usb: pl2303: fix+improve the divsor based baud rate encoding method
Based on the formula in the code description, Reinhard Max and me have investigated the devices behavior / functional principle of the divisor based baud rate encoding method. It turned out, that (although beeing a good starting point) the current code has some flaws. It doesn't work correctly for a wide range of baud rates and the divisor resolution can be improved. It also doesn't report the actually set baud rate. This patch fixes and improves the code for the divisor based baud rate encoding method a lot. It can now be used for the whole range of baud rates from 46 baud to 24M baud with a very good divisor resolution and userspace can read back the resulting baud rate. It also documents the formula used for encoding and the hardware behavior (including special cases). The basic algorithm, rounding and several code comments/explanations are provided by Reinhard Max. I've added some minor fixes, the handling of the special cases and further code/algorithm descriptions. Signed-off-by: NFrank Schäfer <fschaefer.oss@googlemail.com> Signed-off-by: NReinhard Max <max@suse.de> Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
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