Cstyle UEFI Guide: Chapter 20 IGD Opregion Interface && IGD opregion PROTOCOL

ACPI IGD opregioninterface is to use SCI to achieve IGD Driver,os,bios communication between the bridge, IGD Opregion protocol is the backbone of the UEFI BIOS building bridge.
how to communicate with the OS in the Legacy implementation mode:
opregion Memory Layout:

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The public ACPI methods provided by Mailbox1 is able to find each of the corresponding method from ACPI spec Appendix B Video Extensions. It provides a standard ACPI-defined communication channel for the IGD graphics devices. The Mailbox2 duty provides port information IGD graphic driver How to communicate with the BIOS via SWSCI.Mailbox3,asle (ASL event) support. The mailbox for communicating all driver features related information between thedriver and the Sbios
ASL Storage Register,Used to store IGD opregion header address:
IGD and BIOS communication (SCI):
Swsci REGISTER,IGD sends sci to ICH via DMI to triggertcosci_sts bit in its GPE0 register to trigger the real SCI event. And then through the ASL defined in the BIOS_lxx method to handle, at the same time throughMailbox2 to deliver the message. The main is before the trigger SCI pre-filled mailbox the corresponding parameters, including Function-code and other information, and then trigger SCI, in L_xx method which will read Mailbox2 the corresponding fields, to do the corresponding processing. Finally, some other fields return the result and status of the processing. (Tco:total cost of ownership. Ref Intel EDS)    Under the new scheme, rather than writing to the SWSMI bit (bit 0 of the Swsmi registerat offset 0xe0 in the PCI configuration space of the graphics device), the graphics driverwrites to the Swsci register (bit 0, offset 0xe8 assu Ming Swsci is configured for scioperation) .    This register serves both purposes:1) support selection of SMI or Sci Event Source (smiscisel-bit15) 2) SCI Event trigger (Gsscie–bit 0) to generate a SW SCI event, software (System BIOS /graphics driver) should program bit15 (Smiscisel) to 1. This was typically programmed once (assuming SMIs is nevertriggered). On a write transition of 0->1 of bit 0 of this register, the GMCH sends a single scimessage the DMI link to ICH. ICH would set the DMISCI bit in it tco1_sts registerand tcosci_sts bit in it GPE0 register, upon receiving this message F Rom DMI. Thecorresponding SCI event handler in BIOS are to being defined as an _lxx method. Once written as 1, software must write a "0" to this bit To clear it. All other write transitions (1->0, 0->0, 1->1) would be ignored. Writes would likewise beignored if bit 0.To generate an SW SMI event, software should program bit 0 and trigger Smithrough writes to Swsmi Register (see SWSMI Register for programming details) .   

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Video BIOS Table (VBT)    This is our common so-called use of BMP tool to configure the VBIOS/GOP of a bin file, will finally be wrapped in the BIOS ROM, in the post process in which the BIOS to read into the memory, with the variousMailbox (typically created by ASL CODE,IGDOPRN.ASL) together form IGD opregion table, which is usually done by IGD opregion PROTOCOL &acpi protocl. The resulting table is read by the video BIOS and device driver to obtain various information about the current hardware: FlatPanel Timings, Generic Mode Timing, GPIO pins, clock, etc. Table can be dynamically changed in post based on user's setpoint or Platformpolicy in Setup (table is saved inefi_global_nvs_area->igdopregionaddress:igdopregion which is read by ACPI driver). In order to achieve the user's different customization (reference IGDOPROM.C).
IGD opregion PROTOCOLthe protocol mentioned here is used to create igdopregiontable DXE Driver, which provides relevant information for VBIOS/GOP and IGD driver. It mainly reads the bin file generated by the BMP configuration from the FV and the Platform policy and the setup value to customize the various parameters of the system and assists ACPI driver to create the ACPI table (GNVS->ASLB).
All right, here's the record. For more information, see ACPI Spc,intel IGDOPRN SPC, and related Spc.
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Cstyle UEFI Guide: Chapter 20 IGD Opregion Interface && IGD opregion PROTOCOL