Re: [PATCH] clocksource/drivers/riscv: Refuse to probe on T-Head

[Samuel Holland <samuel@sholland.org>]

Hi Palmer,

On 2/9/23 17:23, Palmer Dabbelt wrote:
> From: Palmer Dabbelt <palmer@rivosinc.com>
> 
> As of d9f15a9de44a ("Revert "clocksource/drivers/riscv: Events are
> stopped during CPU suspend"") this driver no longer functions correctly
> for the T-Head firmware.  That shouldn't impact any users, as we've got

The current situation is that the C9xx CLINT binding was just accepted,
so the CLINT is not yet described in any devicetree. So at least with
upstream OpenSBI, which needs the CLINT DT node, the SBI timer extension
never worked at all.

> a functioning driver that's higher priority, but let's just be safe and
> ban it from probing at all.
> 
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> ---
> This feel super ugly to me, but I'm not sure how to do this more
> cleanly.  I'm not even sure if it's necessary, but I just ran back into
> the driver reviewing some other patches so I figured I'd say something.

This is not necessary as long as we add the riscv,timer node with the
riscv,timer-cannot-wake-cpu property before we add the CLINT node. So it
should not be a problem for any C9xx platform going forward.

Regards,
Samuel

> ---
>  drivers/clocksource/timer-riscv.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index a0d66fabf073..d2d0236d1ae6 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -139,6 +139,22 @@ static int __init riscv_timer_init_dt(struct device_node *n)
>  	if (cpuid != smp_processor_id())
>  		return 0;
>  
> +	/*
> +	 * The T-Head firmware does not route timer interrups to the core
> +	 * during non-retentive suspend.  This is allowed by the specifications
> +	 * (no interrupts are required to wake up the core during non-retentive
> +	 * suspend), but most systems don't behave that way and Linux just
> +	 * assumes that interrupts work.
> +	 *
> +	 * There's another timer for the T-Head sytems that behave this way
> +	 * that is already probed by default, but just to be sure skip
> +	 * initializing the SBI driver as it'll just break things later.
> +	 */
> +	if (sbi_get_mvendorid() == THEAD_VENDOR_ID) {
> +		pr_debug_once("Skipping SBI timer on T-Head due to missed wakeups");
> +		return 0;
> +	}
> +
>  	domain = NULL;
>  	child = of_get_compatible_child(n, "riscv,cpu-intc");
>  	if (!child) {