Add blog post about #322

docs/blog/2020-10-03-bootloader-fix.md

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+---
+title: Fixing the Mysterious Broken Bootloader
+author: Nick Winans
+author_title: Contributor
+author_url: https://github.com/Nicell
+author_image_url: https://avatars1.githubusercontent.com/u/9439650
+tags: [bootloader, keyboards, firmware, oss, ble]
+---
+
+Recently I was able to fix the "stuck in the bootloader" issue in
+[#322](https://github.com/zmkfirmware/zmk/pull/322) that had been plaguing us
+for quite some time. I want to go over what the issue was, how the issue was
+diagnosed, and how it was fixed.
+
+## Background
+
+What exactly is the "stuck in the bootloader" issue? Seemingly randomly, users'
+keyboards would suddenly stop working and when they would reset their keyboard
+they would get put into the bootloader instead of back into the firmware. This
+would require the user to re-flash the firmware again to get into the firmware.
+That wouldn't be so bad except for the fact that once this occurs, every reset
+would require the user to re-flash the firmware again. The only way to really
+fix this issue was to re-flash the bootloader itself, which is a huge pain.
+
+Going into this, all we knew was that this issue was most likely introduced
+somewhere in the [#133](https://github.com/zmkfirmware/zmk/pull/133), which
+added Bluetooth profile management. We've had quite a few attempts at trying to
+recreate the issue, but we never were able to get it to happen consistently.
+
+## Diagnosing the issue
+
+This issue had been happening sporadically for the past month, and I finally
+decided to dig in to see what was going on. We started in the Discord and
+discussed what was common between all of the people who have experienced this
+issue. Everyone who had this issue reported that they did quite a bit of profile
+switching. This lined up with the possible connection to the Bluetooth profile
+management pull request.
+
+### Pinpointing the cause
+
+I had a hunch that this was related to the settings system. The settings system
+is used by profile Bluetooth switching, and the settings system works directly
+with the system flash. Based on this hunch, I tried spamming the RGB underglow
+cycle behavior on my main keyboard. Sure enough after a couple minutes, I got
+stuck in the bootloader. I was even able to reproduce it again.
+
+This was an important discovery for two reasons. First, I was able to recreate
+the issue consistently, which meant I could set up logging and more closely
+monitor what the board was doing. Second, this more or less proved that it was
+specifically the settings system at fault. Both Bluetooth profile switching and
+RGB underglow cycling trigger it, and the one common piece is they save their
+state to settings.
+
+### Settings system overview
+
+To understand what's going wrong, we first need to understand how the settings
+system works. Here's a diagram to explain the flash space that the settings
+system holds for our nRF52840 based boards (nice!nano, nRFMicro, BlueMicro).
+
+![Settings Diagram](https://i.imgur.com/DF2t3Oq.png)
+
+The settings flash space lives at the end of the flash of the chip. In this case
+it starts at `0xF8000` and is `0x8000` bytes long, which is 32KB in more
+comprehensible units. Then due to the chip's architecture, this flash space is
+broken into pages, which are `0x1000` bytes in size (4KB).
+
+The backend that carries out the settings save and read operation in ZMK is
+called NVS. NVS calls these pages sectors. Due to how flash works, you can't
+write to the same bytes multiple times without erasing them first, and to erase
+bytes, you need to erase the entire sector of flash. This means when NVS writes
+to the settings flash if there's no erased space available for the new value, it
+will need to erase a sector.
+
+### Logging discoveries
+
+So first I enabled logging of the NVS module by adding
+`CONFIG_NVS_LOG_LEVEL_DBG=y` to my `.conf` file. I repeated the same test of
+spamming RGB underglow effect cycle and the resulting logs I got were this:
+
+```
+[00:00:00.000,671] <inf> fs_nvs: 8 Sectors of 4096 bytes
+[00:00:00.000,671] <inf> fs_nvs: alloc wra: 3, f70
+[00:00:00.000,671] <inf> fs_nvs: data wra: 3, f40
+// A bunch of effect cycle spam
+[00:02:34.781,188] <dbg> fs_nvs: Erasing flash at fd000, len 4096
+// A bunch more effect cycle spam
+[00:06:42.219,970] <dbg> fs_nvs: Erasing flash at ff000, len 4096
+// A bunch more effect cycle spam
+// KABOOM - bootloader issue
+```
+
+So at start up, we can see that the 8 sectors of 4KB are found by NVS properly,
+however, I wasn't sure what the second and third lines meant, but we'll get back
+to that. Nonetheless the next two logs from NVS showed erasing the sector at
+`0xFD000` and then erasing the `0xFF000` sector.
