Re: [PATCH] serial8250 on tegra hsuart: recover from spurious interrupts due to tegra2 silicon bug

[Ilpo Järvinen]

On Mon, 21 Nov 2022, Richard Leitner wrote:

> Hi,
> 
> On Fri, Jul 13, 2018 at 11:32:42AM +0000, David R. Piegdon wrote:
> > Hi,
> > a while back I sent a few mails regarding spurious interrupts in the
> > UARTA (hsuart) block of the Tegra2 SoC, when using the 8250 driver for
> > it instead of the hsuart driver. After going down a pretty deep
> > debugging/testing hole, I think I found a patch that fixes the issue. So
> > far testing in a reboot-cycle suggests that the error frequency dropped
> > from >3% of all reboots to at least <0.05% of all reboots. Tests
> > continue to run over the weekend.
> > 
> > The patch below already is a second iteration; the first did not reset
> > the MCR or contain the lines below '// clear interrupts'. This resulted
> > in no more spurious interrupts, but in a few % of spurious interrupts
> > that were recovered the UART block did not receive any characters any
> > more. So further resetting was required to fully reacquire operational
> > state of the UART block.
> > 
> > I'd love any comments/suggestions on this!
> 
> I'd like to follow up on this ancient patch as we are using it
> successfully for a few years with different kernel versions on a
> tegra20 SOM (tamonten) now and I'm currently cleaning up our tree.
> 
> David, have you done any work in regarding this issue since 2018?
> 
> What would be needed to get this solution mainline?

It seems that the code would belong to ->handle_irq() rather than 
8250_core. Do the affected device belong under 8250_tegra.c? If they do, 
then just create .handle_irq for it and detect this condition there after 
call to serial8250_handle_irq().

> The recipient of this mail are from the initial thread [1] and
> a current get_maintainers.pl run.
> 
> regards;rl
> 
> [1] https://patchwork.ozlabs.org/project/linux-tegra/patch/4676ea34-69ce-5422-1ded-94218b89f7d9@xxxxxxxx/
> 
> > 
> > Cheers,
> > 
> > David
> > 
> > diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> > index e8819aa20415..1d76eebefd4e 100644
> > --- a/drivers/tty/serial/8250/8250_core.c
> > +++ b/drivers/tty/serial/8250/8250_core.c
> > @@ -140,6 +140,38 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
> >  				"serial8250: too much work for irq%d\n", irq);
> >  			break;
> >  		}
> > +
> > +#ifdef CONFIG_ARCH_TEGRA_2x_SOC
> > +		if (!handled && (port->type == PORT_TEGRA)) {
> > +			/*
> > +			 * Fix Tegra 2 CPU silicon bug where sometimes
> > +			 * "TX holding register empty" interrupts result in a
> > +			 * bad (metastable?) state in Tegras HSUART IP core.
> > +			 * Only way to recover seems to be to reset all
> > +			 * interrupts as well as the TX queue and the MCR.
> > +			 * But we don't want to loose any outgoing characters,
> > +			 * so only do it if the RX and TX queues are empty.
> > +			 */
> > +			unsigned char lsr = port->serial_in(port, UART_LSR);

serial_lsr_in(), make sure you take the port's lock btw.

> > +			const unsigned char fifo_empty_mask =
> > +						(UART_LSR_TEMT | UART_LSR_THRE);
> > +			if (((lsr & (UART_LSR_DR | fifo_empty_mask)) ==
> > +							fifo_empty_mask)) {

uart_lsr_tx_empty(lsr) && !(lsr & UART_LSR_DR)

fifo_empty_mask can be dropped.

-- 
 i.

> > +				port->serial_out(port, UART_IER, 0);
> > +				port->serial_out(port, UART_MCR, 0);
> > +				serial8250_clear_and_reinit_fifos(up);
> > +				port->serial_out(port, UART_MCR, up->mcr);
> > +				port->serial_out(port, UART_IER, up->ier);
> > +				// clear interrupts
> > +				serial_port_in(port, UART_LSR);
> > +				serial_port_in(port, UART_RX);
> > +				serial_port_in(port, UART_IIR);
> > +				serial_port_in(port, UART_MSR);
> > +				up->lsr_saved_flags = 0;
> > +				up->msr_saved_flags = 0;
> > +			}
> > +		}
> > +#endif
> >  	} while (l != end);
> >  
> >  	spin_unlock(&i->lock);
>