Message-Id: <20170805011855.1027-2-brendanhiggins@google.com>
Date:   Fri,  4 Aug 2017 18:18:52 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     corbet@....net, robh+dt@...nel.org, mark.rutland@....com,
        arnd@...db.de, gregkh@...uxfoundation.org, minyard@....org,
        joel@....id.au, benh@...nel.crashing.org, benjaminfair@...gle.com
Cc:     linux-doc@...r.kernel.org, devicetree@...r.kernel.org,
        openipmi-developer@...ts.sourceforge.net, openbmc@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        Brendan Higgins <brendanhiggins@...gle.com>
Subject: [PATCH v2 1/4] ipmi: bt-i2c: added documentation for bt-i2c drivers

Added device tree binding documentation for ipmi-bt-i2c (host) and
ipmi-bmc-bt-i2c (BMC) and documentation for the Block Transfer over I2C
(bt-i2c) protocol.

Signed-off-by: Brendan Higgins <brendanhiggins@...gle.com>
---
Changes for v2:
  - Fixed a typo
  - Reworded a sentence to make it clear that I was talking about IBM's BT-BMC
  - Deleted an unnecessary discussion of the host side interface (unnecessary
    since the OpenIPMI stuff already has its own documentation).
---
 Documentation/bt-i2c.txt                           | 109 +++++++++++++++++++++
 .../devicetree/bindings/ipmi/ipmi-bt-i2c.txt       |  21 ++++
 .../bindings/ipmi_bmc/ipmi-bmc-bt-i2c.txt          |  21 ++++
 3 files changed, 151 insertions(+)
 create mode 100644 Documentation/bt-i2c.txt
 create mode 100644 Documentation/devicetree/bindings/ipmi/ipmi-bt-i2c.txt
 create mode 100644 Documentation/devicetree/bindings/ipmi_bmc/ipmi-bmc-bt-i2c.txt

