drm/xe: Document Xe PM component

Replace outdated information with a proper PM documentation.
Already establish the rules for the runtime PM get and put that
Xe needs to follow.

Also add missing function documentation to all the "exported" functions.

v2: updated after Francois' feedback.
    s/grater/greater (Matt)
v3: detach D3 from runtime_pm
    remove opportunistic S0iX (Anshuman)

Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Acked-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com> #v2
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222163937.138342-1-rodrigo.vivi@intel.com

drivers/gpu/drm/xe/xe_pm.c

@@ -25,21 +25,47 @@
 /**
  * DOC: Xe Power Management
  *
- * Xe PM shall be guided by the simplicity.
- * Use the simplest hook options whenever possible.
- * Let's not reinvent the runtime_pm references and hooks.
- * Shall have a clear separation of display and gt underneath this component.
+ * Xe PM implements the main routines for both system level suspend states and
+ * for the opportunistic runtime suspend states.
  *
- * What's next:
+ * System Level Suspend (S-States) - In general this is OS initiated suspend
+ * driven by ACPI for achieving S0ix (a.k.a. S2idle, freeze), S3 (suspend to ram),
+ * S4 (disk). The main functions here are `xe_pm_suspend` and `xe_pm_resume`. They
+ * are the main point for the suspend to and resume from these states.
  *
- * For now s2idle and s3 are only working in integrated devices. The next step
- * is to iterate through all VRAM's BO backing them up into the system memory
- * before allowing the system suspend.
+ * PCI Device Suspend (D-States) - This is the opportunistic PCIe device low power
+ * state D3, controlled by the PCI subsystem and ACPI with the help from the
+ * runtime_pm infrastructure.
+ * PCI D3 is special and can mean D3hot, where Vcc power is on for keeping memory
+ * alive and quicker low latency resume or D3Cold where Vcc power is off for
+ * better power savings.
+ * The Vcc control of PCI hierarchy can only be controlled at the PCI root port
+ * level, while the device driver can be behind multiple bridges/switches and
+ * paired with other devices. For this reason, the PCI subsystem cannot perform
+ * the transition towards D3Cold. The lowest runtime PM possible from the PCI
+ * subsystem is D3hot. Then, if all these paired devices in the same root port
+ * are in D3hot, ACPI will assist here and run its own methods (_PR3 and _OFF)
+ * to perform the transition from D3hot to D3cold. Xe may disallow this
+ * transition by calling pci_d3cold_disable(root_pdev) before going to runtime
+ * suspend. It will be based on runtime conditions such as VRAM usage for a
+ * quick and low latency resume for instance.
  *
- * Also runtime_pm needs to be here from the beginning.
+ * Runtime PM - This infrastructure provided by the Linux kernel allows the
+ * device drivers to indicate when the can be runtime suspended, so the device
+ * could be put at D3 (if supported), or allow deeper package sleep states
+ * (PC-states), and/or other low level power states. Xe PM component provides
+ * `xe_pm_runtime_suspend` and `xe_pm_runtime_resume` functions that PCI
+ * subsystem will call before transition to/from runtime suspend.
  *
- * RC6/RPS are also critical PM features. Let's start with GuCRC and GuC SLPC
- * and no wait boost. Frequency optimizations should come on a next stage.
+ * Also, Xe PM provides get and put functions that Xe driver will use to
+ * indicate activity. In order to avoid locking complications with the memory
+ * management, whenever possible, these get and put functions needs to be called
+ * from the higher/outer levels.
+ * The main cases that need to be protected from the outer levels are: IOCTL,
+ * sysfs, debugfs, dma-buf sharing, GPU execution.
+ *
+ * This component is not responsible for GT idleness (RC6) nor GT frequency
+ * management (RPS).
  */
 
 /**
@@ -178,6 +204,12 @@ void xe_pm_init_early(struct xe_device *xe)
 	drmm_mutex_init(&xe->drm, &xe->mem_access.vram_userfault.lock);
 }
 
+/**
+ * xe_pm_init - Initialize Xe Power Management
+ * @xe: xe device instance
+ *
+ * This component is responsible for System and Device sleep states.
+ */
 void xe_pm_init(struct xe_device *xe)
 {
 	/* For now suspend/resume is only allowed with GuC */
@@ -196,6 +228,10 @@ void xe_pm_init(struct xe_device *xe)
 	xe_pm_runtime_init(xe);
 }
 
