linux-kernel - Re: [PATCH v1 tty] 8250: microchip: Add 4 Mbps support in PCI1XXXX UART

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Date: Thu, 8 Feb 2024 02:53:38 +0000
From: <Rengarajan.S@...rochip.com>
To: <andriy.shevchenko@...ux.intel.com>
CC: <jiaqing.zhao@...ux.intel.com>, <Kumaravel.Thiagarajan@...rochip.com>,
	<gregkh@...uxfoundation.org>, <ilpo.jarvinen@...ux.intel.com>,
	<john.ogness@...utronix.de>, <tony@...mide.com>,
	<linux-kernel@...r.kernel.org>, <Tharunkumar.Pasumarthi@...rochip.com>,
	<jirislaby@...nel.org>, <f.fainelli@...il.com>, <tglx@...utronix.de>,
	<UNGLinuxDriver@...rochip.com>, <linux-serial@...r.kernel.org>
Subject: Re: [PATCH v1 tty] 8250: microchip: Add 4 Mbps support in PCI1XXXX
 UART

On Thu, 2024-02-01 at 13:52 +0200, Andy Shevchenko wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you
> know the content is safe
> 
> On Tue, Jan 30, 2024 at 10:52:41AM +0000,
> Rengarajan.S@...rochip.com wrote:
> > On Sun, 2024-01-28 at 17:27 +0200, Andy Shevchenko wrote:
> > > On Thu, Jan 25, 2024 at 03:36:19PM +0530, Rengarajan S wrote:
> 
> ...
> 
> > > > +     /*
> > > > +      * Microchip PCI1XXXX UART supports maximum baud rate up
> > > > to 4
> > > > Mbps
> > > > +      */
> > > > +     if (up->port.type == PORT_MCHP16550A)
> > > > +             max = 4000000;
> > > 
> > > No. Please refactor the way the 8250_port won't be modified.
> > > 
> > > Also you have a define for this constant, use it.
> > 
> > The current UART clk in MCHP Ports in pci1xxxx.c is set to 62.5 MHz
> > in
> > order to support fractional baud rates which enables generation of
> > acceptable baud rate and lower error percentage from any available
> > frequency. With 62.5 MHz the maximum supported baud rate supported
> > as
> > per serial_8250_get_baud_rate is 3.9 Mbps. In order to extend the
> > support to 4 Mbps we had hardcoded the max value to 4 Mbps. Since,
> > baud
> > rate is calculated here we needed to make these changes in
> > 8250_port
> > and could not find a way to handle as part 8250_pci1xxxx. Can you
> > let
> > us know any alternatives to address this upper(max) limit?
> 
> Update port->uartclk accordingly in your driver, see how other 8250_*
> drivers
> do that (e.g., 8250_mid).
> 
> So, it will no go with hack in the 8250_port.


Hi Andy Shevchenko,

The UART clock is currently fixed to 62.5 MHz for supporting fractional
baud rates which allows the user to set any given baud rate with lower
errors. Changing the clock would no longer support the fractional baud
rate support feature. However, will check for any alternate way to add
the support in the 8250_pci1xxxx driver file and will update the patch
accordingly in the next revision.

> 
> --
> With Best Regards,
> Andy Shevchenko
> 
>