linux-kernel - Re: [PATCH] hwmon: (aspeed-pwm-tacho) Using falling edge.

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Message-ID: <20210624124428.GB1670703@roeck-us.net>
Date:   Thu, 24 Jun 2021 05:44:28 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Billy Tsai <billy_tsai@...eedtech.com>
Cc:     jdelvare@...e.com, joel@....id.au, andrew@...id.au,
        linux-hwmon@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        BMC-SW@...eedtech.com
Subject: Re: [PATCH] hwmon: (aspeed-pwm-tacho) Using falling edge.

On Thu, Jun 24, 2021 at 11:58:21AM +0800, Billy Tsai wrote:
> The tach shouldn't use both edges to measure. When the tach input
> duty cycle isn't 50% the return value will inaccurate.
> 
A tachometer doesn't have a duty cycle. A pwm has a duty cycle, but that
is completely independent of the pwm duty cycle used to set the fan speed.
So this patch does not really make sense with the above explanation.

The impact of this patch is likely that the reported fan speed is reduced
by 50%. It may well be that the driver currently reports twice the real fan
speed. I have no idea if that is the case, but if it is it should not be
conditional. The description above states "when the tach input cycle isn't
50%", suggesting that this is conditional on some other configuration.
I don't know what that might be either.

So, sorry, I can't accept this patch without a more detailed and accurate
description and explanation why it is needed.

> Signed-off-by: Billy Tsai <billy_tsai@...eedtech.com>
> ---
>  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 3d8239fd66ed..0a70a0e22acf 100644
> --- a/drivers/hwmon/aspeed-pwm-tacho.c
> +++ b/drivers/hwmon/aspeed-pwm-tacho.c
> @@ -158,7 +158,7 @@
>   * 10: both
>   * 11: reserved.
>   */
> -#define M_TACH_MODE 0x02 /* 10b */
> +#define M_TACH_MODE 0x00 /* 10b */

That comment is now wrong.

Guenter

>  #define M_TACH_UNIT 0x0210
>  #define INIT_FAN_CTRL 0xFF
>  
> -- 
> 2.25.1
>