing the chip type to avoid 0 as done in
>>> ac0c26bae662 is the wrong way to fix an issue like that.
>>>
>>
>> That doesn't really answer my question. It does not explain why it was
>> necessary to change the chip ID base for other drivers from 0 to 1,
>> but not for the drivers in this series. I fail to see the difference,
>> and I have to assume that others looking into the code will have the
>> same problem.
>>
> 
> Just to follow up: I am not going to apply this series until I understand
> why the chip ID range had to be changed from 0.. to 1.. for other hardware
> monitoring drivers (lm25066, nct6775) but not for the drivers changed
> in this series. I have been telling people that chip IDs need to start
> with 1 if i2c_get_match_data() is used. I'll need understand when and
> why this is needed to be able to provide guidance to other developers.
> 

I was hoping one of those patch authors that made those 0->1 changes
would speak up (+Rob), I can't know what their thinking was, only
offer my best guess as I did above.

Andrew

> Guenter