On Tue, Aug 11, 2020 at 12:38:41PM +0200, pet...@infradead.org wrote:
> On Tue, Aug 11, 2020 at 11:13:13AM +0100, Will Deacon wrote:
> > Using magic-sysrq via a keyboard interrupt over the serial console results
> > in
> > the following lockdep splat with the PL011 UART driver on v5.8. I can
> > reproduce
> > the issue under QEMU with arm64 defconfig + PROVE_LOCKING.
> >
> > Any chance somebody could take a look, please? It's a little annoying,
> > because it means when I uses magic-sysrq to increase the loglevel prior
> > to testing something else, lockdep gets disabled as a result.
> >
>
> Going by msm_serial, the thing to do is something like this:
>
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 8efd7c2a34fe..1717790ece2b 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -308,8 +308,9 @@ static void pl011_write(unsigned int val, const struct
> uart_amba_port *uap,
> */
> static int pl011_fifo_to_tty(struct uart_amba_port *uap)
> {
> - u16 status;
> unsigned int ch, flag, fifotaken;
> + int sysrq;
> + u16 status;
>
> for (fifotaken = 0; fifotaken != 256; fifotaken++) {
> status = pl011_read(uap, REG_FR);
> @@ -344,10 +345,12 @@ static int pl011_fifo_to_tty(struct uart_amba_port *uap)
> flag = TTY_FRAME;
> }
>
> - if (uart_handle_sysrq_char(&uap->port, ch & 255))
> - continue;
> + spin_unlock(&uap->port.lock);
> + sysrq = uart_handle_sysrq_char(&uap->port, ch & 255);
> + spin_lock(&uap->port.lock);
>
> - uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
> + if (!sysrq)
> + uart_insert_char(&uap->port, ch, UART011_DR_OE, ch,
> flag);
> }
>
> return fifotaken;
Cheers, that seems to do the trick:
Tested-by: Will Deacon <w...@kernel.org>
but what I don't understand is why I haven't run into this before, and why
nobody else seems to be reporting it!
I'll try some older kernels to see if it ever worked.
Will
