[PATCH] lib: sbi: Apply budget restriction when polling Zkr CSR state transition

[Nicholas Piggin]

On Tue, May 19, 2026 at 10:50:14PM +0000, Evgeny Voevodin wrote:
> Zkr architecture doesn't define a time limit on state transitions
> which results in hanging on unresponsive or event-driven platforms.
> To prevent this, we need to limit polling iterations and fall back
> in case the budget is over, and stack guard keeps its initial value.
> The budget is configurable with CONFIG_SBI_INIT_ZKR_POLL_BUDGET,
> defaulting to 1000 iterations.
> Successful reads do not consume a try.
> 
> Signed-off-by: Evgeny Voevodin <evvoevod at tenstorrent.com>
> ---
>  lib/sbi/Kconfig    | 12 ++++++++++++
>  lib/sbi/sbi_init.c | 12 +++++++++---
>  2 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
> index c6cc04bc..a11f788c 100644
> --- a/lib/sbi/Kconfig
> +++ b/lib/sbi/Kconfig
> @@ -6,6 +6,18 @@ config CONSOLE_EARLY_BUFFER_SIZE
>  	int "Early console buffer size (bytes)"
>  	default 256
>  
> +config SBI_INIT_ZKR_POLL_BUDGET
> +	int "Zkr seed polling budget (iterations)"
> +	default 1000
> +	help
> +	  Maximum number of iterations to poll CSR_SEED when initializing
> +	  the stack guard variable. The Zkr specification doesn't define
> +	  a time limit on transitioning to ES16 between polls, which
> +	  makes it impossible to tell whether entropy is being
> +	  accumulated slowly or the entropy source is not functioning.

If entropy source is not functioning it should return DEAD, surely.
Specifications always require a "reasonable" performance, whether that
is explicit or not. That's open to interpretation and application, but
if a word of entropy at boot causes a responsiveness problem, I don't
know if that's a reasonable expectation in the spec that needs to be
accounted for.

So I think a bit more information would be good. If if this is a
workaround for a particular platform that might be okay but I think it
should be framed as such.

Thanks,
Nick