linux-kernel - Re: [PATCH v3 3/3] serial: core: Allow skipping old serial port initialization

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Date:   Thu, 15 Mar 2018 15:04:28 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Daniel Kurtz <djkurtz@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     adurbin@...omium.org, linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, Jiri Slaby <jslaby@...e.com>,
        Kees Cook <keescook@...omium.org>,
        Matthias Brugger <mbrugger@...e.com>,
        David Howells <dhowells@...hat.com>,
        Allen Pais <allen.lkml@...il.com>, Sean Young <sean@...s.org>,
        Douglas Anderson <dianders@...omium.org>,
        Matt Redfearn <matt.redfearn@...s.com>,
        Jeffy Chen <jeffy.chen@...k-chips.com>,
        Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        "open list:ACPI" <linux-acpi@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>
Subject: Re: [PATCH v3 3/3] serial: core: Allow skipping old serial port
 initialization

On Wed, 2018-03-14 at 20:04 -0600, Daniel Kurtz wrote:
> The old_serial_port global array in 8250_core is supposed to hold an
> entry
> for each serial port on the system that cannot be discovered via a
> standard enumeration mechanism (aka ACPI/PCI/DTS).  The array is
> populated
> at compile-time from the value specified in the SERIAL_PORT_DFNS
> macro.
> This macro is defined in arch/serial.h.
> 
> For x86, this macro is currently unconditionally initialized to supply
> four ioport UARTs (0x3F8, 0x2F8, 0x3E8, 0x2E8).
> 
> However, not all x86 CPUs have these four ioport UARTs.  For example,
> the
> UARTs on AMD Carrizo and later are separate memory mapped Designware
> IP
> blocks.
> 
> Fairly early in boot the console_initcall univ8250_console_init
> iterates
> over this array and installs these old UARTs into the global array
> serial8250_ports.  Further, it attempts to register them for use as
> the console.  In other words, if, for example, the kernel commandline
> has
> console=ttyS0, the console will be switched over to one of these
> non-existent UARTs.  Only later, when the real UART drivers are probed
> and their devices are instantiated will the console switch back over
> to
> the proper UART.
> 
> This is noticeable when using earlycon, since part of the serial
> console
> log will appear to disappear (when the bogus old takes over) and then
> re-appear (when the real UART finally gets registered for the
> console).
> 
> The problem is even more noticable when *not* using earlycon, since in
> this case the entire console output is missing, having been
> incorrectly
> played back to the non-existing serial port.
> 
> Create a global variable to allow skipping old serial port
> initialization
> and wire it up to the AMDCZ ACPI SPCR quirk and the special amdcz
> earlycon
> setup handler.

I don't like this approach at all.
But unfortunately I have nothing to propose.
Just felt like I have to share my opinion on this.

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy