linux-kernel - Re: [PATCH] hwmon: (aspeed-pwm-tacho) increase fan tach period (again)

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Message-ID: <20220715233408.GA2181024@roeck-us.net>
Date:   Fri, 15 Jul 2022 16:34:08 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Paul Fertser <fercerpav@...il.com>
Cc:     Jean Delvare <jdelvare@...e.com>, Joel Stanley <joel@....id.au>,
        Patrick Venture <venture@...gle.com>,
        Andrew Jeffery <andrew@...id.au>, linux-hwmon@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: (aspeed-pwm-tacho) increase fan tach period
 (again)

On Thu, Jul 14, 2022 at 05:23:44PM +0300, Paul Fertser wrote:
> The old value allows measuring fan speeds down to about 970 RPM and
> gives timeout for anything less than that. It is problematic because it
> can also be used as an indicator for fan failure or absence.
> 
> Despite having read the relevant section of "ASPEED AST2500/AST2520 A2
> Datasheet – V1.7" multiple times I wasn't able to figure out what
> exactly "fan tach period" and "fan tach falling point of period" mean
> (both are set by the driver from the constant this patch is amending).
> 
> Experimentation with a Tioga Pass OCP board (AST2500 BMC) showed that
> value of 0x0108 gives time outs for speeds below 1500 RPM and the value
> offered by the patch is good for at least 750 RPM (the fans can't spin
> any slower so the lower bound is unknown). Measuring with the fans
> spinning takes about 30 ms, sometimes down to 18 ms, so about the same
> as with the previous value.
> 
> This constant was last changed in
> 762b1e88801357770889d013c5d20fe110d1f456.
> 
> Signed-off-by: Paul Fertser <fercerpav@...il.com>

Patrick - any feedback ? Would it be possible to test this change with
Quanta-q71l ? Or do you envision no problems ?

Thanks,
Guenter

> ---
>  drivers/hwmon/aspeed-pwm-tacho.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/aspeed-pwm-tacho.c b/drivers/hwmon/aspeed-pwm-tacho.c
> index 3cb88d6fbec0..d11f674e3dc3 100644
> --- a/drivers/hwmon/aspeed-pwm-tacho.c
> +++ b/drivers/hwmon/aspeed-pwm-tacho.c
> @@ -159,7 +159,7 @@
>   * 11: reserved.
>   */
>  #define M_TACH_MODE 0x02 /* 10b */
> -#define M_TACH_UNIT 0x0210
> +#define M_TACH_UNIT 0x0420
>  #define INIT_FAN_CTRL 0xFF
>  
>  /* How long we sleep in us while waiting for an RPM result. */