Re: [PATCH] USB: serial: ch341: reimplement line-speed handling

[Johan Hovold <johan@kernel.org>]

On Fri, Nov 01, 2019 at 06:24:10PM +0100, Johan Hovold wrote:
> 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:
> 
> 	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@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: Jonathan Olds <jontio@i4free.co.nz>
> Tested-by: Jonathan Olds <jontio@i4free.co.nz>
> Cc: Michael Dreher <michael@5dot1.de>
> Signed-off-by: Johan Hovold <johan@kernel.org>
> ---
>  drivers/usb/serial/ch341.c | 97 +++++++++++++++++++++++++++++---------
>  1 file changed, 75 insertions(+), 22 deletions(-)

I've applied this one for 5.5 now (not 5.4 as I mistakingly said earlier).

Johan