[PATCH v3 15/18] drivers/fsi: Add FSI bus documentation

[robh@kernel.org (Rob Herring)]

On Wed, Feb 01, 2017 at 10:53:55AM -0600, Christopher Bostic wrote:
> From: Chris Bostic <cbostic@us.ibm.com>
> 
> Add details on the basic functions of the FSI serial bus.
> 
> Signed-off-by: Chris Bostic <cbostic@us.ibm.com>
> ---
>  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
> +bi-directional.
> +
> +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.

This document now isn't really more that a description of FSI. You need 
to define how the above bus structure gets reflected in DT. I assume the 
root is the FSI master, each child node is a link, a child can be a hub 
with its children being more links. What is the format of reg property 
is another thing to define.

> +
> +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...
> -- 
> 1.8.2.2
>