[PATCH 6.6 278/567] usb: cdc-acm: Restore CAP_BRK functionnality to CH343

Greg Kroah-Hartman
2026-03-23T15:06:38Z

6.6-stable review patch. If anyone has any objections, please let me know.

------------------

From: Marc Zyngier <maz@kernel.org>

commit 14ae24cba291bddfdc296bbcbfd00cd09d0498ef upstream.

The CH343 USB/serial adapter is as buggy as it is popular (very).
One of its quirks is that despite being capable of signalling a
BREAK condition, it doesn't advertise it.

This used to work nonetheless until 66aad7d8d3ec5 ("usb: cdc-acm:
return correct error code on unsupported break") applied some
reasonable restrictions, preventing breaks from being emitted on
devices that do not advertise CAP_BRK.

Add a quirk for this particular device, so that breaks can still
be produced on some of my machines attached to my console server.

Fixes: 66aad7d8d3ec5 ("usb: cdc-acm: return correct error code on unsupported break")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable <stable@kernel.org>
Cc: Oliver Neukum <oneukum@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Oliver Neukum <oneukum@suse.com>
Link: https://patch.msgid.link/20260301124440.1192752-1-maz@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/usb/class/cdc-acm.c | 5 +++++
drivers/usb/class/cdc-acm.h | 1 +
2 files changed, 6 insertions(+)

--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -1379,6 +1379,8 @@ made_compressed_probe:
acm->ctrl_caps = h.usb_cdc_acm_descriptor->bmCapabilities;
if (quirks & NO_CAP_LINE)
acm->ctrl_caps &= ~USB_CDC_CAP_LINE;
+ if (quirks & MISSING_CAP_BRK)
+ acm->ctrl_caps |= USB_CDC_CAP_BRK;
acm->ctrlsize = ctrlsize;
acm->readsize = readsize;
acm->rx_buflimit = num_rx_buf;
@@ -2002,6 +2004,9 @@ static const struct usb_device_id acm_id
.driver_info = IGNORE_DEVICE,
},

+ /* CH343 supports CAP_BRK, but doesn't advertise it */
+ { USB_DEVICE(0x1a86, 0x55d3), .driver_info = MISSING_CAP_BRK, },
+
/* control interfaces without any protocol set */
{ USB_INTERFACE_INFO(USB_CLASS_COMM, USB_CDC_SUBCLASS_ACM,
USB_CDC_PROTO_NONE) },
--- a/drivers/usb/class/cdc-acm.h
+++ b/drivers/usb/class/cdc-acm.h
@@ -113,3 +113,4 @@ struct acm {
#define CLEAR_HALT_CONDITIONS BIT(5)
#define SEND_ZERO_PACKET BIT(6)
#define DISABLE_ECHO BIT(7)
+#define MISSING_CAP_BRK BIT(8)