[PATCH 5.7 05/24] USB: serial: ch341: fix lockup of devices with limited prescaler

Greg Kroah-Hartman
2020-06-09T17:59:01Z

From: Johan Hovold <johan@kernel.org>

commit c432df155919582a3cefa35a8f86256c830fa9a4 upstream.

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/usb/serial/ch341.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)

--- 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 spe
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
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 us
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;
}