Given it's a developer tool and most workstations actually deliver 1.5A, I would not lower the limit.
Ramping is the icing on the cake.

I'm worried that it's fragile. Just to give you an idea, when I plug suzy-qable into the front port of a Z840 and pull 1.5A, I measure 4.4V VBUS (that's fine). But when I connect through a 7 ft extension cable, VBUS drops below 4.0V and gets into a cycling loop. I'm think many devs may have a similar extension cable config and will complain if I break their debug functionality.

Note that we don't ramp USB-C ports by default, though I guess we could implement something just for DTS sources. A CL implementing my original proposal is here: https://chromium-review.googlesource.com/#/c/chromiumos/platform/ec/+/693295/, but it's not ready to land, because I need to figure out what to do with servo_v4.

> I'm worried that it's fragile. Just to give you an idea, 
> when I plug suzy-qable into the front port of a Z840 and pull 1.5A, 
> I measure 4.4V VBUS (that's fine). But when I connect through a 7 ft extension cable, 
> VBUS drops below 4.0V and gets into a cycling loop. 
> I'm think many devs may have a similar extension cable config and 
> will complain if I break their debug functionality.

That's why for Ryu, I bought 40 powered hubs with 12W ports (and good quality USB cables) and distributed them... because the other half of the developers will complain that their battery dies when powered through the cable.


> Note that we don't ramp USB-C ports by default,

I know.

> though I guess we could implement something just for DTS sources.

This is nice to have as it mostly solve this issue (and we can probably go up to 2.x A)

I updated https://chromium-review.googlesource.com/#/c/chromiumos/platform/ec/+/693295/ to ramp DTS sources up to 2.4A, except for 3.0 Rp, which will ramp to 3.0A.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/ec/+/165f7d6f3bad4d49f977e1c5efad326f11007bf0

commit 165f7d6f3bad4d49f977e1c5efad326f11007bf0
Author: Shawn Nematbakhsh <shawnn@chromium.org>
Date: Thu Oct 19 19:56:42 2017

charge_ramp: Ramp USB-C DTS sources

suzy-qable advertises 1.5A, but its actual capability depends on the
host USB port it is attached to. Since suzy-qable is ubiquitous and
other DTS sources may behave in the same way, ramp the input current
limit in order to find a reasonable maximum.

BUG= chromium:770296 
BRANCH=None
TEST=Attach suzy-qable to kevin and reef, verify that neither OCs and EC
console via cr50 is available on reef. Also verify donette chargers
kevin at 3A and does not ramp.

Change-Id: Idd0683ede3a44111a01da6b4faab52f388ee82fd
Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/693295
Commit-Ready: Shawn N <shawnn@chromium.org>
Tested-by: Shawn N <shawnn@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

[modify] https://crrev.com/165f7d6f3bad4d49f977e1c5efad326f11007bf0/include/charge_manager.h
[modify] https://crrev.com/165f7d6f3bad4d49f977e1c5efad326f11007bf0/test/charge_manager.c
[modify] https://crrev.com/165f7d6f3bad4d49f977e1c5efad326f11007bf0/include/usb_pd.h
[modify] https://crrev.com/165f7d6f3bad4d49f977e1c5efad326f11007bf0/common/usb_pd_protocol.c
[modify] https://crrev.com/165f7d6f3bad4d49f977e1c5efad326f11007bf0/common/charge_ramp.c
[modify] https://crrev.com/165f7d6f3bad4d49f977e1c5efad326f11007bf0/board/host/board.h
[modify] https://crrev.com/165f7d6f3bad4d49f977e1c5efad326f11007bf0/common/charge_manager.c

USB-C DTS source will now start at 500mA and ramp up to the advertised limit.

If charge ramp is not enabled on a board, USB-C DTS will be stuck at 500mA.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/ec/+/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6

commit 3b06a92ced8d2d9f9773176da12b7fcb8db68ef6
Author: Shawn Nematbakhsh <shawnn@chromium.org>
Date: Mon Feb 12 19:49:28 2018

charge_ramp: Ramp USB-C DTS sources

suzy-qable advertises 1.5A, but its actual capability depends on the
host USB port it is attached to. Since suzy-qable is ubiquitous and
other DTS sources may behave in the same way, ramp the input current
limit in order to find a reasonable maximum.

BUG= chromium:770296 
BRANCH=None
TEST=Attach suzy-qable to kevin and reef, verify that neither OCs and EC
console via cr50 is available on reef. Also verify donette chargers
kevin at 3A and does not ramp.

Change-Id: I1a73a17f4228c4590a40d5209db7e98a5857ae0a
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Original-Commit-Id: 165f7d6f3bad4d49f977e1c5efad326f11007bf0
Original-Change-Id: Idd0683ede3a44111a01da6b4faab52f388ee82fd
Original-Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/693295
Original-Commit-Ready: Shawn N <shawnn@chromium.org>
Original-Tested-by: Shawn N <shawnn@chromium.org>
Original-Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/914651

[modify] https://crrev.com/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6/include/charge_manager.h
[modify] https://crrev.com/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6/test/charge_manager.c
[modify] https://crrev.com/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6/include/usb_pd.h
[modify] https://crrev.com/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6/common/usb_pd_protocol.c
[modify] https://crrev.com/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6/common/charge_ramp.c
[modify] https://crrev.com/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6/board/host/board.h
[modify] https://crrev.com/3b06a92ced8d2d9f9773176da12b7fcb8db68ef6/common/charge_manager.c