[PATCH v4 8/8] ACPI: property: Document _CRS CSI-2 and DisCo for Imaging support

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Document how ACPI _CRS CSI-2 and DisCo for Imaging works. It's non-trivial
so such documentation can be useful.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
---
 drivers/acpi/mipi.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/drivers/acpi/mipi.c b/drivers/acpi/mipi.c
index 7d35351553cc7..b944ae7646a8f 100644
--- a/drivers/acpi/mipi.c
+++ b/drivers/acpi/mipi.c
@@ -3,6 +3,43 @@
  * MIPI DisCo for Imaging support.
  *
  * Copyright (C) 2023 Intel Corporation
+ *
+ * _CRS CSI-2 descriptors, as defined starting from ACPI 6.4 [1], contain
+ * information on cross-device CSI-2 bus configuration. The descriptors are
+ * located under transmitter devices, and the receiver devices have no direct
+ * way to access them even if the information in these descriptors is equally
+ * important for receivers. This information is amended with MIPI DisCo for
+ * Imaging [2] specification that defines _DSD data nodes and properties.
+ *
+ * The support for these is based on two-fold approach, firstly renaming
+ * properties where semantics matches and secondly gathering information to
+ * generate properties using information gathered from various sources. The
+ * former is trivial (see acpi_properties_prepare_mipi() at the end of the
+ * file) whereas the latter requires a more elaborate explanation.
+ *
+ * acpi_bus_scan_crs_csi2() scans an ACPI bus for devices with _CRS CSI-2
+ * descriptors and stores them to a linked list. This is done as traversing just
+ * this list is much smaller task than the entire DSDT. This list is then used
+ * to figure out how much memory is needed for swnodes related to a given ACPI
+ * device (handle). Further on, the same function sets the property values for
+ * the properties the values of which are obtained from the _CRS CSI-2
+ * descriptor. The information is stored into another list where the information
+ * can be looked up based on device's acpi_handle as the struct acpi_device
+ * isn't available yet at this point (and could not, as cross-device references
+ * need to be set up before the devices are available for drivers to probe).
+ *
+ * For each struct acpi_device, acpi_init_swnodes() further obtains information
+ * required to find out the values for the rest of the properties needed by
+ * drivers. This includes all port and endpoint properties as the node
+ * structures used by DT graphs and DisCo for Imaging are different. Finally the
+ * function registers software nodes for the device and sets the secondary
+ * pointer for the ACPI device's fwnode.
+ *
+ * Access to data the structures is serialised using acpi_scan_lock in scan.c.
+ *
+ * [1] https://uefi.org/sites/default/files/resources/ACPI_Spec_6_4_Jan22.pdf
+ *
+ * [2] https://www.mipi.org/specifications/mipi-disco-imaging
  */
 
 #include <linux/acpi.h>
-- 
2.30.2