[PATCH 4/4] Documentation: Add documentation for the PCI switch PEX8xxx I2C driver

[Rajat Jain]

Signed-off-by: Rajat Jain <rajatxjain@xxxxxxxxx>
Signed-off-by: Rajat Jain <rajatjain@xxxxxxxxxxx>
Signed-off-by: Guenter Roeck <groeck@xxxxxxxxxxx>
---
 Documentation/PCI/pex8xxx_i2c.txt |  134 +++++++++++++++++++++++++++++++++++++
 1 file changed, 134 insertions(+)
 create mode 100644 Documentation/PCI/pex8xxx_i2c.txt

diff --git a/Documentation/PCI/pex8xxx_i2c.txt b/Documentation/PCI/pex8xxx_i2c.txt
new file mode 100644
index 0000000..9195242
--- /dev/null
+++ b/Documentation/PCI/pex8xxx_i2c.txt
@@ -0,0 +1,134 @@
+================================================================================
+                    The PEX8xxx I2C Interface driver
+
+		 Rajat Jain <rajatjain@xxxxxxxxxxx> - Sep 2014
+================================================================================
+
+0. Why have an I2C interface to a PCIe switch?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Other than the regular PCI express interface, most modern PCIe switches (e.g.
+from IDT and PLX) have an I2C based secondary interface. This interface allows
+access to all the registers of the switch. Some of these registers may not even
+be accessible over the regular PCI interface. Also, there are certain registers
+that can be written to, using only the I2C interface and may only be read-only
+using the PCI interface.
+
+This I2C interface is often used in designs involving these switches, and can
+be used for a variety of use cases where the switch needs to be configured
+independent of the PCI subsystem (and likely before PCI enumeration). Some
+examples:
+
+* Dividing a PCIe switch into multiple "virtual" switches. Using this feature,
+  a switch could be connected to 2 root ports for instance, each managing its
+  own PCI hierarchy, and the traffic from one virtual switch does not leak into
+  another.
+
+* Managing Transparent / Non-transparent bridging, and changing them on-the-fly.
+  There are ports that can be converted into "Non-transparent" bridge ports.
+  Essentially this is used to create different domains (not visible to
+  software). In a dynamic distributed system, it may be desirable to change a
+  transparent bridge to non-transparent or vice versa, for example, to handle a
+  failover situation.
+
+* Buggy hardware / Bad EEPROM configuration. There may be cases where an errata
+  involving register writes need to be applied before enumerating over PCI.
+  Also these switches are typically attached to an EEPROM that is supposed to
+  initialize the switch. If that EEPROM is not present, or contains bad
+  initialization data, this I2C interface can be used to fix that.
+
+* Changing switch configuration on the fly. In a multi-homed or complex
+  distributed systemsystem, there may be a need to change the switch
+  configuration (eg. change the upstream port, or the port or lane
+  configuration etc) to address run time scenarios (CPU plug out etc).
+
+1. What devices does this driver support?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+PEX8xxx represents a family of PCI Express switches from the vendor PLX.
+(http://www.plxtech.com/products/expresslane/switches). Currently this driver
+supports the following PLX switch devices:
+PEX8614
+PEX8618
+PEX8713
+
+2. What does this "PEX8xxx I2C Interface driver" do?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This driver is an I2C client driver that allows talking to the said PEX8XXX
+PCIe switches over the I2C interface. In a nutshell, it currently provides:
+
+* API calls to read / write the PEX8xxx switch device.
+* A sysfs interface, to read / write the PEX8xxx switch device.
+
+The API calls are self explanatory (all reads / writes are 32 bit wide, but
+the argument "byte_mask" can be used to selectively mask out the bytes):
+
+int pex8xxx_read(struct i2c_client *client, u8 stn, u8 mode, u8 byte_mask,
+                 u8 port, u32 reg, u32 *val);
+int pex8xxx_write(struct i2c_client *client, u8 stn, u8 mode, u8 byte_mask,
+                  u8 port, u32 reg, u32 val)
+
+The arguments correspond to the arguments as described in the Chapter 7
+"I2C/SMBus Slave Interface Operation" of the all the switch datasheets.
+http://www.plxtech.com/products/expresslane/pex8614#technicaldocumentation
+http://www.plxtech.com/products/expresslane/pex8618#technicaldocumentation
+http://www.plxtech.com/products/expresslane/pex8713#technicaldocumentation
+
+The sysfs interface is described in the next section.
+
+3. The PEX8xxx I2C driver sysfs Interface
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The sysfs interface allows to read / write / dump the registers of any given
+port of the pex8xxx switch. Note that all reads / writes are 32 bit wide. For
+all pex8xxx devices, the following sysfs attributes are provided by this driver
+in the directory /sys/bus/i2c/drivers/pex8xxx/<i2c-client-device>/
+
+* "port_num" (RW) - The port number whose registers are to be read / written.
+* "reg_addr" (RW) - The register offset (within the port register space) that
+                    is to be read / written.
+* "reg_value"(RW) - When read, it gives the value of the register at offset
+                    "reg_addr" in the port "port_num" of the switch. When
+                    written, it writes the value to the same register.
+* "port_config_regs" (RO) - A binary dump of the 4KB register address space of
+                    the port "port_num".
+
+In addition, some devices (currently PEX8713) support and require additional
+parameters, and hence these will appear for PEX8713 only currently:
+
+* "port_mode" (RW) - Denotes the port mode to use to talk to the switch. Valid
+                     values are: "transparent" / "nt-link" / "nt-virtual" /
+                     "dma".
+* "port_stn" (RW) -  Port Station number.
+
+Note all the attributes preserve their value unless explicitly changed. Thus if
+port_num is set to 5, then all subsequent reads / writes will be directed to
+that port unless the port_num attribute is explicitly changed.
+
+4. Examples using sysfs interface
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+* Instantiate the device (refer "Documentation/i2c/instantiating-devices").
+  Valid strings of device names for this driver are "pex8614", "pex8618" and
+  "pex8713".
+  Example (pex8614 at I2C address 0x38 on I2C bus 55):
+  echo pex8614 0x38 > /sys/bus/i2c/devices/i2c-55/new_device
+
+* Verify if device was instantiated:
+  cd /sys/bus/i2c/drivers/pex8xxx/55-0038
+  cat name (should give pex8614)
+
+* Dump all registers of port number 4:
+  echo 4 > port_num
+  od -x port_config_regs
+
+* Get register values at offsets 0x230, 0x234, 0x238 of port number 1:
+  echo 1 > port_num
+  echo 0x230 > reg_addr
+  cat reg_value
+  echo 0x234 > reg_addr
+  cat reg_value
+  echo 0x238 > reg_addr
+  cat reg_value
+
+* Write value 0x101 at offset 0x530 of port number 5:
+  echo 5 > port_num
+  echo 0x530 > reg_addr
+  echo 0x101 > reg_value
-- 
1.7.9.5

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