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Subject[PATCH v3 15/18] drivers/fsi: Add FSI bus documentation
From: Chris Bostic <>

Add details on the basic functions of the FSI serial bus.

Signed-off-by: Chris Bostic <>
Documentation/devicetree/bindings/fsi/fsi.txt | 54 +++++++++++++++++++++++++++
1 file changed, 54 insertions(+)
create mode 100644 Documentation/devicetree/bindings/fsi/fsi.txt

diff --git a/Documentation/devicetree/bindings/fsi/fsi.txt b/Documentation/devicetree/bindings/fsi/fsi.txt
new file mode 100644
index 0000000..7fa2394
--- /dev/null
+++ b/Documentation/devicetree/bindings/fsi/fsi.txt
@@ -0,0 +1,54 @@
+FSI: Flexible Support processor Interface
+FSI is a two line serial bus capable of running at speeds up to 166 MHz.
+The lines consist of a clock responsible for synchronizing the target device
+(slave) with the master which is responsible for all transactions on the bus.
+The master owns the clock line and is the only side allowed to change its
+state. The second line, SDA, is a data line that conveys information to/from
+the slave who samples based on the clock line. The data line is
+The master initiates communication by sending a command to the slave and
+depending on the type of command will allow the slave to control the bus
+to return requested data. All commands are CRC protected. The slave upon
+receipt of a command will determine if the CRC is correct and discard
+the data if noise has corrupted the line. In the same manner the master
+will verify the CRC received from the slave.
+Types of commands:
+Read 32 bit: Read a 32 bit word from a specified address on the slave.
+Read 16 bit: Read a 16 bit 'half word' from a specified address on the slave.
+read 8 bit: Read a byte from a specified address on the slave.
+Write 32,16,8 bit: Write to a specified address on the slave with the provided
+ data.
+BREAK: Initialize the slave's logic to receive commands.
+TERM: Terminate the slave's error lockout to resume communications
+ after an error on the bus is detected.
+D-POLL: Poll the slave to determine when it is no longer buy processing
+ a previous command.
+I-POLL: Interrupt signal check. Master queries slave to see if any
+ interrupts are asserting.
+High fanout capability:
+FSI buses can be chained together in 'hub' configurations to expand the
+available communications channels and thus allow connetion to more slaves.
+Typical implementation
+ FSI master ----- slave with local FSI master (hub) ------- downstream slave
+Each two line combination of a clock and data line is collectively referred
+to as a 'FSI link'. Depending on hardware the primary FSI master may support
+up to 64 links. Hub FSI masters can support at most 8 links. Total number
+of supported slaves can grow exponentially depending on how many hubs are
+placed in the path. Presently only two hubs in the chain are allowed but
+in the future this may be expanded.
+The slave hardware logic responsible for decoding FSI master commands is
+contained in a CFAM (Common Field replaceable unit Access Macro). Up to
+4 slaves or CFAMs can be connected on each FSI link. CFAMs in addition
+to the slave logic (or engine) can contain other functions that allow access
+via FSI. Common additional functionality includes I2C masters, GPIO
+controllers, UARTs, etc...
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