[Skiboot] [PATCH] cpu: supply ibm,dec-bits via devicetree

[Oliver O'Halloran]

ISAv3 adds a mode to increase the size of the decrementer from 32 bits.
The enlarged decrementer can be between 32 and 64 bits wide with the
exact value being implementation dependent. This patch adds support for
detecting the size of the large decrementer and populating each CPU node
with the "ibm,dec-bits" property.

Signed-off-by: Oliver O'Halloran <oohall at gmail.com>
---
 core/cpu.c          | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/processor.h |  1 +
 2 files changed, 49 insertions(+)

diff --git a/core/cpu.c b/core/cpu.c
index ffc264f90907..32b70e0c3713 100644
--- a/core/cpu.c
+++ b/core/cpu.c
@@ -524,10 +524,55 @@ void init_boot_cpu(void)
 	list_head_init(&global_job_queue);
 }
 
+static void enable_ld(bool on)
+{
+	u64 lpcr = mfspr(SPR_LPCR);
+
+	if (on)
+		lpcr |= SPR_LPCR_P9_LD;
+	else
+		lpcr &= ~SPR_LPCR_P9_LD;
+
+	mtspr(SPR_LPCR, lpcr);
+}
+
+#define HIGH_BIT (1ull << 63)
+
+static int find_dec_bits(void)
+{
+	int bits = 65; /* we always decrement once */
+	u64 mask = ~0ull;
+
+	if (proc_gen < proc_gen_p9)
+		return 32;
+
+	/* The ISA doesn't specify the width of the decrementer register so we
+	 * need to discover it. When in large mode (LPCR.LD = 1) reads from the
+	 * DEC SPR are sign extended to 64 bits and writes are truncated to the
+	 * physical register width. We can use this behaviour to detect the
+	 * width by starting from an all 1s value and left shifting until we
+	 * read a value from the DEC with it's high bit cleared.
+	 */
+
+	enable_ld(true);
+
+	do {
+		bits--;
+		mask = mask >> 1;
+		mtspr(SPR_DEC, mask);
+	} while (mfspr(SPR_DEC) & HIGH_BIT);
+
+	enable_ld(false);
+
+	prlog(PR_DEBUG, "CPU: decrementer bits %d\n", bits);
+	return bits;
+}
+
 void init_all_cpus(void)
 {
 	struct dt_node *cpus, *cpu;
 	unsigned int thread, new_max_pir = 0;
+	int dec_bits = find_dec_bits();
 
 	cpus = dt_find_by_path(dt_root, "/cpus");
 	assert(cpus);
@@ -582,6 +627,9 @@ void init_all_cpus(void)
 		/* Add associativity properties */
 		add_core_associativity(t);
 
+		/* Add the decrementer width property */
+		dt_add_property_cells(cpu, "ibm,dec-bits", dec_bits);
+
 		/* Adjust max PIR */
 		if (new_max_pir < (pir + cpu_thread_count - 1))
 			new_max_pir = pir + cpu_thread_count - 1;
diff --git a/include/processor.h b/include/processor.h
index 1236c779aac8..48bbf903df58 100644
--- a/include/processor.h
+++ b/include/processor.h
@@ -96,6 +96,7 @@
 #define SPR_LPCR_P8_PECE2	PPC_BIT(49)   /* Wake on external interrupts */
 #define SPR_LPCR_P8_PECE3	PPC_BIT(50)   /* Wake on decrementer */
 #define SPR_LPCR_P8_PECE4	PPC_BIT(51)   /* Wake on MCs, HMIs, etc... */
+#define SPR_LPCR_P9_LD		PPC_BIT(46)   /* Large decrementer mode bit */
 
 
 /* Bits in TFMR - control bits */
-- 
2.5.5