The Application Processor (sometimes called the host) communicates with the EC by issuing host commands, which are identified by a command ID and version number, and then reading a response. When a host command is issued through
ectool, two or three software components are involved:
ectool, the user-space binary,
cros-ecKernel driver, and
We'll go into detail of each of these, as well as the traffic on the wire, in the following sections.
ectool contains wrapper functions for the host commands exposed by the EC, providing a CLI. They call one of the transport-specific
ec_command implementations in the
util/comm-*.c files to send and receive from the EC.
In most cases,
ectool communicates via the
cros-ec Kernel driver, rather than directly from userspace. It sends raw commands to the Kernel driver, which sends them on to the EC, bypassing a lot of the other Kernel driver functionality.
There are other CrOS EC-related Kernel drivers, which use host commands to act as adapters to existing Linux APIs. For example, sensors from the EC are mapped to the Linux Industrial I/O system.
Now we come to the protocol itself. All transactions take this general form:
struct ec_host_requestcontaining the command ID, data length, and a checksum; and
struct ec_host_responsecontaining the result code, data length, and a checksum; and
The host packet is received on the EC by some chip-specific code which checks its transport-specific header, then passes it on to the common host command code, starting at
host_packet_receive. The common code validates the packet and then sends it on to the handler function (annotated with the
DECLARE_HOST_COMMAND macro), which runs in the
HOSTCMD task. The handler can set a response by modifying its arguments struct, which is sent back to the host via the chip-specific code.
While this is happening, the EC needs to indicate to the host that it is busy processing and not yet ready to give a response. How it does this depends on the transport method used (see Transport-specific details below).
There are two different concepts of “version” involved in host commands: version of the overarching protocol, and versions of individual commands.
There have been three protocol versions so far, and this document describes version 3. Version 1 was superseded by 2 before it shipped, so no devices use it anymore. Version 2 is generally deprecated, but you might still encounter it occasionally.
Which version is in use can be determined using the
EC_CMD_GET_PROTOCOL_INFO command. This was only introduced in version 3, however, so if errors,
EC_CMD_HELLO should be sent in version 2. If the hello command succeeds, the EC speaks version 2.
Individual commands also have versions, independent of the protocol version they're being called with. Different versions of a command may have different parameter or response formats.
EC_CMD_GET_CMD_VERSIONS returns the versions of the given command supported by the EC. These version numbers start at 0.
Although the command and response formats are the same across all transports, some details of how they are transmitted differ, which may be of interest when implementing the EC side of the protocol on a new chip.
I2C is very flexible with its timing, so when the EC receives a packet from the host, it should stretch the clock, holding it low until it is ready for the host to read the response.
If the host tries to read more bytes than were in the response, the EC should respond with an obvious filler byte (such as 0xEC). For example, if a command that normally returns 50 bytes errors, its response will only be 8 bytes (the size of the response struct). The host will probably try to read 50 bytes anyway, so the EC should send the 8 bytes of the struct followed by 42 copies of the filler byte.
The SPI bus is similar to I2C, but with two major exceptions. First, there‘s a minimum speed on the SPI bus. If slave devices don’t respond quickly enough, the master will assume they're broken and give up. Second, every transaction is bidirectional. When bits are being clocked from master to slave on the MOSI line, the master will simultaneously read bits in the other direction on the MISO line.
Hardware devices can usually handle this, and often some hardware-based flow control used to “stretch” the transaction by a bit or byte if the slave device needs a little extra time to respond to the master's demands.
When exchanging messages with the EC on the SPI bus, the EC‘s host commands are communicated using our own software flow-control scheme, because most of the embedded controllers either aren’t fast enough or don't have any support for hardware flow-control.
It works like this: When the AP sends a byte to the EC, if the EC doesn‘t have a response queued up in advance, a default byte is returned. The EC preconfigures that default response byte to indicate its status (ready, busy, waiting for more input, etc.). Once the AP has sent a complete command message, it continues clocking bytes to the EC (which the EC ignores) and just looks at the response byte that comes back. Once the EC has parsed the AP’s command and is ready to reply, it sends a “start of frame” byte, followed by the actual response. The AP continues to read and ignore bytes from the EC until it sees the start of frame byte, and then it knows that the EC's response is starting with the next byte.
Once the response packet has been read, any additional reads should return
The EC should set
EC_LPC_STATUS_PROCESSING in its command status register after receiving a host packet and before it has a response ready.