ACPI / CPPC: set a non-zero value for transition_latency

Compute the expected transition latency for frequency transitions
using the values from the PCCT tables when the desired perf
register is in PCC.
Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Reviewed-by: Alexey Klimov <alexey.klimov@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/cppc_acpi.c

@@ -85,7 +85,7 @@ static void __iomem *pcc_comm_addr;
 static int pcc_subspace_idx = -1;
 static bool pcc_channel_acquired;
 static ktime_t deadline;
-static unsigned int pcc_mpar, pcc_mrtt;
+static unsigned int pcc_mpar, pcc_mrtt, pcc_nominal;
 
 /* pcc mapped address + header size + offset within PCC subspace */
 #define GET_PCC_VADDR(offs) (pcc_comm_addr + 0x8 + (offs))
@@ -473,7 +473,6 @@ static int register_pcc_channel(int pcc_subspace_idx)
 			return -ENODEV;
 		}
 
-
 		/*
 		 * cppc_ss->latency is just a Nominal value. In reality
 		 * the remote processor could be much slower to reply.
@@ -483,6 +482,7 @@ static int register_pcc_channel(int pcc_subspace_idx)
 		deadline = ns_to_ktime(usecs_lat * NSEC_PER_USEC);
 		pcc_mrtt = cppc_ss->min_turnaround_time;
 		pcc_mpar = cppc_ss->max_access_rate;
+		pcc_nominal = cppc_ss->latency;
 
 		pcc_comm_addr = acpi_os_ioremap(cppc_ss->base_address, cppc_ss->length);
 		if (!pcc_comm_addr) {
@@ -1048,3 +1048,45 @@ int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
 	return ret;
 }
 EXPORT_SYMBOL_GPL(cppc_set_perf);
+
+/**
+ * cppc_get_transition_latency - returns frequency transition latency in ns
+ *
+ * ACPI CPPC does not explicitly specifiy how a platform can specify the
+ * transition latency for perfromance change requests. The closest we have
+ * is the timing information from the PCCT tables which provides the info
+ * on the number and frequency of PCC commands the platform can handle.
+ */
+unsigned int cppc_get_transition_latency(int cpu_num)
+{
+	/*
+	 * Expected transition latency is based on the PCCT timing values
+	 * Below are definition from ACPI spec:
+	 * pcc_nominal- Expected latency to process a command, in microseconds
+	 * pcc_mpar   - The maximum number of periodic requests that the subspace
+	 *              channel can support, reported in commands per minute. 0
+	 *              indicates no limitation.
+	 * pcc_mrtt   - The minimum amount of time that OSPM must wait after the
+	 *              completion of a command before issuing the next command,
+	 *              in microseconds.
+	 */
+	unsigned int latency_ns = 0;
+	struct cpc_desc *cpc_desc;
+	struct cpc_register_resource *desired_reg;
+
+	cpc_desc = per_cpu(cpc_desc_ptr, cpu_num);
+	if (!cpc_desc)
+		return CPUFREQ_ETERNAL;
+
+	desired_reg = &cpc_desc->cpc_regs[DESIRED_PERF];
+	if (!CPC_IN_PCC(desired_reg))
+		return CPUFREQ_ETERNAL;
+
+	if (pcc_mpar)
+		latency_ns = 60 * (1000 * 1000 * 1000 / pcc_mpar);
+
+	latency_ns = max(latency_ns, (pcc_nominal + pcc_mrtt) * 1000);
+
+	return latency_ns;
+}
+EXPORT_SYMBOL_GPL(cppc_get_transition_latency);

drivers/cpufreq/cppc_cpufreq.c

@@ -98,6 +98,7 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
 	policy->max = cpu->perf_caps.highest_perf;
 	policy->cpuinfo.min_freq = policy->min;
 	policy->cpuinfo.max_freq = policy->max;
+	policy->cpuinfo.transition_latency = cppc_get_transition_latency(cpu_num);
 	policy->shared_type = cpu->shared_type;
 
 	if (policy->shared_type == CPUFREQ_SHARED_TYPE_ANY)

include/acpi/cppc_acpi.h

@@ -131,5 +131,6 @@ extern int cppc_get_perf_ctrs(int cpu, struct cppc_perf_fb_ctrs *perf_fb_ctrs);
 extern int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls);
 extern int cppc_get_perf_caps(int cpu, struct cppc_perf_caps *caps);
 extern int acpi_get_psd_map(struct cpudata **);
+extern unsigned int cppc_get_transition_latency(int cpu);
 
 #endif /* _CPPC_ACPI_H*/