196 lines
8.7 KiB
Markdown
196 lines
8.7 KiB
Markdown
---
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title: Fixing the Mysterious Broken Bootloader
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author: Nick Winans
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author_title: Contributor
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author_url: https://github.com/Nicell
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author_image_url: https://avatars1.githubusercontent.com/u/9439650
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tags: [bootloader, keyboards, firmware, oss, ble]
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---
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Recently I was able to fix the "stuck in the bootloader" issue in
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[#322](https://github.com/zmkfirmware/zmk/pull/322) that had been plaguing us
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for quite some time. I want to go over what the issue was, how the issue was
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diagnosed, and how it was fixed.
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## Background
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What exactly is the "stuck in the bootloader" issue? Seemingly randomly, users'
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keyboards would suddenly stop working and when they would reset their keyboard
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they would get put into the bootloader instead of back into the firmware. This
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would require the user to re-flash the firmware again to get into the firmware.
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That wouldn't be so bad except for the fact that once this occurs, every reset
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would require the user to re-flash the firmware again. The only way to really
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fix this issue was to re-flash the bootloader itself, which is a huge pain.
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Going into this, all we knew was that this issue was most likely introduced
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somewhere in the [#133](https://github.com/zmkfirmware/zmk/pull/133), which
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added Bluetooth profile management. We've had quite a few attempts at trying to
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recreate the issue, but we never were able to get it to happen consistently.
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## Diagnosing the issue
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This issue had been happening sporadically for the past month, and I finally
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decided to dig in to see what was going on. We started in the Discord and
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discussed what was common between all of the people who have experienced this
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issue. Everyone who had this issue reported that they did quite a bit of profile
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switching. This lined up with the possible connection to the Bluetooth profile
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management pull request.
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### Pinpointing the cause
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I had a hunch that this was related to the settings system. The settings system
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is used by profile Bluetooth switching, and the settings system works directly
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with the system flash. Based on this hunch, I tried spamming the RGB underglow
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cycle behavior on my main keyboard. Sure enough after a couple minutes, I got
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stuck in the bootloader. I was even able to reproduce it again.
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This was an important discovery for two reasons. First, I was able to recreate
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the issue consistently, which meant I could set up logging and more closely
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monitor what the board was doing. Second, this more or less proved that it was
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specifically the settings system at fault. Both Bluetooth profile switching and
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RGB underglow cycling trigger it, and the one common piece is they save their
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state to settings.
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### Settings system overview
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To understand what's going wrong, we first need to understand how the settings
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system works. Here's a diagram to explain the flash space that the settings
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system holds for our nRF52840 based boards (nice!nano, nRFMicro, BlueMicro).
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The settings flash space lives at the end of the flash of the chip. In this case
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it starts at `0xF8000` and is `0x8000` bytes long, which is 32KB in more
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comprehensible units. Then due to the chip's architecture, this flash space is
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broken into pages, which are `0x1000` bytes in size (4KB).
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The backend that carries out the settings save and read operation in ZMK is
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called NVS. NVS calls these pages sectors. Due to how flash works, you can't
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write to the same bytes multiple times without erasing them first, and to erase
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bytes, you need to erase the entire sector of flash. This means when NVS writes
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to the settings flash if there's no erased space available for the new value, it
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will need to erase a sector.
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### Logging discoveries
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So first I enabled logging of the NVS module by adding
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`CONFIG_NVS_LOG_LEVEL_DBG=y` to my `.conf` file. I repeated the same test of
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spamming RGB underglow effect cycle and the resulting logs I got were this:
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```
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[00:00:00.000,671] <inf> fs_nvs: 8 Sectors of 4096 bytes
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[00:00:00.000,671] <inf> fs_nvs: alloc wra: 3, f70
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[00:00:00.000,671] <inf> fs_nvs: data wra: 3, f40
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// A bunch of effect cycle spam
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[00:02:34.781,188] <dbg> fs_nvs: Erasing flash at fd000, len 4096
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// A bunch more effect cycle spam
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[00:06:42.219,970] <dbg> fs_nvs: Erasing flash at ff000, len 4096
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// A bunch more effect cycle spam
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// KABOOM - bootloader issue
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```
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So at start up, we can see that the 8 sectors of 4KB are found by NVS properly,
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however, I wasn't sure what the second and third lines meant, but we'll get back
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to that. Nonetheless the next two logs from NVS showed erasing the sector at
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`0xFD000` and then erasing the `0xFF000` sector.