+
+![Erased Sectors](https://i.imgur.com/DmLycMJ.png)
+
+It's really odd that the third to last sector and the last sector are erased,
+and then shortly after the bootloader issue is hit. I really had no explanation
+for this behavior.
+
+### Reaching out to Zephyr
+
+At this point, I nor anyone else working on the ZMK project knew enough about
+NVS to explain what was going on here. [Pete
+Johanson](https://github.com/petejohanson), project founder, reached out on the
+Zephyr Project's Slack (ZMK is built on top of Zephyr if you weren't aware).
+Justin B and Laczen assisted by first explaining that those `alloc wra` and
+`data wra` logs from earlier are showing what data NVS found at startup.
+
+More specifically, `data wra` should be `0` when it first starts up on a clean
+flash. As we can see from my earlier logging on a clean flash I was instead
+getting `f40`. NVS is finding data in our settings sectors when they should be
+blank! We were then given the advice to double check our bootloader.
+
+### The Adafruit nRF52 Bootloader
+
+Most of the boards the contributors of ZMK use have the [Adafruit nRF52
+Bootloader](https://github.com/adafruit/Adafruit_nRF52_Bootloader), which allows
+for extremely easy flashing by dragging and dropping `.uf2` files onto the board
+as a USB drive. Every bootloader takes up a portion of the flash, and in the
+README explains that the first `0x26000` is reserved for the bootloader with the
+nRF52840, and we've properly allocated that.
+
+However, there isn't a full explanation of the flash allocation of the
+bootloader in the README. There's a possibility that the bootloader is using
+part of the same flash area we're using. I reached out on the Adafruit Discord,
+and [Dan Halbert](https://github.com/dhalbert) pointed me towards the [linker
+map](https://github.com/adafruit/Adafruit_nRF52_Bootloader/blob/master/linker/nrf52840.ld)
+of the nRF52840. Let's take a look.
+
+```
+FLASH (rx) : ORIGIN = 0xF4000, LENGTH = 0xFE000-0xF4000-2048 /* 38 KB */
+
+BOOTLOADER_CONFIG (r): ORIGIN = 0xFE000 - 2048, LENGTH = 2048
+
+/** Location of mbr params page in flash. */
+MBR_PARAMS_PAGE (rw) : ORIGIN = 0xFE000, LENGTH = 0x1000
+
+/** Location of bootloader setting in flash. */
+BOOTLOADER_SETTINGS (rw) : ORIGIN = 0xFF000, LENGTH = 0x1000
+```
+
+Here's a diagram to show this a bit better.
+
+![Adafruit Bootloader Diagram](https://i.imgur.com/TEOA31m.png)
+
+We've found the issue! As you can see from the red bar (representing our
+settings flash area), we've put the settings flash area _right on top_ of the
+Adafruit bootloader's flash space. Oops!
+
+This also shines some light on why NVS erased `0xFD000` and `0xFF000` sectors.
+It's possible there was no flash written to `0xFD000` because the bootloader
+didn't use up all of that space it has, and then there possibly weren't any
+bootloader settings set yet, so `0xFF000` could be used and erased by NVS too.
+
+After erasing `0xFF000`, NVS probably next erased a rather important part of the
+bootloader that resulted in this issue at hand. In my opinion, we're pretty
+lucky that it didn't delete an even more vital part of the bootloader. At least
+we could still get to it, so that we could re-flash the bootloader easily!
+
+## The solution
+
+Now that we've found the issue, we can pretty easily fix this. We'll need to
+move the settings flash area back so that it doesn't overlap with the
+bootloader. First we calculate the size of the of flash area the bootloader is using.
+
+```
+0x100000 (end of flash) - 0x0F4000 (start of bootloader) = 0xC000 (48KB)
+```
+
+So the bootloader is using the last 48KB of the flash, this means all we need to
+do is shift back the settings area and code space `0xC000` bytes. We'll apply
+this to all of the `.dts` files for the boards that were affected by this issue.
+
+```diff
+		code_partition: partition@26000 {
+			label = "code_partition";
+-			reg = <0x00026000 0x000d2000>;
++			reg = <0x00026000 0x000c6000>;
+		};
+
+
+-		storage_partition: partition@f8000 {
++		storage_partition: partition@ec000 {
+			label = "storage";
+-			reg = <0x000f8000 0x00008000>;
++			reg = <0x000ec000 0x00008000>;
+		};
+```
+
+And with those changes, we should no longer run into this issue! In the process
+of these changes, we lost 48KB of space for application code, but we're only
+using around 20% of it anyways. 🎉