cpu: supply ibm,dec-bits via devicetree

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@gmail.com>
Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
[stewart@linux.vnet.ibm.com: rename enable_ld() to enable_large_dec()]
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

1 files changed, 48 insertions, 0 deletions

diff --git a/core/cpu.c b/core/cpu.c
index ffc264f..8a7a43f 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_large_dec(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_large_dec(true);
+
+	do {
+		bits--;
+		mask = mask >> 1;
+		mtspr(SPR_DEC, mask);
+	} while (mfspr(SPR_DEC) & HIGH_BIT);
+
+	enable_large_dec(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;