Fix flash wear leveling sector calculation

[Diff-fusion]

Description

Currently the calculation of the last usable flash sector doesn't work properly if last_sector starts beyond the end of the actual flash size:

qmk_firmware/platforms/chibios/drivers/wear_leveling/wear_leveling_efl.c

Lines 87 to 92 in 172c349

	for (flash_sector_t i = 0; i < last_sector; ++i) {
	first_sector = last_sector - i - 1;
	if (flashGetSectorOffset(flash, first_sector) >= flash_size) {
	last_sector = first_sector;
	continue;
	}

Here the last_sector is calculated from the first_sector but also i is included in the calculation. But i is incremented in each loop iteration and thus an increasing offset is added.
The solution here is to first calculate the number of the last usable sector and then use another loop to calculate the first sector.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[sigprof]

Looks like this code won't work on STM32F4xx (and any other chips with non-uniform flash sectors), because flashGetSectorOffset(flash, desc->sectors_count) would read beyond the end of the desc->sectors array. So the code which finds the actual number of available flash sectors needs to be careful to avoid passing invalid sector numbers to ChibiOS functions.

[sigprof]

Applying WEAR_LEVELING_EFL_OMIT_LAST_SECTOR_COUNT first looks wrong (probably whoever added this code assumed that desc->sectors_count should be correct, but this might not be the case). There are no real users of that option in the repo though.

[sigprof]

This code would trigger undefined behavior on the first iteration if EFL uses the desc->sectors array (e.g., on STM32F4xx), because the valid range of sector numbers is [0…desc->sectors_count-1]. The original code (before it had been broken by the change which added WEAR_LEVELING_EFL_OMIT_LAST_SECTOR_COUNT) did not have this problem, because it subtracted at least 1 from desc->sectors_count.

Also should we guard against more pathological cases, like no free sectors, or even a sector being partially out of range of the reported flash size? Although with the current error handling the only option is apparently to halt with a message that nobody would see.

[Diff-fusion]

Thanks for the review. I fixed the issues you mentioned and added some additional checks.
One remaining issue is that flash sectors that are used for the firmware code could be used by the wear_leveling driver. I don't see any checks to prevent that. But I'm not that experienced with QMK and don't know what would be the best way to do this.
Error handling is a bit problematic. Exiting with chSysHalt is better than just crashing. But maybe compile time checks could be used to prevent some of the error cases.