linux-kernel - Re: [PATCH] lm75: add lm75b detection

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Date:   Sun, 27 Oct 2019 16:03:39 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Rain Wang <Rain_Wang@...il.com>, Jean Delvare <jdelvare@...e.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lm75: add lm75b detection

On 10/26/19 1:10 AM, Rain Wang wrote:
> The National Semiconductor LM75B is very similar as the
> LM75A, but it has no ID byte register 7, and unused registers
> return 0xff rather than the last read value like LM75A.
> 
> Signed-off-by: Rain Wang <rain_wang@...il.com>

I am quite hesitant to touch the detect function for this chip.
Each addition increases the risk for false positives. What is the
use case ?

Thanks,
Guenter

> ---
>   drivers/hwmon/lm75.c | 17 +++++++++++++++--
>   1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
> index 5e6392294c03..a718f9eb4d72 100644
> --- a/drivers/hwmon/lm75.c
> +++ b/drivers/hwmon/lm75.c
> @@ -760,6 +760,7 @@ static int lm75_detect(struct i2c_client *new_client,
>   	int i;
>   	int conf, hyst, os;
>   	bool is_lm75a = 0;
> +	bool is_lm75b = 0;
>   
>   	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
>   				     I2C_FUNC_SMBUS_WORD_DATA))
> @@ -780,8 +781,12 @@ static int lm75_detect(struct i2c_client *new_client,
>   	 * register 7, and unused registers return 0xff rather than the
>   	 * last read value.
>   	 *
> +	 * The National Semiconductor LM75B is very similar as the
> +	 * LM75A, but it has no ID byte register 7, and unused registers
> +	 * return 0xff rather than the last read value like LM75A.
> +	 *
>   	 * Note that this function only detects the original National
> -	 * Semiconductor LM75 and the LM75A. Clones from other vendors
> +	 * Semiconductor LM75 and the LM75A/B. Clones from other vendors
>   	 * aren't detected, on purpose, because they are typically never
>   	 * found on PC hardware. They are found on embedded designs where
>   	 * they can be instantiated explicitly so detection is not needed.
> @@ -806,6 +811,13 @@ static int lm75_detect(struct i2c_client *new_client,
>   		is_lm75a = 1;
>   		hyst = i2c_smbus_read_byte_data(new_client, 2);
>   		os = i2c_smbus_read_byte_data(new_client, 3);
> +	} else if (i2c_smbus_read_byte_data(new_client, 4) == 0xff
> +		 && i2c_smbus_read_byte_data(new_client, 5) == 0xff
> +		 && i2c_smbus_read_byte_data(new_client, 6) == 0xff) {
> +		/* LM75B detection */
> +		is_lm75b = 1;
> +		hyst = i2c_smbus_read_byte_data(new_client, 2);
> +		os = i2c_smbus_read_byte_data(new_client, 3);
>   	} else { /* Traditional style LM75 detection */
>   		/* Unused addresses */
>   		hyst = i2c_smbus_read_byte_data(new_client, 2);
> @@ -839,7 +851,8 @@ static int lm75_detect(struct i2c_client *new_client,
>   			return -ENODEV;
>   	}
>   
> -	strlcpy(info->type, is_lm75a ? "lm75a" : "lm75", I2C_NAME_SIZE);
> +	strlcpy(info->type, is_lm75a ?
> +		"lm75a" : (is_lm75b ? "lm75b" : "lm75"), I2C_NAME_SIZE);
>   
>   	return 0;
>   }
>