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

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Message-ID: <20180705145458.7e7d9b9f@xhacker.debian>
Date:   Thu, 5 Jul 2018 14:54:58 +0800
From:   Jisheng Zhang <Jisheng.Zhang@...aptics.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.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, 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

Thanks