Re: [PATCH v2 3/6] USB: serial: ch341: Limit prescaler on quirky chips

[Johan Hovold <johan@kernel.org>]

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@kernel.org>
> 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@vger.kernel.org>	# 5.5
> Reported-by: Michael Hanselmann <public@hansmi.ch>
> Signed-off-by: Johan Hovold <johan@kernel.org>
> ---
>  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