lib: sbi: Apply budget restriction when polling Zkr CSR state transition 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_ZKR_POLL_BUDGET, defaulting to 1000 iterations. Successful reads do not consume a try. Signed-off-by: Evgeny Voevodin <evvoevod@tenstorrent.com> Reviewed-by: Anup Patel <anup@brainfault.org> Link: https://lore.kernel.org/r/20260519225014.244672-1-evvoevod@tenstorrent.com Signed-off-by: Anup Patel <anup@brainfault.org>

diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
index c6cc04b..89245a2 100644
--- a/lib/sbi/Kconfig
+++ b/lib/sbi/Kconfig

@@ -6,6 +6,18 @@
 	int "Early console buffer size (bytes)"
 	default 256
 
+config 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.
+	  This also limits the wait time on systems with an event-driven
+	  entropy source. A successful read doesn't consume a try.
+
 config SBI_ECALL_TIME
 	bool "Timer extension"
 	default y

diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
index 4e34bbc..38f9bf2 100644
--- a/lib/sbi/sbi_init.c
+++ b/lib/sbi/sbi_init.c

@@ -280,20 +280,26 @@
 	if (sbi_hart_has_extension(scratch, SBI_HART_EXT_ZKR)) {
 		unsigned long guard_val = 0;
 		int chunks = sizeof(unsigned long) / sizeof(uint16_t);
-		bool res = true;
+		unsigned int tries = CONFIG_ZKR_POLL_BUDGET;
+		bool res = false;
 
-		while (chunks) {
+		while (chunks && tries) {
 			unsigned long seed = csr_swap(CSR_SEED, 0);
 			unsigned long opst = seed & SEED_OPTS_MASK;
+			res = false;
 
 			if (opst == SEED_OPTS_DEAD) {
-				res = false;
 				break;
 			}
 			if (opst == SEED_OPTS_ES16) {
 				guard_val = (guard_val << 16) | (seed & SEED_ENTROPY_MASK);
 				chunks--;
+				res = true;
+				/* Successful read doesn't consume a try */
+				tries++;
 			}
+
+			tries--;
 			continue;
 		}
 		if (res)