From c432df155919582a3cefa35a8f86256c830fa9a4 Mon Sep 17 00:00:00 2001

From: Johan Hovold <johan@kernel.org>

Date: Thu, 14 May 2020 11:36:45 +0200

Subject: [PATCH] USB: serial: ch341: fix lockup of devices with limited

 prescaler

Git-commit: c432df155919582a3cefa35a8f86256c830fa9a4

References: git-fixes

Patch-mainline: v5.8-rc1

Michael Hanselmann reports that

	[a] subset of all CH341 devices stop responding to bulk

	transfers, usually after the third byte, when the highest

	prescaler bit (0b100) is set. There is one exception, namely a

	prescaler of exactly 0b111 (fact=1, ps=3).

Fix this by forcing a lower base clock (fact = 0) whenever needed.

This specifically makes the standard rates 110, 134 and 200 bps work

again with these devices.

Fixes: 35714565089e ("USB: serial: ch341: reimplement line-speed handling")

Cc: stable <stable@vger.kernel.org>	# 5.5

Reported-by: Michael Hanselmann <public@hansmi.ch>

Link: https://lore.kernel.org/r/20200514141743.GE25962@localhost

Signed-off-by: Johan Hovold <johan@kernel.org>

Signed-off-by: Oliver Neukum <oneukum@suse.com>

---

 drivers/usb/serial/ch341.c | 15 ++++++++++++---

 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c

index f2b93f4554a7..89675ee29645 100644

--- a/drivers/usb/serial/ch341.c

+++ b/drivers/usb/serial/ch341.c

@@ -73,6 +73,8 @@

 #define CH341_LCR_CS6          0x01

 #define CH341_LCR_CS5          0x00

+#define CH341_QUIRK_LIMITED_PRESCALER	BIT(0)

+

 static const struct usb_device_id id_table[] = {

 	{ USB_DEVICE(0x4348, 0x5523) },

 	{ USB_DEVICE(0x1a86, 0x7523) },

@@ -160,9 +162,11 @@ static const speed_t ch341_min_rates[] = {

  *		2 <= div <= 256 if fact = 0, or

  *		9 <= div <= 256 if fact = 1

  */

-static int ch341_get_divisor(speed_t speed)

+static int ch341_get_divisor(struct ch341_private *priv)

 {

 	unsigned int fact, div, clk_div;

+	speed_t speed = priv->baud_rate;

+	bool force_fact0 = false;

 	int ps;

 	/*

@@ -188,8 +192,12 @@ static int ch341_get_divisor(speed_t speed)

 	clk_div = CH341_CLK_DIV(ps, fact);

 	div = CH341_CLKRATE / (clk_div * speed);

+	/* Some devices require a lower base clock if ps < 3. */

+	if (ps < 3 && (priv->quirks & CH341_QUIRK_LIMITED_PRESCALER))

+		force_fact0 = true;

+

 	/* Halve base clock (fact = 0) if required. */

-	if (div < 9 || div > 255) {

+	if (div < 9 || div > 255 || force_fact0) {

 		div /= 2;

 		clk_div *= 2;

 		fact = 0;

@@ -228,7 +236,7 @@ static int ch341_set_baudrate_lcr(struct usb_device *dev,

 	if (!priv->baud_rate)

 		return -EINVAL;

-	val = ch341_get_divisor(priv->baud_rate);

+	val = ch341_get_divisor(priv);

 	if (val < 0)

 		return -EINVAL;

@@ -333,6 +341,7 @@ static int ch341_detect_quirks(struct usb_serial_port *port)

 			    CH341_REG_BREAK, 0, buffer, size, DEFAULT_TIMEOUT);

 	if (r == -EPIPE) {

 		dev_dbg(&port->dev, "break control not supported\n");

+		quirks = CH341_QUIRK_LIMITED_PRESCALER;

 		r = 0;

 		goto out;

 	}

-- 

2.35.3