USB: serial: ch341: fix lockup of devices with limited prescaler

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>

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index f2b93f4..89675ee 100644 (file)
--- 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;
        }