Re: [PATCH v2 3/3] serial: core: Allow skipping old serial port initialization

[Kees Cook]

On Wed, Mar 14, 2018 at 2:44 PM, Daniel Kurtz <djkurtz@xxxxxxxxxxxx> 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.
>
> Signed-off-by: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>
> ---
> Changes since v1:
>  * Rename variable to serial8250_skip_old_ports
>  * Also set variable in acpi/spcr. to support no-earlycon case.
>
>  drivers/acpi/spcr.c                  | 2 ++
>  drivers/tty/serial/8250/8250_core.c  | 6 ++++++
>  drivers/tty/serial/8250/8250_early.c | 1 +
>  include/linux/serial_8250.h          | 6 ++++++
>  4 files changed, 15 insertions(+)
>
> diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
> index 52d840d0e05b..f59591283410 100644
> --- a/drivers/acpi/spcr.c
> +++ b/drivers/acpi/spcr.c
> @@ -14,6 +14,7 @@
>  #include <linux/acpi.h>
>  #include <linux/console.h>
>  #include <linux/kernel.h>
> +#include <linux/serial_8250.h>
>  #include <linux/serial_core.h>
>
>  /*
> @@ -208,6 +209,7 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
>         }
>
>         if (amdcz_present(table)) {
> +               serial8250_skip_old_ports = true;
>                 if (enable_earlycon)
>                         uart = "amdcz";
>         }

Sorry for being dense. What tree is this against? I can't find mention
of amdcz in Linus's tree nor linux-next.

> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 9342fc2ee7df..a04da392a251 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -66,6 +66,9 @@ static unsigned int skip_txen_test; /* force skip of txen test at init time */
>  #define SERIAL_PORT_DFNS
>  #endif
>
> +bool serial8250_skip_old_ports;
> +EXPORT_SYMBOL(serial8250_skip_old_ports);
> +
>  static const struct old_serial_port old_serial_port[] = {
>         SERIAL_PORT_DFNS /* defined in asm/serial.h */
>  };
> @@ -537,6 +540,9 @@ static void __init serial8250_isa_init_ports(void)
>         if (share_irqs)
>                 irqflag = IRQF_SHARED;
>
> +       if (serial8250_skip_old_ports)
> +               return;
> +
>         for (i = 0, up = serial8250_ports;
>              i < ARRAY_SIZE(old_serial_port) && i < nr_uarts;
>              i++, up++) {
> diff --git a/drivers/tty/serial/8250/8250_early.c b/drivers/tty/serial/8250/8250_early.c
> index c6bf971a6038..288d2be82990 100644
> --- a/drivers/tty/serial/8250/8250_early.c
> +++ b/drivers/tty/serial/8250/8250_early.c
> @@ -202,6 +202,7 @@ static int __init early_amdcz_setup(struct earlycon_device *dev,
>  {
>         struct uart_port *port = &dev->port;
>
> +       serial8250_skip_old_ports = true;
>         port->uartclk = 48000000;
>
>         return early_serial8250_setup(dev, opt);
> diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
> index a27ef5f56431..02570edaddd8 100644
> --- a/include/linux/serial_8250.h
> +++ b/include/linux/serial_8250.h
> @@ -136,6 +136,12 @@ struct uart_8250_port {
>         struct uart_8250_em485 *em485;
>  };
>
> +#ifdef CONFIG_SERIAL_8250
> +extern bool serial8250_skip_old_ports;
> +#else
> +static const bool serial8250_skip_old_ports;
> +#endif

Where does serial8250_skip_old_ports get used where CONFIG_SERIAL_8250
isn't defined? (i.e. why is the #ifdef needed here?)

> +
>  static inline struct uart_8250_port *up_to_u8250p(struct uart_port *up)
>  {
>         return container_of(up, struct uart_8250_port, port);
> --
> 2.16.2.804.g6dcf76e118-goog
>

Otherwise, sure, sounds good. :)

-Kees

-- 
Kees Cook
Pixel Security