On Tue, Mar 31, 2020 at 11:37:19PM +0000, Michael Hanselmann wrote:
> 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). Limit these to the lower prescaler values at the cost of timing
> precision.
>
> Rates above 46875 baud use the same timings as the normal algorithm.
> Below that the worst difference between desired and actual baud rate is
> 2.17 percentage points. The worst difference is 1.06 p.p. when only
> looking at divisors differing from the normal algorithm.
>
> Signed-off-by: Michael Hanselmann <public@xxxxxxxxx>
> ---
> drivers/usb/serial/ch341.c | 70 ++++++++++++++++++++++++++++----------
> 1 file changed, 52 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
> index 9c839f67c3d4..67a5d4c3df42 100644
> --- a/drivers/usb/serial/ch341.c
> +++ b/drivers/usb/serial/ch341.c
> @@ -24,6 +24,8 @@
> #define DEFAULT_BAUD_RATE 9600
> #define DEFAULT_TIMEOUT 1000
>
> +#define CH341_QUIRK_LIMITED_PRESCALER 0x01
> +
> /* flags for IO-Bits */
> #define CH341_BIT_RTS (1 << 6)
> #define CH341_BIT_DTR (1 << 5)
> @@ -195,35 +197,67 @@ static int ch341_calc_divisor(speed_t speed, unsigned int ps,
> * 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)
> {
> + speed_t speed;
> unsigned int fact, div, clk_div;
> int ps;
>
> + speed = priv->baud_rate;
> +
> /*
> * Clamp to supported range, this makes the (ps < 0) and (div < 2)
> * sanity checks below redundant.
> */
> speed = clamp(speed, 46U, 3000000U);
>
> - /*
> - * Start with highest possible base clock and find a divisor for the
> - * requested baud rate.
> - */
> - for (ps = 3; ps >= 0; --ps) {
> - if (ch341_calc_divisor(speed, ps, 1U, &div, &clk_div) == 0) {
> - fact = 1;
> - break;
> - }
> -
> + if (priv->quirks & CH341_QUIRK_LIMITED_PRESCALER) {
> /*
> - * Prefer half base clock (fact = 0) before trying lower
> - * prescaler values. This makes the receiver more tolerant to
> - * errors.
> + * A subset of all CH34x devices stop responding to bulk
> + * transfers when configured with certain prescaler values.
> + *
> + * fact=0, ps=0..3: Works
> + * fact=1, ps=0..2: Unreliable
> + * fact=1, ps=3: Works
> + *
> + * Limit these devices to working prescaler values at the cost
> + * of precision for speeds up to 46875 baud above which
> + * fact = 1 with ps = 3 is used.
> */
> - if (ch341_calc_divisor(speed, ps, 0U, &div, &clk_div) == 0) {
> + if (ch341_calc_divisor(speed, 3U, 1U, &div, &clk_div) == 0) {
> + ps = 3U;
> + fact = 1U;
> + } else {
> fact = 0;
> - break;
> +
> + for (ps = 3; ps >= 0; --ps) {
> + if (ch341_calc_divisor(speed, ps, fact,
> + &div, &clk_div) == 0)
> + break;
> + }
> + }
> + } else {
> + /*
> + * Start with highest possible base clock and find a divisor
> + * for the requested baud rate.
> + */
> + for (ps = 3; ps >= 0; --ps) {
> + if (ch341_calc_divisor(speed, ps, 1U,
> + &div, &clk_div) == 0) {
> + fact = 1;
> + break;
> + }
> +
> + /*
> + * Prefer half base clock (fact = 0) before trying
> + * lower prescaler values. This makes the receiver more
> + * tolerant to errors.
> + */
> + if (ch341_calc_divisor(speed, ps, 0U,
> + &div, &clk_div) == 0) {
> + fact = 0;
> + break;
> + }
> }
> }
>
> @@ -250,7 +284,7 @@ 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;
>
> @@ -353,7 +387,7 @@ static int ch341_detect_quirks(struct usb_device *dev)
> if (r == -EPIPE) {
> dev_dbg(&dev->dev, "%s - reading break condition register"
> " failed (%d)\n", __func__, r);
> - r = 0;
> + r = CH341_QUIRK_LIMITED_PRESCALER;
> goto out;
> }
This can be implemented in a more compact way which makes the algorithm
easier to follow and doesn't depend on your patch 1/6 either. With a
smaller fix we can get this backported to stable as well.
I've verified that the below works here on top of your (slightly
modified) 2/6. Would you mind giving it a try on your end as well?
Johan
>From ea23730616b6406101d4efeb12e1ae2312dd20c9 Mon Sep 17 00:00:00 2001
From: Johan Hovold <johan@xxxxxxxxxx>
Date: Thu, 14 May 2020 11:36:45 +0200
Subject: [PATCH] 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@xxxxxxxxxxxxxxx> # 5.5
Reported-by: Michael Hanselmann <public@xxxxxxxxx>
Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
---
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;
}
--
2.26.2
