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From: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org
Subject: Re: [PATCH v2 3/3] serial: 8250_dw: add fractional divisor support
Date: Thu, 5 Jul 2018 14:54:58 +0800	[thread overview]
Message-ID: <20180705145458.7e7d9b9f@xhacker.debian> (raw)
In-Reply-To: <20180705143921.6a8aeb50@xhacker.debian>

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

WARNING: multiple messages have this Message-ID (diff)
From: Jisheng.Zhang@synaptics.com (Jisheng Zhang)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH v2 3/3] serial: 8250_dw: add fractional divisor support
Date: Thu, 5 Jul 2018 14:54:58 +0800	[thread overview]
Message-ID: <20180705145458.7e7d9b9f@xhacker.debian> (raw)
In-Reply-To: <20180705143921.6a8aeb50@xhacker.debian>

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

  reply	other threads:[~2018-07-05  6:54 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-04  8:59 [PATCH v2 0/3] serial: 8250_dw: add fractional divisor support Jisheng Zhang
2018-07-04  8:59 ` Jisheng Zhang
2018-07-04  9:00 ` [PATCH v2 1/3] serial: 8250: let serial8250_get_divisor() get uart_port * as param Jisheng Zhang
2018-07-04  9:00   ` Jisheng Zhang
2018-07-04 10:00   ` Andy Shevchenko
2018-07-04 10:00     ` Andy Shevchenko
2018-07-04  9:02 ` [PATCH v2 2/3] serial: 8250: introduce get_divisor() and set_divisor() hook Jisheng Zhang
2018-07-04  9:02   ` Jisheng Zhang
2018-07-04 10:00   ` Andy Shevchenko
2018-07-04 10:00     ` Andy Shevchenko
2018-07-04  9:03 ` [PATCH v2 3/3] serial: 8250_dw: add fractional divisor support Jisheng Zhang
2018-07-04  9:03   ` Jisheng Zhang
2018-07-04 16:08   ` Andy Shevchenko
2018-07-04 16:08     ` Andy Shevchenko
2018-07-04 16:08     ` Andy Shevchenko
2018-07-05  6:39     ` Jisheng Zhang
2018-07-05  6:39       ` Jisheng Zhang
2018-07-05  6:54       ` Jisheng Zhang [this message]
2018-07-05  6:54         ` Jisheng Zhang
2018-07-06 17:39         ` Andy Shevchenko
2018-07-06 17:39           ` Andy Shevchenko
2018-07-06 17:37       ` Andy Shevchenko
2018-07-06 17:37         ` Andy Shevchenko
2018-07-09  6:04         ` Jisheng Zhang
2018-07-09  6:04           ` Jisheng Zhang
2018-07-09  6:04           ` Jisheng Zhang
2018-07-09  6:15           ` Andy Shevchenko
2018-07-09  6:15             ` Andy Shevchenko

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