[PATCH] hwmon: (aquacomputer_d5next) Add selective 200ms delay after sending ctrl report

[Aleksa Savic <savicaleksa83@gmail.com>]

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;
 }
 
-- 
2.41.0