linux-kernel - Re: [PATCH v2 3/3] serial: 8250_dw: add fractional divisor support

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Date:   Fri, 06 Jul 2018 20:39:30 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Jisheng Zhang <Jisheng.Zhang@...aptics.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH v2 3/3] serial: 8250_dw: add fractional divisor support

On Thu, 2018-07-05 at 14:54 +0800, Jisheng Zhang wrote:
> On Thu, 5 Jul 2018 14:39:21 +0800 Jisheng Zhang wrote:
> 
> <snip>
> 
> > >   
> > > > +	serial_port_out(p, UART_LCR, up->lcr | UART_LCR_DLAB);
> > > > +	serial_dl_write(up, quot);    
> > > 
> > > At some point it would be a helper, I think. We can call
> > > serial8250_do_set_divisor() here. So, perhaps we might export
> > > it.  
> > 
> > serial8250_do_set_divisor will drop the frac, that's not we want ;)
> > 
> 
> And most importantly, serial8250_do_set_divisor() will set a wrong
> BRD(I)
> for fractional capable DW uarts. For example, clk = 25MHZ, baud =
> 115200.
> 
> In fractional capable DW uarts, we should set BRD(I) as
> 25000000/(16*115200) = 13
> 
> but serial8250_do_set_divisor() will set BRD(I) as
> DIV_ROUND_CLOSEST(25*1000000, 16*115200)) = 14

How come? It doesn't do any calculus (for DW 8250), it just writes few
registers based on input.

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy