linux-kernel - [PATCH] HID: i2c-hid: Revert to await reset ACK before reading report descriptor

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Message-ID: <20240331132332.6694-1-kl@kl.wtf>
Date: Sun, 31 Mar 2024 15:23:32 +0200
From: Kenny Levinsen <kl@...wtf>
To: Jiri Kosina <jikos@...nel.org>,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc: Douglas Anderson <dianders@...omium.org>,
	Hans de Goede <hdegoede@...hat.com>,
	linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Kenny Levinsen <kl@...wtf>
Subject: [PATCH] HID: i2c-hid: Revert to await reset ACK before reading report descriptor

In af93a167eda9, i2c_hid_parse was changed to continue with reading the
report descriptor before waiting for reset to be acknowledged.

This has lead to two regressions:

1. We fail to handle reset acknowledgement if it happens while reading
   the report descriptor. The transfer sets I2C_HID_READ_PENDING, which
   causes the IRQ handler to return without doing anything.

   This affects both a Wacom touchscreen and a Sensel touchpad.

2. On a Sensel touchpad, reading the report descriptor this quickly
   after reset results in all zeroes or partial zeroes.

The issues were observed on the Lenovo Thinkpad Z16 Gen 2.

The change in question was made based on a Microsoft article[0] stating
that Windows 8 *may* read the report descriptor in parallel with
awaiting reset acknowledgement, intended as a slight reset performance
optimization. Perhaps they only do this if reset is not completing
quickly enough for their tastes?

As the code is not currently ready to read registers in parallel with a
pending reset acknowledgement, and as reading quickly breaks the report
descriptor on the Sensel touchpad, revert to waiting for reset
acknowledgement before proceeding to read the report descriptor.

[0]: https://learn.microsoft.com/en-us/windows-hardware/drivers/hid/plug-and-play-support-and-power-management

Fixes: af93a167eda9 ("HID: i2c-hid: Move i2c_hid_finish_hwreset() to after reading the report-descriptor")
Signed-off-by: Kenny Levinsen <kl@...wtf>
---
 drivers/hid/i2c-hid/i2c-hid-core.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/drivers/hid/i2c-hid/i2c-hid-core.c b/drivers/hid/i2c-hid/i2c-hid-core.c
index 2df1ab3c31cc..be5d661dde41 100644
--- a/drivers/hid/i2c-hid/i2c-hid-core.c
+++ b/drivers/hid/i2c-hid/i2c-hid-core.c
@@ -737,7 +737,9 @@ static int i2c_hid_parse(struct hid_device *hid)
 		ret = i2c_hid_start_hwreset(ihid);
 		if (ret)
 			msleep(1000);
+		ret = i2c_hid_finish_hwreset(ihid);
 	} while (tries-- > 0 && ret);
+	mutex_unlock(&ihid->reset_lock);

 	if (ret)
 		goto abort_reset;
@@ -767,16 +769,8 @@ static int i2c_hid_parse(struct hid_device *hid)
 		}
 	}

-	/*
-	 * Windows directly reads the report-descriptor after sending reset
-	 * and then waits for resets completion afterwards. Some touchpads
-	 * actually wait for the report-descriptor to be read before signalling
-	 * reset completion.
-	 */
-	ret = i2c_hid_finish_hwreset(ihid);
 abort_reset:
 	clear_bit(I2C_HID_RESET_PENDING, &ihid->flags);
-	mutex_unlock(&ihid->reset_lock);
 	if (ret)
 		goto out;

-- 
2.44.0