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Sender: Guenter Roeck <groeck7@gmail.com>
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Date: Thu, 6 Jun 2024 10:17:39 -0700
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Subject: Re: [PATCH 00/31] Remove use of i2c_match_id in HWMON
From: Guenter Roeck <linux@roeck-us.net>
To: Andrew Davis <afd@ti.com>
Cc: Rob Herring <robh@kernel.org>, 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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On 4/18/24 06:42, Guenter Roeck wrote:
> On Tue, Apr 16, 2024 at 12:08:50PM -0500, Andrew Davis wrote:
>> 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.
>>
> 
> I can see three possibilities.
> 
> - Chip IDs must start with 1 if i2c_get_match_data() is used, as I was told
>    previously. If so, this series is wrong.
> - It is ok for chip IDs to start with 0. If so, what I have been told
>    previously is wrong, and the patches changing chip IDs to start with 1
>    can and should be partially reverted to avoid confusion.
> - Chip IDs must sometimes, but not always, start with 1. If so, the
>    conditions will have to be documented to help driver developers decide
>    the valid starting value and to be able to determine if all the patches
>    in this series follow the rules.
> 
> Someone will have to step up and explain to me which one it is.
> 

Unfortunately, that step-in never happened. Instead, I keep getting patches
similar to this series. So, I applied the series, and I'll submit
patches to update all drivers to have the chip ID enum start with 0,
and/or to remove the complexity introduced under the assumption that it must
not be 0 / NULL.

Guenter