Re: [PATCH v2 2/3] Input: aw86938 - add driver for Awinic AW86938

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 2/2/26 12:04 PM, Dmitry Torokhov wrote:
> On Mon, Feb 02, 2026 at 11:19:36AM +0100, Konrad Dybcio wrote:
>> On 2/2/26 11:14 AM, Luca Weiss wrote:
>>> Hi Konrad,
>>>
>>> On Mon Feb 2, 2026 at 11:12 AM CET, Konrad Dybcio wrote:
>>>> On 2/1/26 2:49 AM, Dmitry Torokhov wrote:
>>>>> Hi Griffin,
>>>>>
>>>>> On Wed, Jan 28, 2026 at 04:51:14PM +0100, Griffin Kroah-Hartman wrote:
>>>>>> @@ -717,9 +746,19 @@ static int aw86927_detect(struct aw86927_data *haptics)
>>>>>>  
>>>>>>  	chip_id = be16_to_cpu(read_buf);
>>>>>>  
>>>>>> -	if (chip_id != AW86927_CHIPID) {
>>>>>> -		dev_err(haptics->dev, "Unexpected CHIPID value 0x%x\n", chip_id);
>>>>>> -		return -ENODEV;
>>>>>> +	switch (haptics->model) {
>>>>>> +	case AW86927:
>>>>>> +		if (chip_id != AW86927_CHIPID) {
>>>>>> +			dev_err(haptics->dev, "Unexpected CHIPID value 0x%x\n", chip_id);
>>>>>> +			return -ENODEV;
>>>>>> +		}
>>>>>
>>>>> If we are able to query chip ID why do we need to have separate
>>>>> compatibles? I would define chip data structure with differences between
>>>>> variants and assign and use it instead of having separate compatible.
>>>>
>>>> dt-bindings guidelines explicitly call for this, a chipid comparison
>>>> then works as a safety net
>>>
>>> Are you saying, that
>>>
>>> 1. we should enforce dt-bindings == CHIP_ID (what's currently done)
>>
>> This
> 
> No. If there is a compatible chip with different ID (for whatever reason
> - maybe there is additional functionality that either board does not
> need or the driver does not implement) we absolutely should not refuse
> to bind the driver.
> 
> Hint: this thing is called _compatible_ for a reason.

Right, the reason you have in mind is fulfilled by fallback compatibles

(i.e. "vendor,actualchipname", "vendor,similarchipname" where the driver
only considers the latter becuase the software interface hasn't changed)

> 
>>
>>>
>>> or
>>>
>>> 2. we should have both compatibles with no handling based on compatible,
>>>    but only use CHIP_ID at runtime to change behavior
>>
>> This is spaghetti
> 
> I really do not understand the aversion of DT maintainers to generic
> compatibles. We see this in I2C HID where we keep adding compatibles
> for what could be described via device properties.

This is because it's the only way to allow for retroactive changes that
do not require changing firmware. That's why ACPI carries new identifiers
for even very slightly different devices too. Once the firmware containing
(ACPI tables / DTB) is put on a production device, it is generally not
going to ever change.

CHIP_ID registers are a good tool to validate that the author of the
firmware table is doing the right thing, but solely relying on them
encourages creating a "vendor,haptic" compatible, which I'm sure you'll
agree is totally meaningless.

That's especially if the naming scheme makes no sense and you can't
even factor out a common wildcard-name (which also happens to be the case
quite often)

Plus a compatible is used to restrict/modify the set of allowed/required
properties, so having an "actual" compatible is required for schema
validation to work

Konrad