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It's really odd that the third to last sector and the last sector are erased,
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and then shortly after the bootloader issue is hit. I really had no explanation
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for this behavior.
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### Reaching out to Zephyr
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At this point, I nor anyone else working on the ZMK project knew enough about
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NVS to explain what was going on here. [Pete
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Johanson](https://github.com/petejohanson), project founder, reached out on the
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Zephyr Project's Slack (ZMK is built on top of Zephyr if you weren't aware).
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Justin B and Laczen assisted by first explaining that those `alloc wra` and
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`data wra` logs from earlier are showing what data NVS found at startup.
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More specifically, `data wra` should be `0` when it first starts up on a clean
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flash. As we can see from my earlier logging on a clean flash I was instead
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getting `f40`. NVS is finding data in our settings sectors when they should be
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blank! We were then given the advice to double check our bootloader.
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### The Adafruit nRF52 Bootloader
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Most of the boards the contributors of ZMK use have the [Adafruit nRF52
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Bootloader](https://github.com/adafruit/Adafruit_nRF52_Bootloader), which allows
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for extremely easy flashing by dragging and dropping `.uf2` files onto the board
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as a USB drive. Every bootloader takes up a portion of the flash, and in the
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README explains that the first `0x26000` is reserved for the bootloader with the
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nRF52840, and we've properly allocated that.
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However, there isn't a full explanation of the flash allocation of the
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bootloader in the README. There's a possibility that the bootloader is using
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part of the same flash area we're using. I reached out on the Adafruit Discord,
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and [Dan Halbert](https://github.com/dhalbert) pointed me towards the [linker
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map](https://github.com/adafruit/Adafruit_nRF52_Bootloader/blob/master/linker/nrf52840.ld)
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of the nRF52840. Let's take a look.
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```
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FLASH (rx) : ORIGIN = 0xF4000, LENGTH = 0xFE000-0xF4000-2048 /* 38 KB */
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BOOTLOADER_CONFIG (r): ORIGIN = 0xFE000 - 2048, LENGTH = 2048
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/** Location of mbr params page in flash. */
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MBR_PARAMS_PAGE (rw) : ORIGIN = 0xFE000, LENGTH = 0x1000
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/** Location of bootloader setting in flash. */
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BOOTLOADER_SETTINGS (rw) : ORIGIN = 0xFF000, LENGTH = 0x1000
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```
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Here's a diagram to show this a bit better.
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We've found the issue! As you can see from the red bar (representing our
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settings flash area), we've put the settings flash area _right on top_ of the
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Adafruit bootloader's flash space. Oops!
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This also shines some light on why NVS erased `0xFD000` and `0xFF000` sectors.
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It's possible there was no flash written to `0xFD000` because the bootloader
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didn't use up all of that space it has, and then there possibly weren't any
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bootloader settings set yet, so `0xFF000` could be used and erased by NVS too.
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After erasing `0xFF000`, NVS probably next erased a rather important part of the
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bootloader that resulted in this issue at hand. In my opinion, we're pretty
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lucky that it didn't delete an even more vital part of the bootloader. At least
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we could still get to it, so that we could re-flash the bootloader easily!
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## The solution
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Now that we've found the issue, we can pretty easily fix this. We'll need to
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move the settings flash area back so that it doesn't overlap with the
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bootloader. First we calculate the size of the of flash area the bootloader is using.
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```
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0x100000 (end of flash) - 0x0F4000 (start of bootloader) = 0xC000 (48KB)
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```
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So the bootloader is using the last 48KB of the flash, this means all we need to
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do is shift back the settings area and code space `0xC000` bytes. We'll apply
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this to all of the `.dts` files for the boards that were affected by this issue.
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```diff
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code_partition: partition@26000 {
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label = "code_partition";
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- reg = <0x00026000 0x000d2000>;
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+ reg = <0x00026000 0x000c6000>;
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};
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- storage_partition: partition@f8000 {
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+ storage_partition: partition@ec000 {
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label = "storage";
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- reg = <0x000f8000 0x00008000>;
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+ reg = <0x000ec000 0x00008000>;
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};
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```
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And with those changes, we should no longer run into this issue! In the process
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of these changes, we lost 48KB of space for application code, but we're only
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using around 20% of it anyways. 🎉
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