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

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Date:   Thu, 15 Mar 2018 16:24:56 +0000
From:   Daniel Kurtz <djkurtz@...omium.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        adurbin@...omium.org, linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, jslaby@...e.com,
        Kees Cook <keescook@...omium.org>, mbrugger@...e.com,
        dhowells@...hat.com, allen.lkml@...il.com, sean@...s.org,
        Doug Anderson <dianders@...omium.org>, matt.redfearn@...s.com,
        Jeffy <jeffy.chen@...k-chips.com>,
        marc_gonzalez@...madesigns.com, linux-acpi@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH v3 3/3] serial: core: Allow skipping old serial port initialization

On Thu, Mar 15, 2018 at 7:04 AM Andy Shevchenko <
andriy.shevchenko@...ux.intel.com> wrote:

> 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.

I'm not very proud of this either, but I don't know how else to set this
flag.
I proposed a Kconfig option earlier, but that also met resistance.
Perhaps another kernel command line option?


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