fpga: dfl-pci: locate DFLs by PCIe vendor specific capability

[linux-2.6-microblaze.git] / Documentation / fpga / dfl.rst

diff --git a/Documentation/fpga/dfl.rst b/Documentation/fpga/dfl.rst
index 0404fe6..ea8cefc 100644 (file)
--- a/Documentation/fpga/dfl.rst
+++ b/Documentation/fpga/dfl.rst
@@ -501,6 +501,33 @@ Developer only needs to provide a sub feature driver with matched feature id.
 FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-pr.c)
 could be a reference.
 
+Location of DFLs on a PCI Device
+===========================
+The original method for finding a DFL on a PCI device assumed the start of the
+first DFL to offset 0 of bar 0.  If the first node of the DFL is an FME,
+then further DFLs in the port(s) are specified in FME header registers.
+Alternatively, a PCIe vendor specific capability structure can be used to
+specify the location of all the DFLs on the device, providing flexibility
+for the type of starting node in the DFL.  Intel has reserved the
+VSEC ID of 0x43 for this purpose.  The vendor specific
+data begins with a 4 byte vendor specific register for the number of DFLs followed 4 byte
+Offset/BIR vendor specific registers for each DFL. Bits 2:0 of Offset/BIR register
+indicates the BAR, and bits 31:3 form the 8 byte aligned offset where bits 2:0 are
+zero.
+
+        +----------------------------+
+        |31     Number of DFLS      0|
+        +----------------------------+
+        |31     Offset     3|2 BIR  0|
+        +----------------------------+
+                      . . .
+        +----------------------------+
+        |31     Offset     3|2 BIR  0|
+        +----------------------------+
+
+Being able to specify more than one DFL per BAR has been considered, but it
+was determined the use case did not provide value.  Specifying a single DFL
+per BAR simplifies the implementation and allows for extra error checking.
 
 Open discussion
 ===============