linux-kernel - [PATCH] Fix default q_len for usb_f

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Message-Id: <9DFB1605-63A5-46DB-A5A4-B59B315D8115@msweet.org>
Date:   Mon, 28 Dec 2020 12:18:24 -0500
From:   Michael R Sweet <msweet@...eet.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Michael Sweet <msweet@...eet.org>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Felipe Balbi <balbi@...nel.org>
Subject: [PATCH] Fix default q_len for usb_f_printer gadget driver

The usb_f_printer gadget driver uses a default q_len value of *0* which prevents
any IO from occurring.  Moreover, once the driver is instantiated it is
impossible to change the q_len value.

The following patch uses a default q_len value of 10 which matches the legacy
g_printer gadget driver.  This minimizes the possibility that you end up with a
non-working printer gadget.  It is still possible to set the q_len to a
different value using the configfs path of the same name.

Signed-off-by: Michael R Sweet <msweet@...eet.org>
---
drivers/usb/gadget/function/f_printer.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
index 9c7ed2539ff7..4f3161005e4f 100644
--- a/drivers/usb/gadget/function/f_printer.c
+++ b/drivers/usb/gadget/function/f_printer.c
@@ -50,6 +50,8 @@
#define GET_PORT_STATUS		1
#define SOFT_RESET		2

+#define DEFAULT_Q_LEN		10 /* same as legacy g_printer gadget */
+
static int major, minors;
static struct class *usb_gadget_class;
static DEFINE_IDA(printer_ida);
@@ -1317,6 +1319,9 @@ static struct usb_function_instance *gprinter_alloc_inst(void)
	opts->func_inst.free_func_inst = gprinter_free_inst;
	ret = &opts->func_inst;

+	/* Make sure q_len is initialized, otherwise the bound device can't support read/write! */
+	opts->q_len = DEFAULT_Q_LEN;
+
	mutex_lock(&printer_ida_lock);

	if (ida_is_empty(&printer_ida)) {
-- 
2.17.1