Commit 35714565 authored by Johan Hovold's avatar Johan Hovold

USB: serial: ch341: reimplement line-speed handling

The current ch341 divisor algorithm was known to give inaccurate results
for certain higher line speeds. Jonathan Olds <jontio@i4free.co.nz>
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:

	baudrate = 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.
requesting 2950000 Bd resulted in a rate of 2 MBd instead of 3 MBd and
thus 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@5dot1.de> 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-calculationReported-by: default avatarJonathan Olds <jontio@i4free.co.nz>
Tested-by: default avatarJonathan Olds <jontio@i4free.co.nz>
Cc: Michael Dreher <michael@5dot1.de>
Signed-off-by: default avatarJohan Hovold <johan@kernel.org>
parent ebd09f1c
...@@ -48,12 +48,6 @@ ...@@ -48,12 +48,6 @@
#define CH341_BIT_DCD 0x08 #define CH341_BIT_DCD 0x08
#define CH341_BITS_MODEM_STAT 0x0f /* all bits */ #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 /* Break support - the information used to implement this was gleaned from
* the Net/FreeBSD uchcom.c driver by Takanori Watanabe. Domo arigato. * the Net/FreeBSD uchcom.c driver by Takanori Watanabe. Domo arigato.
*/ */
...@@ -144,37 +138,96 @@ static int ch341_control_in(struct usb_device *dev, ...@@ -144,37 +138,96 @@ static int ch341_control_in(struct usb_device *dev,
return 0; 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:
*
* baudrate = 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, static int ch341_set_baudrate_lcr(struct usb_device *dev,
struct ch341_private *priv, u8 lcr) struct ch341_private *priv, u8 lcr)
{ {
short a; int val;
int r; int r;
unsigned long factor;
short divisor;
if (!priv->baud_rate) if (!priv->baud_rate)
return -EINVAL; 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; return -EINVAL;
factor = 0x10000 - factor;
a = (factor & 0xff00) | divisor;
/* /*
* CH341A buffers data until a full endpoint-size packet (32 bytes) * CH341A buffers data until a full endpoint-size packet (32 bytes)
* has been received unless bit 7 is set. * 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) if (r)
return r; return r;
......
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