linux-kernel - Re: [PATCH] serial: 8250: probe all 16550A variants by default

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Date:   Mon, 25 May 2020 16:40:28 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     gregkh@...uxfoundation.org, linux-serial@...r.kernel.org,
        jslaby@...e.com, lukas@...ner.de, heikki.krogerus@...ux.intel.com,
        vigneshr@...com, linux-kernel@...r.kernel.org, fido_max@...ox.ru,
        radu-andrei.bulie@....com
Subject: Re: [PATCH] serial: 8250: probe all 16550A variants by default

On Mon, May 25, 2020 at 04:02:38PM +0300, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> On NXP T1040, the UART is typically detected as 16550A_FSL64. After said
> patch, it gets detected as plain 16550A and the Linux console is
> completely garbled and missing characters.
> 
> So clearly, introducing the SERIAL_8250_16550A_VARIANTS config option
> has broken many existing users because it has changed the default
> behavior. Restore that by adding a 'default y' to this option. Users who
> care about 20 ms shorter boot time can always disable it, but stop
> wasting many debugging hours for people who don't care all that much.

While this sounds like a good enough fix of the regression, the proper one
seems to have a separate quirk driver which avoids auto probe and uses
compatible strings or whatever that board can supply. 8250_port historically
has a burden of old and buggy hardware, that's why the above mentioned patch
exposed that.

That said, I have no objections, but had to note about possible approaches.

> Fixes: dc56ecb81a0a ("serial: 8250: Support disabling mdelay-filled probes of 16550A variants")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> ---
>  drivers/tty/serial/8250/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/tty/serial/8250/Kconfig b/drivers/tty/serial/8250/Kconfig
> index af0688156dd0..89c7ecb55619 100644
> --- a/drivers/tty/serial/8250/Kconfig
> +++ b/drivers/tty/serial/8250/Kconfig
> @@ -63,6 +63,7 @@ config SERIAL_8250_PNP
>  config SERIAL_8250_16550A_VARIANTS
>  	bool "Support for variants of the 16550A serial port"
>  	depends on SERIAL_8250
> +	default y
>  	help
>  	  The 8250 driver can probe for many variants of the venerable 16550A
>  	  serial port. Doing so takes additional time at boot.
> -- 
> 2.25.1
> 

-- 
With Best Regards,
Andy Shevchenko