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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

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@linux.intel.com>
Intel Finland Oy