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@xxxxxxxxxxxxxxx> --- drivers/acpi/mipi.c | 37 +++++++++++++++++++++++++++++++++++++ 1 file changed, 37 insertions(+) diff --git a/drivers/acpi/mipi.c b/drivers/acpi/mipi.c index 3cb698b094ac1..24cef9d501ebc 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
