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Sender: Guenter Roeck <groeck7@gmail.com>
Message-ID: <5b988c94-0de3-95fd-de39-66b52a59cdd6@roeck-us.net>
Date:   Sat, 29 Jul 2023 06:44:32 -0700
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To:     Aleksa Savic <savicaleksa83@gmail.com>, linux-hwmon@vger.kernel.org
Cc:     stable@vger.kernel.org, Jack Doan <me@jackdoan.com>,
        Jean Delvare <jdelvare@suse.com>, linux-kernel@vger.kernel.org
References: <20230729112732.5516-1-savicaleksa83@gmail.com>
From:   Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH] hwmon: (aquacomputer_d5next) Add selective 200ms delay
 after sending ctrl report
In-Reply-To: <20230729112732.5516-1-savicaleksa83@gmail.com>
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On 7/29/23 04:27, Aleksa Savic wrote:
> Add a 200ms delay after sending a ctrl report to Quadro,
> Octo, D5 Next and Aquaero to give them enough time to
> process the request and save the data to memory. Otherwise,
> under heavier userspace loads where multiple sysfs entries
> are usually set in quick succession, a new ctrl report could
> be requested from the device while it's still processing the
> previous one and fail with -EPIPE.
> 
> Reported by a user on Github [1] and tested by both of us.
> 
> [1] https://github.com/aleksamagicka/aquacomputer_d5next-hwmon/issues/82
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Aleksa Savic <savicaleksa83@gmail.com>
> ---
>   drivers/hwmon/aquacomputer_d5next.c | 25 +++++++++++++++++++++++++
>   1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/hwmon/aquacomputer_d5next.c b/drivers/hwmon/aquacomputer_d5next.c
> index a997dbcb563f..9cb55d51185a 100644
> --- a/drivers/hwmon/aquacomputer_d5next.c
> +++ b/drivers/hwmon/aquacomputer_d5next.c
> @@ -652,6 +652,31 @@ static int aqc_send_ctrl_data(struct aqc_data *priv)
>   	ret = hid_hw_raw_request(priv->hdev, priv->secondary_ctrl_report_id,
>   				 priv->secondary_ctrl_report, priv->secondary_ctrl_report_size,
>   				 HID_FEATURE_REPORT, HID_REQ_SET_REPORT);
> +	if (ret < 0)
> +		return ret;
> +
> +	/*
> +	 * Wait 200ms before returning to make sure that the device actually processed both reports
> +	 * and saved ctrl data to memory. Otherwise, an aqc_get_ctrl_data() call made shortly after
> +	 * may fail with -EPIPE because the device is still busy and can't provide data. This can
> +	 * happen when userspace tools, such as fancontrol or liquidctl, write to sysfs entries in
> +	 * quick succession.
> +	 *
> +	 * 200ms was found to be the sweet spot between fixing the issue and not significantly
> +	 * prolonging the call. Quadro, Octo, D5 Next and Aquaero are currently known to be
> +	 * affected.
> +	 */
> +	switch (priv->kind) {
> +	case quadro:
> +	case octo:
> +	case d5next:
> +	case aquaero:
> +		msleep(200);
> +		break;
> +	default:
> +		break;
> +	}
> +
>   	return ret;
>   }
>   

This would force writes to sleep even if there is no subsequent operation.
Please make this conditional by saving the most recent access time and wait
on the subsequent operation. I would also suggest to store the wait time
in struct aqc_data to avoid the switch statement in the data path. An example
for a driver doing something similar is drivers/hwmon/pmbus/zl6100.c.

Thanks,
Guenter