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EC-3PO: The EC console interpreter

Introduction

Today, the Chromium OS Embedded Controller (EC) has a very rich debug console that is very helpful and has features including command history, editing, timestamps, channels and much more. However, all of these features currently use up valuable flash space that a number of our boards desperately need. We constantly run into this problem where boards are running out of space and people have to hack out a lot of code just so the image will fit. It’s been occurring with more frequency lately and I imagine it will continue to occur as we add more features to our EC code base. What we could do instead is move all of that console functionality out to a separate utility and turn the EC console into a binary mode which would only speak in host command packets and debug output packets. EC-3PO would serve as the interpreter translating from the traditional EC console that we all know and love to host command packets sent down to the EC and vice versa.

Benefits

The benefits to be gained are numerous and can all occur without changing people’s existing workflow. The only slight impact might be that we instruct people to emerge hdctools every so often. All people would notice would that the EC images would be getting smaller and/or more console features.

Testing & FAFT

Currently, FAFT runs its tests while trying to parse strings from the EC console. This method can be fairly fragile as debug output can be interleaved with the console input. A lot of items could be improved by switching to this host command packet interface.
  • Communicating in packets makes testing easier and more robust.
    • When FAFT is running, the EC could be in a binary mode where it only communicates in host command packets. These packets are easier to parse, create, and filter on.
    • With filtering, you get the added bonus of not having unwanted debug output.
    • It allows us to really test the host command interface which is how the EC talks to the AP anyways.
  • Better testing of existing host command handlers.
    • By speaking in host command packets, we can reuse the existing host command handlers which is nice since we’ll be using the same handlers that are used to respond to the AP.
  • FAFT would no longer have to worry about the console dropping characters.
    • We can add error checking to the interpreter which would automatically retry errors. This alleviates FAFT from trying to check if the EC had properly received a line of input. (CTRL+L)
With better and more reliable tests, we can improve the quality of our EC codebase.

Space Savings

By moving the console functionality off of the EC, we would be able to shave off a considerable amount of bytes from the EC images. People wouldn’t have to worry as much about creating a debug console print with the fear of bloating up the image size. Smaller stack requirements by changing printf formatting to only count bytes while moving common strings off the EC. Additionally, most of these savings will come for free as it will apply to every EC with a console. We won’t be restricted by the type of chip.

A richer EC console

  • We could do things like on-the-fly console channel filtering.
  • Coloring specific channels such as “mark all USB PD messages in green”.
  • Adding colors in general.
  • Adding temporary console commands.
  • Longer command history which survives EC reboot
  • Searching command history
  • Redirecting debug output to log files (which causes no interleaving of command and debug output)
  • Bang commands (!foo)

Better debuggability

Sometimes, there will be an issue with the EC (or believed to be an EC issue) such as the keyboard locking up on certain keys or rows. At times like that, it would be nice to have an EC console to see what’s going on. Other times maybe having a servo connected might make the issue not present itself.
  • We could do cool things like having an EC console without having to hook up servo.
  • Run ectool from the chroot using a PTY interface.

Deployment Strategy

There are many facets to this feature, but here’s a deployment strategy which will gradually take us from the current EC console today, to a future where the console is completely removed from the EC. The goal will be to make this change as transparent as possible to developers.

Phase 1: Insertion

[Merged to ToT on 2016-Feb-07]
Phase 1 will most likely consist of getting EC-3PO in place in between servo and the EC while not modifying the behavior of the console too much. In this phase, we can replicate the console interface and achieve the following things.
  • Replicate command editing.
  • Save command history.
  • Add error checking to the console commands.

Phase 2: Assimilation

Phase 2 will start to introduce the host command packet communication.
  • Printing will be done via packets and assembled in EC-3PO.
  • Console commands now are sent using the host command packets.
    • This will be incremental as console commands are converted.
  • Add debug output filtering and redirection/logging.

Phase 3: Expansion

Phase 3 will expand the feature set of EC-3PO.
  • Add PTY interface to ectool.
  • Add on-device console without servod.
  • Colored output.
  • Command history search.
  • Bang commands (!foo)

High Level Design & Highlights


EC Interface

Each host command is a 16-bit command value. Commands which take parameters or return response data specify structs for that data. See include/ec_commands.h for the current format of request and replies. Currently, there are no changes made to the format of the host request and response structures.

On the EC, we essentially need to create a UART host command handler. This handler will be watching the console input for a byte sequence to switch into this host command mode. The starting sequence for an incoming host command packet will be 0xDA, a byte signifying EC_COMMAND_PROTOCOL_3. Once this byte is seen, the EC will transition to its “host command processing mode” and receive the host command. It will then process the host command, send the binary host response, and then transition back into normal mode. Ideally, there should be some locking of the UART to prevent other writes while this is taking place.

By the end of the assimilation (Phase 2), there would be no “normal mode” and everything would be in the binary host command mode.

Host Interface

The host interface is where the bulk of the work will be. This will be converting from the received host commands to console prints as well as converting the console inputs to host commands. It will also be responsible for replicating the console. This includes things like moving back and forth for command editing.

The interpreter should also open a PTY and dut-control should return this PTY as the ec_uart_pty. This is to ensure that the change is as transparent as possible to developers.

Features

The following are an explanation of a few of the planned features.

Command Error Checking

EC-3PO and the EC can perform error checking when sending the commands to the EC. This feature would be implemented prior to switching to the binary format. The interpreter can package the command in the following manner.
  • 2 Ampersands
  • 2 hex digits representing the length of the command
  • 2 hex digits representing the CRC-8 of the command
  • 1 Ampersand
  • The command itself
  • 2 newline characters. 
This is robust because no commands currently start with '&'. If the EC does not see ‘&&’, then one of the ampersands has been dropped. If the EC doesn’t see an ampersand after 4 hex digits, it either dropped a hex digit or the ampersand. Two newlines, so dropping one of those is alright. Once the EC gets the command and newline, it can verify the command string hasn't been corrupted.

