The current ch341 divisor algorithm was known to give inaccurate results for certain higher line speeds. Jonathan Olds <jontio@xxxxxxxxxxxx> investigated this, determined the basic equations used to derive the divisors and confirmed them experimentally [1]. The equations Jonathan used could be generalised further to: baud = 48000000 / (2^(12 - 3 * ps - fact) * div), where 0 <= ps <= 3, 0 <= fact <= 1, 2 <= div <= 256 if fact = 0, or 9 <= div <= 256 if fact = 1 which will also give better results for lower rates. Notably the error is reduced for the following standard rates: 1152000 (4.0% instead of 15% error) 921600 (0.16% instead of -7.5% error) 576000 (-0.80% instead of -5.6% error) 200 (0.16% instead of -0.69% error) 134 (-0.05% instead of -0.63% error) 110 (0.03% instead of -0.44% error) but also for many non-standard ones. The current algorithm also suffered from rounding issues (e.g. 2950000 was rounded to 2 Mbaud instead of 3 Mbaud resulting in a -32% instead of 1.7% error). The new algorithm was inspired by the current vendor driver even if that one only handles two higher rates that require fact=1 by hard coding the corresponding divisors [2]. Michael Dreher <michael@xxxxxxxx> also did a similar generalisation of Jonathan's work and has published his results with a very good summary that provides further insights into how this device works [3]. [1] https://lkml.kernel.org/r/000001d51f34$bad6afd0$30840f70$@co.nz [2] http://www.wch.cn/download/CH341SER_LINUX_ZIP.html [3] https://github.com/nospam2000/ch341-baudrate-calculation Reported-by: Jonathan Olds <jontio@xxxxxxxxxxxx> Tested-by: Jonathan Olds <jontio@xxxxxxxxxxxx> Cc: Michael Dreher <michael@xxxxxxxx> Signed-off-by: Johan Hovold <johan@xxxxxxxxxx> --- drivers/usb/serial/ch341.c | 97 +++++++++++++++++++++++++++++--------- 1 file changed, 75 insertions(+), 22 deletions(-) diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c index 3bb1fff02bed..29f83ce1696e 100644 --- a/drivers/usb/serial/ch341.c +++ b/drivers/usb/serial/ch341.c @@ -48,12 +48,6 @@ #define CH341_BIT_DCD 0x08 #define CH341_BITS_MODEM_STAT 0x0f /* all bits */ -/*******************************/ -/* baudrate calculation factor */ -/*******************************/ -#define CH341_BAUDBASE_FACTOR 1532620800 -#define CH341_BAUDBASE_DIVMAX 3 - /* Break support - the information used to implement this was gleaned from * the Net/FreeBSD uchcom.c driver by Takanori Watanabe. Domo arigato. */ @@ -144,37 +138,96 @@ static int ch341_control_in(struct usb_device *dev, return 0; } +#define CH341_CLKRATE 48000000 +#define CH341_CLK_DIV(ps, fact) (1 << (12 - 3 * (ps) - (fact))) +#define CH341_MIN_RATE(ps) (CH341_CLKRATE / (CH341_CLK_DIV((ps), 1) * 512)) + +static const speed_t ch341_min_rates[] = { + CH341_MIN_RATE(0), + CH341_MIN_RATE(1), + CH341_MIN_RATE(2), + CH341_MIN_RATE(3), +}; + +/* + * The device line speed is given by the following equation: + * + * baud = 48000000 / (2^(12 - 3 * ps - fact) * div), where + * + * 0 <= ps <= 3, + * 0 <= fact <= 1, + * 2 <= div <= 256 if fact = 0, or + * 9 <= div <= 256 if fact = 1 + */ +static int ch341_get_divisor(speed_t speed) +{ + unsigned int fact, div, clk_div; + int ps; + + /* + * 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 (fact = 1) that will give a + * divisor strictly less than 512. + */ + fact = 1; + for (ps = 3; ps >= 0; ps--) { + if (speed > ch341_min_rates[ps]) + break; + } + + if (ps < 0) + return -EINVAL; + + /* Determine corresponding divisor, rounding down. */ + clk_div = CH341_CLK_DIV(ps, fact); + div = CH341_CLKRATE / (clk_div * speed); + + /* Halve base clock (fact = 0) if required. */ + if (div < 9 || div > 255) { + div /= 2; + clk_div *= 2; + fact = 0; + } + + if (div < 2) + return -EINVAL; + + /* + * Pick next divisor if resulting rate is closer to the requested one, + * scale up to avoid rounding errors on low rates. + */ + if (16 * CH341_CLKRATE / (clk_div * div) - 16 * speed >= + 16 * speed - 16 * CH341_CLKRATE / (clk_div * (div + 1))) + div++; + + return (0x100 - div) << 8 | fact << 2 | ps; +} + static int ch341_set_baudrate_lcr(struct usb_device *dev, struct ch341_private *priv, u8 lcr) { - short a; + int val; int r; - unsigned long factor; - short divisor; if (!priv->baud_rate) return -EINVAL; - factor = (CH341_BAUDBASE_FACTOR / priv->baud_rate); - divisor = CH341_BAUDBASE_DIVMAX; - - while ((factor > 0xfff0) && divisor) { - factor >>= 3; - divisor--; - } - if (factor > 0xfff0) + val = ch341_get_divisor(priv->baud_rate); + if (val < 0) return -EINVAL; - factor = 0x10000 - factor; - a = (factor & 0xff00) | divisor; - /* * CH341A buffers data until a full endpoint-size packet (32 bytes) * has been received unless bit 7 is set. */ - a |= BIT(7); + val |= BIT(7); - r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, a); + r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, val); if (r) return r; -- 2.23.0
