Re: [PATCH] hwmon: (jc42) Strengthen detect function

[Jean Delvare <jdelvare@suse.de>]

Hi Guenter,

On Sun, 30 Jun 2024 13:20:28 -0700, Guenter Roeck wrote:
> Configuration register bit 5 must read 0 for all JC42.4 compliant chips.
> Several capability register bits must be set for all TSE2004 compliant
> chips. Use that information to strengthen the detect function.
> 
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
>  drivers/hwmon/jc42.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/jc42.c b/drivers/hwmon/jc42.c
> index 1180af1b1638..a260cff750a5 100644
> --- a/drivers/hwmon/jc42.c
> +++ b/drivers/hwmon/jc42.c
> @@ -413,7 +413,11 @@ static int jc42_detect(struct i2c_client *client, struct i2c_board_info *info)
>  	if (cap < 0 || config < 0 || manid < 0 || devid < 0)
>  		return -ENODEV;
>  
> -	if ((cap & 0xff00) || (config & 0xf800))
> +	if ((cap & 0xff00) || (config & 0xf820))
> +		return -ENODEV;
> +
> +	if ((devid & TSE2004_DEVID_MASK) == TSE2004_DEVID &&
> +	    (cap & 0x00e7) != 0x00e7)
>  		return -ENODEV;
>  
>  	for (i = 0; i < ARRAY_SIZE(jc42_chips); i++) {

I have a user reporting that this change is causing the jc42 driver to
no longer bind to his memory module temperature sensor devices after
updating to kernel v6.11. I asked for a register dump:

     0,8  1,9  2,a  3,b  4,c  5,d  6,e  7,f
00: 7f00 0000 0000 0000 0000 6ac2 091b 3022 

After swapping the bytes, I see that this is a TSE2004-compliant device
(devid = 0x2230) and the capabilities register reads 0x007f. This
doesn't pass the 0x00e7 mask test you added, as bit 7 isn't set in his
case.

The JEDEC standard indeed says that bit 7 should be set, but apparently
this isn't always the case in the real world.

Also note that I looked at the Renesas TSE2004GB2B0 datasheet and it
shows bit 2 (RANGE) as not always set. The ST STTS2004 datasheet shows
bits 0 (EVENT) and 2 (RANGE) as possibly reading 0. So I wonder how
much we can rely on these capability bits being set in the detect
function. Unfortunately I don't have any TS2004-compliant device at
hand to verify, nor do I own register dumps of such devices. Would it
be OK with you if we relax the check to at least ignore bit 7, and
possibly also bits 0 and 2?

Thanks,
-- 
Jean Delvare
SUSE L3 Support