On 19.12.2024 16:30, Geert Uytterhoeven wrote:
> Hi Claudiu,
>
> On Wed, Dec 4, 2024 at 4:58 PM Claudiu <claudiu.beznea@xxxxxxxxx> wrote:
>> From: Claudiu Beznea <claudiu.beznea.uj@xxxxxxxxxxxxxx>
>>
>> In the sh-sci driver, serial ports are mapped to the sci_ports[] array,
>> with earlycon mapped at index zero.
>>
>> The uart_add_one_port() function eventually calls __device_attach(),
>> which, in turn, calls pm_request_idle(). The identified code path is as
>> follows:
>>
>> uart_add_one_port() ->
>> serial_ctrl_register_port() ->
>> serial_core_register_port() ->
>> serial_core_port_device_add() ->
>> serial_base_port_add() ->
>> device_add() ->
>> bus_probe_device() ->
>> device_initial_probe() ->
>> __device_attach() ->
>> // ...
>> if (dev->p->dead) {
>> // ...
>> } else if (dev->driver) {
>> // ...
>> } else {
>> // ...
>> pm_request_idle(dev);
>> // ...
>> }
>>
>> The earlycon device clocks are enabled by the bootloader. However, the
>> pm_request_idle() call in __device_attach() disables the SCI port clocks
>> while earlycon is still active.
>>
>> The earlycon write function, serial_console_write(), calls
>> sci_poll_put_char() via serial_console_putchar(). If the SCI port clocks
>> are disabled, writing to earlycon may sometimes cause the SR.TDFE bit to
>> remain unset indefinitely, causing the while loop in sci_poll_put_char()
>> to never exit. On single-core SoCs, this can result in the system being
>> blocked during boot when this issue occurs.
>>
>> To resolve this, increment the runtime PM usage counter for the earlycon
>> SCI device before registering the UART port.
>>
>> Fixes: 0b0cced19ab1 ("serial: sh-sci: Add CONFIG_SERIAL_EARLYCON support")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@xxxxxxxxxxxxxx>
>
> Thanks for your patch!
>
>> --- a/drivers/tty/serial/sh-sci.c
>> +++ b/drivers/tty/serial/sh-sci.c
>> @@ -3435,7 +3435,24 @@ static int sci_probe_single(struct platform_device *dev,
>> sciport->port.flags |= UPF_HARD_FLOW;
>> }
>>
>> + /*
>> + * In case:
>> + * - this is the earlycon port (mapped on index 0 in sci_ports[]) and
>> + * - it now maps to an alias other than zero and
>> + * - the earlycon is still alive (e.g., "earlycon keep_bootcon" is
>> + * available in bootargs)
>> + *
>> + * we need to avoid disabling clocks and PM domains through the runtime
>> + * PM APIs called in __device_attach(). For this, increment the runtime
>> + * PM reference counter (the clocks and PM domains were already enabled
>> + * by the bootloader). Otherwise the earlycon may access the HW when it
>> + * has no clocks enabled leading to failures (infinite loop in
>> + * sci_poll_put_char()).
>> + */
>> +
>> if (sci_ports[0].earlycon && sci_ports[0].port.mapbase == sci_res->start) {
>
> Now there are two tests for mapbase: here and in sci_probe()...
I'll adjust it!
Thank you for your review,
Claudiu
>
>> + pm_runtime_get_noresume(&dev->dev);
>> +
>> /*
>> * Skip cleanup up the sci_port[0] in early_console_exit(), this
>> * port is the same as the earlycon one.
>
> Gr{oetje,eeting}s,
>
> Geert
>
