Commit 35714565 authored by Johan Hovold's avatar Johan Hovold
Browse files

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-calculation



Reported-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

drivers/usb/serial/ch341.c

+75 −22
Original line number Diff line number Diff line
@@ -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:

 *

 *	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,

				  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;

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