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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Hui Wang <hui.wang@canonical.com>
Subject: [PATCH 4.9 136/175] ACPI: PNP: compare the string length in the matching_id()
Date:   Mon, 28 Dec 2020 13:49:49 +0100
Message-Id: <20201228124859.835762152@linuxfoundation.org>
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From: Hui Wang <hui.wang@canonical.com>

commit b08221c40febcbda9309dd70c61cf1b0ebb0e351 upstream.

Recently we met a touchscreen problem on some Thinkpad machines, the
touchscreen driver (i2c-hid) is not loaded and the touchscreen can't
work.

An i2c ACPI device with the name WACF2200 is defined in the BIOS, with
the current rule in matching_id(), this device will be regarded as
a PNP device since there is WACFXXX in the acpi_pnp_device_ids[] and
this PNP device is attached to the acpi device as the 1st
physical_node, this will make the i2c bus match fail when i2c bus
calls acpi_companion_match() to match the acpi_id_table in the i2c-hid
driver.

WACF2200 is an i2c device instead of a PNP device, after adding the
string length comparing, the matching_id() will return false when
matching WACF2200 and WACFXXX, and it is reasonable to compare the
string length when matching two IDs.

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/acpi/acpi_pnp.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/acpi/acpi_pnp.c
+++ b/drivers/acpi/acpi_pnp.c
@@ -320,6 +320,9 @@ static bool matching_id(const char *idst
 {
 	int i;
 
+	if (strlen(idstr) != strlen(list_id))
+		return false;
+
 	if (memcmp(idstr, list_id, 3))
 		return false;