serial: meson: acquire port->lock in startup()

The uart_ops startup() callback is called without interrupts
disabled and without port->lock locked, relatively late during the
boot process (from the call path of console_on_rootfs()). If the
device is a console, it was already previously registered and could
be actively printing messages.

Since the startup() callback is reading/writing registers used by
the console write() callback (AML_UART_CONTROL), its access must
be synchronized using the port->lock. Currently it is not.

The startup() callback is the only function that explicitly enables
interrupts. Without the synchronization, it is possible that
interrupts become accidentally permanently disabled.

CPU0                           CPU1
meson_serial_console_write     meson_uart_startup
--------------------------     ------------------
spin_lock(port->lock)
val = readl(AML_UART_CONTROL)
uart_console_write()
                               writel(INT_EN, AML_UART_CONTROL)
writel(val, AML_UART_CONTROL)
spin_unlock(port->lock)

Add port->lock synchronization to meson_uart_startup() to avoid
racing with meson_serial_console_write().

Also add detailed comments to meson_uart_reset() explaining why it
is *not* using port->lock synchronization.

Link: https://lore.kernel.org/lkml/[email protected]
Fixes: ff7693d079e5 ("ARM: meson: serial: add MesonX SoC on-chip uart driver")
Reported-by: Marek Szyprowski <[email protected]>
Tested-by: Marek Szyprowski <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Reviewed-by: Jiri Slaby <[email protected]>
Acked-by: Neil Armstrong <[email protected]>
Signed-off-by: John Ogness <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

0 files changed, 0 insertions, 0 deletions