[PATCH 3/7] riscv: Use NULL as a sentinel value for smp_call_function

[Sean Anderson]

Some IPIs may already be pending when U-Boot is started. This could be a
problem if a secondary hart tries to handle an IPI before the boot hart has
initialized the IPI device.

This commit uses NULL as a sentinel for secondary harts so they know when
the IPI is initialized, and it is safe to use the IPI API. The smp addr
parameter is initialized to NULL by board_init_f_init_reserve. Before this,
secondary harts wait in wait_for_gd_init.

This imposes a minor restriction because harts may no longer jump to NULL.
However, given that the RISC-V debug device is likely to be located at
0x400, it is unlikely for any RISC-V implementation to have usable ram
located at 0x0.

Signed-off-by: Sean Anderson <seanga2 at gmail.com>
---

 arch/riscv/lib/smp.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/lib/smp.c b/arch/riscv/lib/smp.c
index ab6d8bd7fa..8c25755330 100644
--- a/arch/riscv/lib/smp.c
+++ b/arch/riscv/lib/smp.c
@@ -18,6 +18,12 @@ static int send_ipi_many(struct ipi_data *ipi, int wait)
 	u32 reg;
 	int ret, pending;
 
+	/* NULL is used as a sentinel value */
+	if (!ipi->addr) {
+		pr_err("smp_function cannot be set to 0x0\n");
+		return -EINVAL;
+	}
+
 	cpus = ofnode_path("/cpus");
 	if (!ofnode_valid(cpus)) {
 		pr_err("Can't find cpus node!\n");
@@ -50,11 +56,16 @@ static int send_ipi_many(struct ipi_data *ipi, int wait)
 			continue;
 #endif
 
-		gd->arch.ipi[reg].addr = ipi->addr;
 		gd->arch.ipi[reg].arg0 = ipi->arg0;
 		gd->arch.ipi[reg].arg1 = ipi->arg1;
 
-		__smp_mb();
+		/*
+		 * Ensure addr only becomes non-NULL when arg0 and arg1 are
+		 * valid. An IPI may already be pending on other harts, so we
+		 * need a way to signal that the IPI device has been
+		 * initialized, and that it is ok to call the function.
+		 */
+		__smp_store_release(&gd->arch.ipi[reg].addr, ipi->addr);
 
 		ret = riscv_send_ipi(reg);
 		if (ret) {
@@ -83,9 +94,16 @@ void handle_ipi(ulong hart)
 	if (hart >= CONFIG_NR_CPUS)
 		return;
 
-	__smp_mb();
+	smp_function = (void (*)(ulong, ulong, ulong))
+			__smp_load_acquire(&gd->arch.ipi[hart].addr);
+	/*
+	 * If the function is NULL, then U-Boot has not requested the IPI. The
+	 * IPI device may not be initialized, so all we can do is wait for
+	 * U-Boot to initialize it and send an IPI
+	 */
+	if (!smp_function)
+		return;
 
-	smp_function = (void (*)(ulong, ulong, ulong))gd->arch.ipi[hart].addr;
 	invalidate_icache_all();
 
 	/*
-- 
2.28.0