ACPI: CPPC: Assume no transition latency if no PCCT

The transition_delay_us (struct cpufreq_policy) is currently defined
as:
  Preferred average time interval between consecutive invocations of
  the driver to set the frequency for this policy.  To be set by the
  scaling driver (0, which is the default, means no preference).
The transition_latency represents the amount of time necessary for a
CPU to change its frequency.

A PCCT table advertises mutliple values:
- 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.
cppc_get_transition_latency() allows to get the max of them.

commit d4f3388 ("cpufreq / CPPC: Set platform specific
transition_delay_us") allows to select transition_delay_us based on
the platform, and fallbacks to cppc_get_transition_latency()
otherwise.

If _CPC objects are not using PCC channels (no PPCT table), the
transition_delay_us is set to CPUFREQ_ETERNAL, leading to really long
periods between frequency updates (~4s).

If the desired_reg, where performance requests are written, is in
SystemMemory or SystemIo ACPI address space, there is no delay
in requests. So return 0 instead of CPUFREQ_ETERNAL, leading to
transition_delay_us being set to LATENCY_MULTIPLIER us (1000 us).

This patch also adds two macros to check the address spaces.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: pierregondois

drivers/acpi/cppc_acpi.c

@@ -100,6 +100,16 @@ static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
 				(cpc)->cpc_entry.reg.space_id ==	\
 				ACPI_ADR_SPACE_PLATFORM_COMM)
 
+/* Check if a CPC register is in SystemMemory */
+#define CPC_IN_SYSTEM_MEMORY(cpc) ((cpc)->type == ACPI_TYPE_BUFFER &&	\
+				(cpc)->cpc_entry.reg.space_id ==	\
+				ACPI_ADR_SPACE_SYSTEM_MEMORY)
+
+/* Check if a CPC register is in SystemIo */
+#define CPC_IN_SYSTEM_IO(cpc) ((cpc)->type == ACPI_TYPE_BUFFER &&	\
+				(cpc)->cpc_entry.reg.space_id ==	\
+				ACPI_ADR_SPACE_SYSTEM_IO)
+
 /* Evaluates to True if reg is a NULL register descriptor */
 #define IS_NULL_REG(reg) ((reg)->space_id ==  ACPI_ADR_SPACE_SYSTEM_MEMORY && \
 				(reg)->address == 0 &&			\
@@ -1456,6 +1466,9 @@ EXPORT_SYMBOL_GPL(cppc_set_perf);
  * transition latency for performance 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.
+ *
+ * If desired_reg is in the SystemMemory or SystemIo ACPI address space,
+ * then assume there is no latency.
  */
 unsigned int cppc_get_transition_latency(int cpu_num)
 {
@@ -1481,7 +1494,9 @@ unsigned int cppc_get_transition_latency(int cpu_num)
 		return CPUFREQ_ETERNAL;
 
 	desired_reg = &cpc_desc->cpc_regs[DESIRED_PERF];
-	if (!CPC_IN_PCC(desired_reg))
+	if (CPC_IN_SYSTEM_MEMORY(desired_reg) || CPC_IN_SYSTEM_IO(desired_reg))
+		return 0;
+	else if (!CPC_IN_PCC(desired_reg))
 		return CPUFREQ_ETERNAL;
 
 	if (pcc_ss_id < 0)