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Subject[PATCH v6 3/3] powerpc/powernv: Preference optimization for SPRs with constant values
Date
There are SPRs whose values don't tend to change over time and invoking
self-save on them, where the values are gotten each time may turn out to
be inefficient. In that case calling a self-restore where passing the
value makes more sense as, if the value is same the memory location
is not updated.
SPRs that dont change are as follows:
SPRN_HSPRG0,
SPRN_LPCR,
SPRN_PTCR,
SPRN_HMEER,
SPRN_HID0,

There are also SPRs whose values change and/or their value may not be
correcty determinable in the kernel. Eg: MSR and PSSCR

The value of LPCR is dynamic based on if the CPU is entered a stop
state during cpu idle versus cpu hotplug.

Therefore in this optimization patch, introducing the concept of
preference for each SPR to choose from in the case both self-save and
self-restore is supported.

The preference bitmask is shown as below:
----------------------------
|... | 2nd pref | 1st pref |
----------------------------
MSB LSB

The preference from higher to lower is from LSB to MSB with a shift of 8
bits.
Example:
Prefer self save first, if not available then prefer self
restore
The preference mask for this scenario will be seen as below.
((FIRMWARE_RESTORE << PREFERENCE_SHIFT) | FIRMWARE_SELF_SAVE)
---------------------------------
|... | Self restore | Self save |
---------------------------------
MSB LSB

Signed-off-by: Pratik Rajesh Sampat <psampat@linux.ibm.com>
---
arch/powerpc/platforms/powernv/idle.c | 88 +++++++++++++++++++++------
1 file changed, 70 insertions(+), 18 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/idle.c b/arch/powerpc/platforms/powernv/idle.c
index e77b31621081..4d896df51582 100644
--- a/arch/powerpc/platforms/powernv/idle.c
+++ b/arch/powerpc/platforms/powernv/idle.c
@@ -43,6 +43,31 @@
#define FIRMWARE_SELF_SAVE 0x2
#define KERNEL_SAVE_RESTORE 0x4

+#define NR_PREFERENCES 2
+#define PREFERENCE_SHIFT 4
+#define PREFERENCE_MASK 0xf
+/*
+ * Bitmask defining the kind of preferences available.
+ * Note : The higher to lower preference is from LSB to MSB, with a shift of
+ * 4 bits.
+ * ----------------------------
+ * | | 2nd pref | 1st pref |
+ * ----------------------------
+ * MSB LSB
+ */
+/* Prefer Restore if available, otherwise unsupported */
+#define PREFER_SELF_RESTORE_ONLY FIRMWARE_RESTORE
+/* Prefer Save if available, otherwise unsupported */
+#define PREFER_SELF_SAVE_ONLY FIRMWARE_SELF_SAVE
+/* Prefer Restore when available, otherwise prefer Save */
+#define PREFER_RESTORE_SAVE ((FIRMWARE_SELF_SAVE << \
+ PREFERENCE_SHIFT)\
+ | FIRMWARE_RESTORE)
+/* Prefer Save when available, otherwise prefer Restore*/
+#define PREFER_SAVE_RESTORE ((FIRMWARE_RESTORE <<\
+ PREFERENCE_SHIFT)\
+ | FIRMWARE_SELF_SAVE)
+
static u32 supported_cpuidle_states;
struct pnv_idle_states_t *pnv_idle_states;
int nr_pnv_idle_states;
@@ -52,6 +77,7 @@ static bool is_ptcr_self_save;

struct preferred_sprs {
u64 spr;
+ u32 preferred_mode;
u32 supported_mode;
};

@@ -66,42 +92,52 @@ struct preferred_sprs {
struct preferred_sprs preferred_sprs[] = {
{
.spr = SPRN_HSPRG0,
+ .preferred_mode = PREFER_RESTORE_SAVE,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = SPRN_LPCR,
+ .preferred_mode = PREFER_SAVE_RESTORE,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = SPRN_PTCR,
+ .preferred_mode = PREFER_RESTORE_SAVE,
.supported_mode = KERNEL_SAVE_RESTORE,
},
{
.spr = SPRN_HMEER,
+ .preferred_mode = PREFER_RESTORE_SAVE,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = SPRN_HID0,
+ .preferred_mode = PREFER_RESTORE_SAVE,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = P9_STOP_SPR_MSR,
+ .preferred_mode = PREFER_SAVE_RESTORE,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = P9_STOP_SPR_PSSCR,
+ .preferred_mode = PREFER_SAVE_RESTORE,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = SPRN_HID1,
+ .preferred_mode = PREFER_RESTORE_SAVE,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = SPRN_HID4,
+ .preferred_mode = PREFER_SELF_RESTORE_ONLY,
.supported_mode = FIRMWARE_RESTORE,
},
{
.spr = SPRN_HID5,
+ .preferred_mode = PREFER_SELF_RESTORE_ONLY,
.supported_mode = FIRMWARE_RESTORE,
}
};
@@ -218,7 +254,9 @@ static int pnv_self_restore_sprs(u64 pir, int cpu, u64 spr)

static int pnv_self_save_restore_sprs(void)
{
- int rc, index, cpu;
+ int rc, index, cpu, k;
+ bool is_initialized;
+ u32 preferred;
u64 pir;
struct preferred_sprs curr_spr;

@@ -234,26 +272,40 @@ static int pnv_self_save_restore_sprs(void)
curr_spr.spr == SPRN_HID4 ||
curr_spr.spr == SPRN_HID5))
continue;
-
- if (curr_spr.supported_mode & FIRMWARE_SELF_SAVE) {
- rc = opal_slw_self_save_reg(pir,
- curr_spr.spr);
- if (rc != 0)
- return rc;
- switch (curr_spr.spr) {
- case SPRN_LPCR:
- is_lpcr_self_save = true;
+ for (k = 0; k < NR_PREFERENCES; k++) {
+ preferred = curr_spr.preferred_mode
+ & PREFERENCE_MASK;
+ if (preferred & curr_spr.supported_mode &
+ FIRMWARE_SELF_SAVE) {
+ is_initialized = true;
+ rc = opal_slw_self_save_reg(pir,
+ curr_spr.spr);
+ if (rc != 0)
+ return rc;
+ switch (curr_spr.spr) {
+ case SPRN_LPCR:
+ is_lpcr_self_save = true;
+ break;
+ case SPRN_PTCR:
+ is_ptcr_self_save = true;
+ break;
+ }
break;
- case SPRN_PTCR:
- is_ptcr_self_save = true;
+ } else if (preferred & curr_spr.supported_mode &
+ FIRMWARE_RESTORE) {
+ is_initialized = true;
+ rc = pnv_self_restore_sprs(pir, cpu,
+ curr_spr.spr);
+ if (rc != 0)
+ return rc;
break;
}
- } else if (curr_spr.supported_mode & FIRMWARE_RESTORE) {
- rc = pnv_self_restore_sprs(pir, cpu,
- curr_spr.spr);
- if (rc != 0)
- return rc;
- } else {
+ preferred_sprs[index].preferred_mode =
+ preferred_sprs[index].preferred_mode >>
+ PREFERENCE_SHIFT;
+ curr_spr = preferred_sprs[index];
+ }
+ if (!is_initialized) {
if (curr_spr.supported_mode & KERNEL_SAVE_RESTORE ||
(cpu_has_feature(CPU_FTR_ARCH_300) &&
(curr_spr.spr == SPRN_HID1 ||
--
2.17.1
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