Re: [PATCH v1 2/2] i3c: master: svc: Prevent IRQ storm from false SLVSTART on NPCM845

[Frank Li]

On Mon, Apr 13, 2026 at 08:50:40AM +0800, Stanley Chu wrote:
> From: Stanley Chu <yschu@xxxxxxxxxxx>
>
> On NPCM845, when a target on the I3C bus gets stuck holding SDA low,
> the controller reports a false Master Request (MR) in-band interrupt
> event. The driver handles this by emitting a STOP condition to restore
> the bus.
>
> However, the hardware quirk SVC_I3C_QUIRK_FALSE_SLVSTART indicates that
> emitting a STOP condition may spuriously set the SLVSTART interrupt
> status bit. In the Master Request case, this creates a feedback loop:
> the STOP triggers a new SLVSTART event, the IRQ handler fires again,
> the controller still reports an MR type, another STOP is emitted, and
> the cycle repeats indefinitely, resulting in an IRQ storm that can lock
> up the CPU.
>
> Clear the SLVSTART status bit explicitly after emitting the STOP in the
> Master Request IBI handler when the SVC_I3C_QUIRK_FALSE_SLVSTART quirk
> is set. This breaks the feedback loop without affecting normal SLVSTART
> processing, which is already guarded in the top-level IRQ handler by
> checking that MSTATUS is in SLVREQ state.
>
> Signed-off-by: Stanley Chu <yschu@xxxxxxxxxxx>
> ---

Reviewed-by: Frank Li <Frank.Li@xxxxxxx>

>  drivers/i3c/master/svc-i3c-master.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/i3c/master/svc-i3c-master.c b/drivers/i3c/master/svc-i3c-master.c
> index 7d88e8fe3742..c16397013334 100644
> --- a/drivers/i3c/master/svc-i3c-master.c
> +++ b/drivers/i3c/master/svc-i3c-master.c
> @@ -655,6 +655,15 @@ static void svc_i3c_master_ibi_isr(struct svc_i3c_master *master)
>  		break;
>  	case SVC_I3C_MSTATUS_IBITYPE_MASTER_REQUEST:
>  		svc_i3c_master_emit_stop(master);
> +
> +		/*
> +		 * If a target gets stuck holding SDA low, the controller reports a MR.
> +		 * On NPCM845, emitting STOP may spuriously set SLVSTART, retriggering
> +		 * the interrupt and re-entering MR handling, leading to an IRQ storm.
> +		 * Clear SLVSTART after STOP to break the loop.
> +		 */
> +		if (svc_has_quirk(master, SVC_I3C_QUIRK_FALSE_SLVSTART))
> +			writel(SVC_I3C_MINT_SLVSTART, master->regs + SVC_I3C_MSTATUS);
>  		break;
>  	default:
>  		break;
> --
> 2.34.1
>