linux-kernel - Re: [PATCH v2 0/1] Revert "serial: 8250: Fix max baud limit in generic 8250 port"

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Date:   Thu, 2 Jul 2020 06:11:52 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Daniel Winkler <danielwinkler@...gle.com>,
        Serge Semin <fancer.lancer@...il.com>,
        linux-serial@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        BlueZ <linux-bluetooth@...r.kernel.org>,
        chromeos-bluetooth-upstreaming 
        <chromeos-bluetooth-upstreaming@...omium.org>,
        stable@...r.kernel.org, abhishekpandit@...omium.org,
        Aaron Sierra <asierra@...-inc.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/1] Revert "serial: 8250: Fix max baud limit in
 generic 8250 port"

On Thu, Jul 02, 2020 at 01:37:13AM +0300, Serge Semin wrote:
> 1) Add a new capability like UART_CAP_NO16DIV and take it into account
>    in the serial8250_get_baud_rate() method.
>  
> I don't have a documentation for the Mediatek UART port, but it seems to me
> that that controller calculates the baud rate differently from the standard
> 8250 port. A standard 8250 port does that by the next formulae:
>   baud = uartclk / (16 * divisor).
> While it seems to me that the Mediatek port uses the formulae like:
>   baud = uartclk / divisor. (Please, correct me if I'm wrong)

8250_bcm2835aux.c seems to suffer from a similar issue and
solves it like this in the ->probe hook:

	/* the HW-clock divider for bcm2835aux is 8,
	 * but 8250 expects a divider of 16,
	 * so we have to multiply the actual clock by 2
	 * to get identical baudrates.
	 */
	up.port.uartclk = clk_get_rate(data->clk) * 2;


> 2) Manually call serial8250_do_set_divisor() in the custom set_termios()
>    callback.
> 
> Just add the uart_update_timeout() and serial8250_do_set_divisor() methods
> invocation into the mtk8250_set_termios() function, which the original commit
> 81bb549fdf14 ("serial: 8250_mtk: support big baud rate") author should have
> done in the first place.

That sound preferable as adding new quirks into core code feels
like a case of midlayer fallacy:

https://blog.ffwll.ch/2016/12/midlayers-once-more-with-feeling.html

Thanks,

Lukas