Re: [PATCH v2 1/7] USB: serial: ftdi_sio: Fix divisor overflow

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On Tue, Jul 12, 2022 at 01:53:00PM +0200, Marek Behún wrote:
> From: Pali Rohár <pali@xxxxxxxxxx>
> 
> The baud rate generating divisor is a 17-bit wide (14 bits integer part
> and 3 bits fractional part).
> 
> Example:
>   base clock = 48 MHz
>   requested baud rate = 180 Baud
>   divisor = 48,000,000 / (16 * 180) = 0b100000100011010.101
> 
>   In this case the MSB gets discarded because of the overflow, and the
>   actually used divisor will be 0b100011010.101 = 282.625, resulting
>   in baud rate 10615 Baud, instead of the requested 180 Baud.
> 
> The best possible thing to do in this case is to generate lowest possible
> baud rate (in the example it would be 183 Baud), by using maximum possible
> divisor.

Actually, the best way to handle this is to add a sanity check for the
lowest supported check as you do in the next patch. That one makes this
change superfluous.

> In case of divisor overflow, use maximum possible divisor.

Johan



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