linux-kernel - Re: [PATCH V5 2/2] iio: proximity: aw9610x: Add support for aw9610x proximity sensor

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Message-ID: <27fb423a-e8f4-44c2-be1a-38d2a42a11b6@kernel.org>
Date: Sun, 28 Jul 2024 10:41:05 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Jonathan Cameron <jic23@...nel.org>
Cc: wangshuaijie@...nic.com, lars@...afoo.de, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, waqar.hameed@...s.com,
 linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, liweilei@...nic.com, kangjiajun@...nic.com
Subject: Re: [PATCH V5 2/2] iio: proximity: aw9610x: Add support for aw9610x
 proximity sensor

On 27/07/2024 17:06, Jonathan Cameron wrote:
> 
>> ...
>>
>>> +static int aw9610x_read_chipid(struct aw9610x *aw9610x)
>>> +{
>>> +	unsigned char cnt = 0;
>>> +	u32 reg_val;
>>> +	int ret;
>>> +
>>> +	while (cnt < AW_READ_CHIPID_RETRIES) {
> Why retries? 
>>> +		ret = aw9610x_i2c_read(aw9610x, REG_CHIPID, &reg_val);
>>> +		if (ret < 0) {
>>> +			cnt++;
>>> +			usleep_range(2000, 3000);
>>> +		} else {
>>> +			reg_val = FIELD_GET(AW9610X_CHIPID_MASK, reg_val);
>>> +			break;
>>> +		}
>>> +	}
>>> +
>>> +	if (reg_val == AW9610X_CHIP_ID)
>>> +		return 0;  
>>
>> So devices are detectable? Encode this in the bindings (oneOf and a
>> fallback compatible) and drop unneeded entry from ID tables.
> 
> Hi Krzysztof,
> 
> I think this is not a good idea.
> 
> Even though these two are detectable, this breaks if along comes a 3rd device
> in the future which is truly compatible with one of these two parts but that
> we don't yet know about (so can't discover). For that part we will want to
> provide a meaningful fallback compatible.
> 
> It needs to fallback to either the 3 channel or the 5 channel chip and handle
> it as appropriate. (Note that this difference is non obvious as right now the
> code pretends there are always 5 channels and that needs fixing).
> 
> If the chips provided a register that told all the chip specific data like
> how many channels, then sure making one fallback to the other would be fine
> as future devices could use those standard registers.
> 
> With just an Id register, we can't discover enough.  Hence these two
> parts should not be listed as compatible with each other.

Sure

Best regards,
Krzysztof