+/**
+ * xe_pm_runtime_fini - Finalize Runtime PM
+ * @xe: xe device instance
+ */
 void xe_pm_runtime_fini(struct xe_device *xe)
 {
 	struct device *dev = xe->drm.dev;
@@ -225,6 +261,12 @@ struct task_struct *xe_pm_read_callback_task(struct xe_device *xe)
 	return READ_ONCE(xe->pm_callback_task);
 }
 
+/**
+ * xe_pm_runtime_suspend - Prepare our device for D3hot/D3Cold
+ * @xe: xe device instance
+ *
+ * Returns 0 for success, negative error code otherwise.
+ */
 int xe_pm_runtime_suspend(struct xe_device *xe)
 {
 	struct xe_bo *bo, *on;
@@ -290,6 +332,12 @@ int xe_pm_runtime_suspend(struct xe_device *xe)
 	return err;
 }
 
+/**
+ * xe_pm_runtime_resume - Waking up from D3hot/D3Cold
+ * @xe: xe device instance
+ *
+ * Returns 0 for success, negative error code otherwise.
+ */
 int xe_pm_runtime_resume(struct xe_device *xe)
 {
 	struct xe_gt *gt;
@@ -341,22 +389,47 @@ int xe_pm_runtime_resume(struct xe_device *xe)
 	return err;
 }
 
+/**
+ * xe_pm_runtime_get - Get a runtime_pm reference and resume synchronously
+ * @xe: xe device instance
+ *
+ * Returns: Any number greater than or equal to 0 for success, negative error
+ * code otherwise.
+ */
 int xe_pm_runtime_get(struct xe_device *xe)
 {
 	return pm_runtime_get_sync(xe->drm.dev);
 }
 
+/**
+ * xe_pm_runtime_put - Put the runtime_pm reference back and mark as idle
+ * @xe: xe device instance
+ *
+ * Returns: Any number greater than or equal to 0 for success, negative error
+ * code otherwise.
+ */
 int xe_pm_runtime_put(struct xe_device *xe)
 {
 	pm_runtime_mark_last_busy(xe->drm.dev);
 	return pm_runtime_put(xe->drm.dev);
 }
 
+/**
+ * xe_pm_runtime_get_if_active - Get a runtime_pm reference if device active
+ * @xe: xe device instance
+ *
+ * Returns: Any number greater than or equal to 0 for success, negative error
+ * code otherwise.
+ */
 int xe_pm_runtime_get_if_active(struct xe_device *xe)
 {
 	return pm_runtime_get_if_active(xe->drm.dev, true);
 }
 
+/**
+ * xe_pm_assert_unbounded_bridge - Disable PM on unbounded pcie parent bridge
+ * @xe: xe device instance
+ */
 void xe_pm_assert_unbounded_bridge(struct xe_device *xe)
 {
 	struct pci_dev *pdev = to_pci_dev(xe->drm.dev);
@@ -371,6 +444,13 @@ void xe_pm_assert_unbounded_bridge(struct xe_device *xe)
 	}
 }
 
+/**
+ * xe_pm_set_vram_threshold - Set a vram threshold for allowing/blocking D3Cold
+ * @xe: xe device instance
+ * @threshold: VRAM size in bites for the D3cold threshold
+ *
+ * Returns 0 for success, negative error code otherwise.
+ */
 int xe_pm_set_vram_threshold(struct xe_device *xe, u32 threshold)
 {
 	struct ttm_resource_manager *man;
@@ -395,6 +475,13 @@ int xe_pm_set_vram_threshold(struct xe_device *xe, u32 threshold)
 	return 0;
 }
 
+/**
+ * xe_pm_d3cold_allowed_toggle - Check conditions to toggle d3cold.allowed
+ * @xe: xe device instance
+ *
+ * To be called during runtime_pm idle callback.
+ * Check for all the D3Cold conditions ahead of runtime suspend.
+ */
 void xe_pm_d3cold_allowed_toggle(struct xe_device *xe)
 {
 	struct ttm_resource_manager *man;