Re: [PATCH v3] powerpc/powernv: Use darn instr for random_seed on p9

On Sat, Aug 5, 2017 at 3:06 AM, Tyrel Datwyler
[off-list ref] wrote:

On 08/03/2017 06:12 PM, Matt Brown wrote:

quoted

This adds the powernv_get_random_darn function which utilises the darn
instruction, introduced in POWER9. The powernv_get_random_darn function
is used as the ppc_md.get_random_seed on P9.

The DARN instruction can potentially throw an error, so we attempt to
register the powernv_get_random_darn function up to 10 times before
failing.

Signed-off-by: Matt Brown <redacted>
---
v3:
      - add repeat attempts to register the ppc_md.get_random_seed
      - fixed the PPC_DARN macro
      - move DARN_ERR definition
      - fixed commit message
v2:
      - remove repeat darn attempts
      - move hook to rng_init
---
 arch/powerpc/include/asm/ppc-opcode.h |  4 ++++
 arch/powerpc/platforms/powernv/rng.c  | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
index c4ced1d..aabd150 100644
--- a/arch/powerpc/include/asm/ppc-opcode.h
+++ b/arch/powerpc/include/asm/ppc-opcode.h

@@ -134,6 +134,7 @@
 #define PPC_INST_COPY                        0x7c00060c
 #define PPC_INST_COPY_FIRST          0x7c20060c
 #define PPC_INST_CP_ABORT            0x7c00068c
+#define PPC_INST_DARN                        0x7c0005e6
 #define PPC_INST_DCBA                        0x7c0005ec
 #define PPC_INST_DCBA_MASK           0xfc0007fe
 #define PPC_INST_DCBAL                       0x7c2005ec

@@ -325,6 +326,9 @@

 /* Deal with instructions that older assemblers aren't aware of */
 #define      PPC_CP_ABORT            stringify_in_c(.long PPC_INST_CP_ABORT)
+#define PPC_DARN(t, l)               stringify_in_c(.long PPC_INST_DARN |  \
+                                             ___PPC_RT(t)       |  \
+                                             (((l) & 0x3) << 16))
 #define      PPC_DCBAL(a, b)         stringify_in_c(.long PPC_INST_DCBAL | \
                                      __PPC_RA(a) | __PPC_RB(b))
 #define      PPC_DCBZL(a, b)         stringify_in_c(.long PPC_INST_DCBZL | \

diff --git a/arch/powerpc/platforms/powernv/rng.c b/arch/powerpc/platforms/powernv/rng.c
index 5dcbdea..83b925c 100644
--- a/arch/powerpc/platforms/powernv/rng.c
+++ b/arch/powerpc/platforms/powernv/rng.c

@@ -16,11 +16,13 @@
 #include <linux/slab.h>
 #include <linux/smp.h>
 #include <asm/archrandom.h>
+#include <asm/cputable.h>
 #include <asm/io.h>
 #include <asm/prom.h>
 #include <asm/machdep.h>
 #include <asm/smp.h>

+#define DARN_ERR 0xFFFFFFFFFFFFFFFFul

 struct powernv_rng {
      void __iomem *regs;

@@ -67,6 +69,21 @@ int powernv_get_random_real_mode(unsigned long *v)
      return 1;
 }

+int powernv_get_random_darn(unsigned long *v)
+{
+     unsigned long val;
+
+     /* Using DARN with L=1 - 64-bit conditioned random number */
+     asm volatile(PPC_DARN(%0, 1) : "=r"(val));
+
+     if (val == DARN_ERR)
+             return 0;
+
+     *v = val;
+
+     return 1;
+}
+
 int powernv_get_random_long(unsigned long *v)
 {
      struct powernv_rng *rng;

@@ -135,8 +152,9 @@ static __init int rng_create(struct device_node *dn)

 static __init int rng_init(void)
 {
+     unsigned long darn_test;
      struct device_node *dn;
-     int rc;
+     int rc, i;

      for_each_compatible_node(dn, NULL, "ibm,power-rng") {
              rc = rng_create(dn);

@@ -150,6 +168,21 @@ static __init int rng_init(void)
              of_platform_device_create(dn, NULL, NULL);
      }

+     if (cpu_has_feature(CPU_FTR_ARCH_300)) {
+             for (i = 0; i < 10; i++) {
+                     if (powernv_get_random_darn(&darn_test)) {
+                             ppc_md.get_random_seed =
+                                     powernv_get_random_darn;
+                             break;

If you return directly here you can avoid the (i == 9) conditional every iteration of the
loop by moving the pr_warn to just outside the loop.

That's true, although it is very unlikely for the
powernv_get_random_darn to fail. So in practice we should never reach
the (i == 9) conditional.
The loop is more of a backup in the rare case that it does fail.

Thanks,
Matt

-Tyrel

quoted

+                     }
+
+                     if (i == 9) {
+                             pr_warn("Failed to use powernv_get_random_darn"\
+                                     "as get_random_seed");
+                     }
+             }
+     }
+
      return 0;
 }
 machine_subsys_initcall(powernv, rng_init);