On Tue, Nov 13, 2018 at 08:19:44PM +0100, Nikolaj Fogh wrote: > On 11/12/18 10:54 AM, Johan Hovold wrote: > > On Wed, Oct 31, 2018 at 09:16:48PM +0100, Nikolaj Fogh wrote: > >> I have experienced that the ftdi_sio driver gives less-than-optimal > >> baud rates as the driver truncates instead of rounds to nearest > >> during baud rate divisor calculation. > >> This patch improves on the baud rate generation. The generated baud > >> rate corresponds to the optimal baud rate achievable with the chip. > >> This is what the windows driver gives as well. > > > > How did you verify this? Did you trace and compare the divisors > > actually requested by the Windows driver, or did you measure the > > resulting rates using a scope? > I verified it by scope. Granted, I only verified it for one baud rate > (961200). Whether it gives the same as the Windows driver in general, > I'm not sure. However, I would think that rounding instead of flooring > would always yield the most accurate result. I'm not so sure in this case. The driver uses "sub-integer" divisors and looks like it depends on truncation rather than rounding. Some background here: https://www.ftdichip.com/Support/Knowledgebase/index.html?whatbaudratesareachieveabl.htm If you want to change these calculations you need to make a stronger case for it and verify that we don't mess up some other rate inadvertently. Thanks, Johan