In the event that the command string was corrupted, the EC can return an error string back of “&&EE”. If the translator reads a line with at least one ampersand followed by one “E”, then an error had occurred and the translator can simply retry the console command. This creates a reliable input to the console, a great win for FAFT.

PTY interface to ectool

Once the UART host command handler is functional, we could add the UART as one of the interfaces to ectool. This would allow ectool to be run from the chroot simply by having ectool communicate over the PTY. Since ectool communicates using host commands anyways, everything should just work™. The benefits of this include faster development of ectool and host commands as well as a more robust interface for FAFT.

On-device EC console without Servo

Once the transition is complete and the console speaks entirely in host commands, it’s no longer necessary to have the console talk solely over the UART. EC-3PO, or a variant, could exist as a standalone application that could be bundled in the system image, just like ectool. It would then send and receive host commands using the same interface that ectool uses whether it be LPC or i2c. This would essentially give us a console without having to hook up servo.

Note for security reasons, this must be locked down to only allow a subset of commands and debug output when the system is ready to ship.

Replacing cprintf()

All prints will need to become packets. In these packets will contain the format string, but all cprintf() has to do is parse to format string to determine how many bytes the parameters take up. Then, cprintf() will send the format string and the parameters to EC-3PO which will create the proper string using that information. That makes cprintf() on the EC smaller and use less stack space than it would have used for formatting.

We could also have a table of common format strings which the EC could just provide an index and the parameters. 

Internal Design

EC-3PO is a python package which aims to migrate the rich debug console from the EC itself to the host. It is composed of two modules: console and interpreter.

 

Console module

The console module provides the interactive console interface between the user and the interpreter. It handles the presentation of the EC console including editing methods as well as session-persistent command history.
The console runs in an infinite loop listening for activity on three things using the select() system call: the PTY served to the user, the command pipe, and the debug pipe. The debug pipe is a unidirectional pipe from the interpreter to the console. From this pipe are debug prints that originate from the attached EC and the console currently writes these strings as is to the user PTY. The command pipe is bidirectional and is used between the console and the interpreter for command traffic. An example transaction would be a host command request and response.

Enhanced EC image negotiation

When the user transmits a character on the PTY, the console begins to scan every byte and perform the appropriate actions. Since not every EC image will support these enhanced features, the console must perform an interrogation to determine what behaviour to take. If the interrogation mode is set to auto, this negotiation takes place every time the enter key is pressed. The interrogation is very simple 2 way handshake. The console sends down a byte, EC_SYN and waits a timeout period to receive a byte, EC_ACK. This timeout period is 300ms for non-enhanced EC images and 1 second for enhanced EC images. Enhanced EC images will try to immediately respond to an EC_SYN with an EC_ACK to indicate to EC-3PO that the current EC images is enhanced. The non-enhanced EC image timeout period is intended to be short because non-enhanced EC images will never reply to the EC_SYN. By keeping this timeout short, we are essentially inserting a slight pause after each command. However, this timeout is long enough for an enhanced EC image to send its reply of EC_ACK. Once the EC_ACK is received by the console, the console increases the timeout to 1 second for stability purposes just in case the enhanced EC image takes a bit longer to send its reply for some reason. This negotiation allows EC-3PO to behave correctly for both enhanced and non-enhanced EC images.

If a user knows that they are not using an enhanced EC image, they can disable the interrogation by issuing a OOBM command. This will cause the console to never initiate a negotiation, eliminating the delay. See the “interrogate” command in the Out of Band Management section for usage.

Enhanced vs. Non-Enhanced EC images

All EC images which don’t explicitly enable the new features (or were built before the features were implemented in the code base) are non-enhanced images. Non-enhanced EC images will be handling the presentation of the console including editing methods (and history if enabled). Therefore, the EC-3PO console and interpreter behave as a simple pipe for this case forwarding everything straight to the EC to handle. For the enhanced EC images, all console presentation(including editing methods) is handled locally by the EC-3PO console. Enhanced EC images will also support all of the other features discussed in this document.

Out of Band Management

The interactive console also has an Out of Band Management (OOBM) interface. This allows commands to be entered that can alter the behavior of the console and interpreter during runtime. From the console, one can bring up the OOBM prompt by pressing ‘%’ . These were originally added for debug purposes.

Supported Commands

  • loglevel <integer>
    • Allows setting the effective loglevel of the the console and interpreter.
  • interrogate <never | always | auto> [enhanced]
    • Allows control of when and how often interrogation occurs.

Interpreter module

The interpreter provides the interpretation layer between the EC UART and the user. Similar to the console module, the interpreter starts an infinite loop of servicing the user and the EC by means of a select() system call. It receives commands through its command pipe, formats the commands for the EC, and sends the command to the EC. It also presents data from the EC to either be displayed via the interactive console module or some other consumer.

The interpreter also keeps track of whether the EC image it’s communicating with is enhanced or not. This is required so that the interpreter can communicate correctly with the EC. For enhanced EC images, the interpreter will pack incoming commands in a particular format. This could be the “packed plaintext” form or the binary host command format. With the packed plaintext form, the interpreter also supports command retrying by monitoring the response of the EC and automatically retrying the command with no input from the user.

Other users

Since the interpreter communicates using pipes, it’s not necessary that the user use the console module. For example, FAFT could directly connect to the interpreter and send down commands and receive command responses instead of having to deal with the PTY and instead just deal with python objects.

Revision History

  • 1.0 (2016-02-11) - Published design document to chromium.org.
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