On Thu, May 14, 2020 at 04:17:43PM +0200, Johan Hovold wrote: > On Tue, Mar 31, 2020 at 11:37:19PM +0000, Michael Hanselmann wrote: > 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; > } I've applied the above fix on top of your quirk-detection patch so that we can get this into 5.8 and backported to stable. Johan