diff --git a/Documentation/bt-i2c.txt b/Documentation/bt-i2c.txt
new file mode 100644
index 000000000000..499931b02e6c
--- /dev/null
+++ b/Documentation/bt-i2c.txt
@@ -0,0 +1,109 @@
+Linux Block Transfer over I2C (bt-i2c) interface description
+============================================================
+
+by Brendan Higgins <brendanhiggins@...gle.com> in 2016
+
+Introduction
+------------
+
+IPMI defines an interface for communication between a CPU, a BMC (Baseboard
+Management Controller), and sensors and various other peripherals. For a more
+complete description of IPMI please see:
+http://www.intel.com/content/www/us/en/servers/ipmi/ipmi-second-gen-interface-spec-v2-rev1-1.html
+
+IPMI defines a *common* message format, as in a set of fields that are common
+across all IPMI messages; they could be viewed as part of the framing
+information for an IPMI message. They include:
+
+	- netfn
+	- lun
+	- cmd
+
+netfn and cmd together define the type of the message; netfn can be viewed as a
+message class and cmd is a subtype of sorts. lun (logical unit number) is used
+for routing between messages between different interfaces. After the last field
+there is usually a variable length payload. Despite these common fields, the
+remainder of the framing varies widely between the IPMI defined hardware
+interfaces; some specify a length as part of the framing which is never more
+than a byte in length; others use a special signal to denote the end of message.
+Some IPMI hardware interfaces, the Block Transfer interface in particular,
+support a sequence number that aids in support of multiple in-flight IPMI
+messages.
+
+IPMI defines SSIF (SMBus System Interface) as the IPMI hardware interface for
+SMBus/I2C. It supports a maximum total message length of 255 bytes that is
+broken up across several SMBus block operations. It does not define a sequence
+field in the IPMI framing making it very difficult to support multiple in flight
+messages (it is also intentionally left out of the specification). SSIF also
+requires the slave device to NACK until it is ready to accept data (technically
+it only specifies that it may NACK until it is ready, but must NACK on attempted
+reads if it does not support SMBus Alert; however, this is an effective
+requirements since a slave device is supposed to start with SMBus Alert
+disabled); this again makes SSIF very difficult to support for some slave
+devices which may not support NACKing arbitrary messages; indeed, at the time of
+writing, the Linux I2C slave driver framework did not have support for sending
+NACKs.
+
+Block Transfer over I2C defines a new IPMI compatible interface that uses Block
+Transfer messages and semantics on top of plain old I2C; it does not assume that
+the I2C slave is capable of NACKing arbitrary messages; however, it is designed
+such that it could take advantage of SMBus Alert so that the master does not
+have to poll (the Linux I2C core slave mode does not currently support SMBus
+Alert, but a patch adding this support is currently on the way).
+
+Protocol Definition
+-------------------
+
+Block Transfer over I2C uses the IPMI defined Block Transfer message format; it
+supports variable length messages with a maximum length of 255 bytes (limited by
+the IPMI Block Transfer length byte).
+
+A Block Transfer over I2C Request is structured as follows:
+
+------------------------------------------------------------------------------------------------------
+| I2C start | slave address / RW bit unset | Block Transfer message | ... (another message or stop ) |
+------------------------------------------------------------------------------------------------------
+
+Multiple requests can be sent before any responses are received. Sequence
+numbers are to be handled by the users of the drivers; thus, no semantics are
+prescribed to their usage; however, the slave driver is required to buffer at
+least 256 requests before dropping requests; this can be used in conjunction
+with sequence numbers to prevent messages from being dropped by the slave.
+
+A Block Transfer over I2C Response is structured as follows:
+
+----------------------------------------------------------------------------------------------------
+| I2C start | slave address / RW bit set | Block Transfer message | ... (another message or stop ) |
+----------------------------------------------------------------------------------------------------
+
+Until a response is ready to send, the slave will respond only with zero bytes.
+If the slave receives a start or a stop before it was done sending a response,
+it will resend the entire response the next time a read is requested; in this
+way, an I2C master may poll for responses by reading a single byte until it is
+non-zero and then perform the read transaction shown above.
+
+In the future, it is planned that the slave will support using SMBus Alert to
+notify the master of an available response, but will never be required.
+
+Driver Interface
+----------------
+
+The device file interface for the slave side is modeled after bt-bmc, a Block
+Transfer over LPC driver for the Aspeed 24xx/255xx.
+
+A read (request) should be large enough to fit a Block Transfer message
+(including the length byte) of 256 bytes; if the provided buffer is smaller, the
+message will be truncated.
+
+A write (response) must be no greater than the maximum valid length of a Block
+Transfer message (including the length byte), 256 bytes; if the provided buffer
+is larger, the write will fail with EINVAL. The driver also enforces a valid
+length byte which must contain the total length of the message not including the
+length byte; thus, the write will fail with EINVAL if the buffer length is less
+than the length field plus one. The driver will also only send length field plus
+one bytes.
+
+The slave device file interface also supports the poll system call; it will
+report when a request is available to read or it is ready to accept a response.
+
+The master side conforms to the OpenIPMI kernel framework.
diff --git a/Documentation/devicetree/bindings/ipmi/ipmi-bt-i2c.txt b/Documentation/devicetree/bindings/ipmi/ipmi-bt-i2c.txt
new file mode 100644
index 000000000000..bd956f2805e4
--- /dev/null
+++ b/Documentation/devicetree/bindings/ipmi/ipmi-bt-i2c.txt
@@ -0,0 +1,21 @@
+Device tree configuration for the ipmi-bt-i2c driver.
+
+Required Properties:
+- compatible 	: should be "ipmi-bt-i2c"
+- reg		: the I2C slave address of the ipmi-bmc-bt-i2c
+
+i2c0: i2c-bus@40 {
+	#address-cells = <1>;
+	#size-cells = <0>;
+	reg = <0x40 0x40>;
+	compatible = "aspeed,ast2400-i2c-bus";
+	clock-frequency = <100000>;
+	status = "disabled";
+	interrupts = <0>;
+	interrupt-parent = <&i2c>;
+
+	bt-i2c-master@41 {
+		compatible = "ipmi-bt-i2c";
+		reg = <0x41>;
+	};
+};
diff --git a/Documentation/devicetree/bindings/ipmi_bmc/ipmi-bmc-bt-i2c.txt b/Documentation/devicetree/bindings/ipmi_bmc/ipmi-bmc-bt-i2c.txt
new file mode 100644
index 000000000000..6e82693ee50e
--- /dev/null
+++ b/Documentation/devicetree/bindings/ipmi_bmc/ipmi-bmc-bt-i2c.txt
@@ -0,0 +1,21 @@
+Device tree configuration for the ipmi-bmc-bt-i2c driver.
+
+Required Properties:
+- compatible 	: should be "ipmi-bmc-bt-i2c"
+- reg		: the I2C slave address of this driver.
+
+i2c0: i2c-bus@40 {
+	#address-cells = <1>;
+	#size-cells = <0>;
+	reg = <0x40 0x40>;
+	compatible = "aspeed,ast2400-i2c-bus";
+	clock-frequency = <100000>;
+	status = "disabled";
+	interrupts = <0>;
+	interrupt-parent = <&i2c>;
+
+	bt-i2c-slave@41 {
+		compatible = "ipmi-bmc-bt-i2c";
+		reg = <0x41>;
+	};
+};
-- 
2.14.0.rc1.383.gd1ce394fe2-goog

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