3mdeb · mkopec · Jan 16, 2024
diff --git a/blog/content/post/2024-0X-XX-usb-c-power-delivery.md b/blog/content/post/2024-0X-XX-usb-c-power-delivery.md
@@ -0,0 +1,109 @@
+---
+title: NovaCustom v1.7.2 / v1.5.2 release # TODO
+abstract: 'A new firmware release for NovaCustom laptops is out! We put a lot
+           of focus on stability and reliability in this release, and this post
+           is about the challenges we faced here.'
+cover: /covers/qubes-openqa.png # TODO
+author: michal.kopec
+layout: post
+published: false
+date: 2024-01-04
+archives: "2024"
+
+tags:
+categories:
+
+---
+
+A couple of months ago we published firmware v1.5.1 for Tiger Lake and v1.7.1
+for Alder Lake based NovaCustom laptops. Unfortunately, this release introduced
+a number of issues that slipped past our validation and failed to correctly
+address the existing problems.
+
+We have just published the v1.7.2 and v1.5.2 "hotfix" releases, that focus
+exclusively on fixing problems and polishing existing features. This release
+also led to some changes in the testing and release process, to ensure that
+these problems don't happen again in the future.
+
+Let's go through the biggest problems and learn how they were fixed, and
+hopefully learn how to avoid them in the future:
+
+## USB-C issues
+
+USB Type-C or USB-C for short is a powerful, versatile connector. As a user I
+appreciate that I can travel with a single charger and cable and will be able
+to charge almost every device I have, or plug my work laptop, personal laptop,
+phone or gaming console into the same docking station and have everything work.
+
+That versatility does, however, depend on every piece being compliant to the
+USB-C specification and behaving as expected when connected to an arbitrary
+accessory. For example, a laptop has to limit the current it draws from the
+charger based on how much the charger says it can provide. And that alone can
+be pretty difficult to get right...
+
+## USB Power Delivery
+
+NovaCustom laptops' USB-C power architecture consists of the physical USB-C
+port, a USB Power Delivery controller that communicates with USB-PD accessories,
+a battery charger in Hybrid Power Boost architecture, the Embedded Controller,
+and voltage regulators that convert input voltage to whatever voltages the
+platform may need. This is a rather standard architecture, and the EC is
+responsible for orchestrating the various components to work together.
+
+When you plug in a USB-C charger, a lot of things happen - in short:
+
+- PD controller detects that a device was plugged in
+- The PD power supply advertises the various voltages it supports
+- PD controller requests the needed voltage (20V) and allows voltage from
+  the USB-C connector to flow into the system
+- Battery charger sees that power is supplied and:
+    - Switches the input power from the battery to the power supply
+    - Begins charging the battery
+
+There is, however, a problem: USB-C power supplies can support various voltages
+and current ratings. Whatever power the charger gives us, we must make sure not
+to exceed.
+
+## Volts, amps, watts
+
+The voltage portion is rather simple in this case: as the laptops have buck
+converters (as opposed to buck-boost) and the system input power is 19V, the
+voltage must be 20V, which is the highest USB-PD voltage available (USB-PD EPR
+is not supported). Therefore the PD controller is programmed to always request
+20V and will not open the input gate for lower voltages.
+
+The current (amperage) part of the equation is a whole different matter. The
+laptops are capable of pulling in excess of 80W, so any weaker power brick will
+shut down or reset due to overcurrent protection. The same thing will happen
+to docking stations, and in this case the dock also disables video and data,
+causing a bad experience for the user.
+
+Clearly, we need a way to limit power draw. The way we went with is Psys power
+limits, which is a feature supported by the hardware. Psys is the power draw
+reported by the battery charger to the CPU voltage regulators, which then use
+that information to throttle power draw appropriately. Psys power limits can
+be programmed by the Embedded Controller based on available input power.
+
+Now, when you plug in a USB-PD power supply, the EC will read out the negotiated
+power contract and program the appropriate power limits in the CPU. That is
+the biggest part of the equation that prevents unwanted dock and charger resets.
+
+But what happens when throttling the CPU alone is not enough? There are some
+cases where we can still exceed that power limit:
+
+- When powering on - the CPU boots at maximum frequency, and has not yet been
+  programmed with the right power limits
+- Other platform components are drawing power - charging fully discharged
+  battery, using the discrete GPU, plugging in a power-hungry accessory...
+
+Clearly we need something more to solve these issues.
+
+## Hybrid Power Boost
+
+There are several battery charger architectures, the most common are Hybrid
+Power Boost and Narrow Voltage DC. NovaCustom laptops come with HPB charger
+units. They all share the following features:
+
+- Buck conversion: They can convert input power down from >19V
+- Power Boost mode: If the power draw is more than the limit programmed by the
+  EC, we