Nick
4 years ago
committed by
Pete Johanson
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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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|
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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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|
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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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 |
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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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 |
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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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 |
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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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