USB-C has the nice feature of making an effectively random choice about which side is supposed to be charged. A phone will happily start charging a power bank. The devices are supposed to negotiate and automatically swap if it doesn't make sense. It doesn't seem to work well.
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in this case it's a wall wart that doesn't even have PD (well, no PD messages are being exchanged)
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I've tried plugging it into a PD power source. absolutely nothing happens. no charging, no PD messages, nothing.
¯\_ (ツ)_/¯
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Which phone model is it? I might actually have the solution to your problem.
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For the Pixel 2 XL, the issue we had is that the LG battery kernel module was broken when built into the kernel instead of loaded as a dynamic kernel module. I'd guess that you're using an OS build with it built into the kernel and maybe it's missing appropriate workarounds.
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The non-XL Pixel 2 has an HTC battery kernel module. It's possible that it has a similar issue. We had a couple fixes applied for the Pixel 2 XL. The issue that I used to have is that fast charging wouldn't kick in with a generic charger or USB-C to the computer.
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yeah that sounds exactly like what i'm experiencing. i think i've only seen fast charging with power banks and such. it's non-XL.
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I don't think we had a commit fixing it for the non-XL variant but I remember it being an issue. It's probably a similar issue. In my experience, both in-tree and out-of-tree Linux kernel driver modules are often broken when built into the kernel instead of dynamically loaded.
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It's easier to build them into the kernel than dealing with building modules and putting them in vendor. For GrapheneOS we can't even do that as a workaround for issues like this because we want dynamic kernel modules disabled. Easiest solution is probably dynamically loading it.
I think your best bet to figure out if this is the issue is disabling the module in your kernel build and disabling the sanity checks for kernel module loading so that the init scripts in vendor can successfully load the stock OS build of the HTC battery module from there.
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