Advice on writing a custom bootloader for stm32 MCU - embedded

Question

Assume there are two boards with stm32 micro-controllers which are connected to each other with rs-485 (each board has a uart-to-rs485 transceiver). This is the connection diagram and the accessible ports:
I want to be able to re-program each board separately using rs-485 wires that are available. Using st system bootloader and boot0 pin is not an option because it requires changing the PCB and re-wiring the system.
So I need to write my own custom bootloader. What I intend to do is to separate the flash memory of the B-1 MCU into three parts:
20KB for bootloader
120KB for B-2 application (as kind of a buffer)
360KB for B-1 application (bootloader jumps to this part after finishing boot mode)
and for B-2, two partitions:
20KB for bootloader
100KB for main application
and using the UART interface of B-1, I can load the .hex files to the specified flash area and tell the MCU what to do with it (either use it as it's own main app or send it to B-2).
Here is a brief algorithm for the bootloader:
// B1: starts from bootloader
for (5 seconds) {
// check for the boot command from UART
if (command received) {
// send boot and reset command to B-2 and receive ack
// start the boot mode
while (boot mode not aborted) {
// receive command packet containing address
if (command == header && address == B1) {
// prompt for the main .hex file and store it in B-1 partition
} else if (command == header && address == B2) {
// prompt for the main .hex file and store it in B-2 partition
// send header and the app to B-2 using rs-485 port
} else if (command == abort) {
break from while loop
}
}
} else {
// do nothing
}
}
// jump to main application
Now I have 2 concerns:
Since there is no gpio connection between B-1 and B-2 to activate boot mode for B-2, is it possible for B-2 board to set a flag in flash memory outside of it's main application area to check for it and use it as a boot mode activation flag?
Is it possible to write the stream of data directly from uart input to the flash memory area of each application? like this:
// Obviously this address is outside the flash area of the current bootloader running
uint8_t appAddress = B2_APP_FLASH_MEMORY_PARTITION_START_ADDRESS;
for (i from 0 to size_of_app) {
hal_uart_receive(&uartPort, appAddress + i, 1, timeout);
}

Answer 1

Since there is no gpio connection between B-1 and B-2 to activate boot mode for B-2, is it possible for B-2 board to set a flag in flash memory outside of it's main application area to check for it and use it as a boot mode activation flag?
I am not sure that you are suggesting, or how your suggestion will solve the problem. Ideally you would have a means form B1 of directly resetting B2 via its /RESET line, but failing that if it is loaded with an application that accepts a reset command or signal over the RS-485, then you can then have it issue a soft-reset to start the bootloader. On Cortex-M devices you can do that through the NVIC.
If you need to communicate information to the B2 bootloader - perhaps to either invoke an update or to bypass that and boot the application, you need not program flash memory for that, you can simply write a boot command or signature via a reserved area of SRAM (best right at the top) that is not initialised by the runtime start-up (or the content of which you capture before such initialisation). Content in SRAM will survive a reset so long as power is maintained, so it can be used to communicate between the application and the bootloader - both ways.
This is of course a bootstrap issue - what if there is no application loaded to accept a reset command, or the application is not valid/complete (programming interruption). Well the relocated application area will have its vector table including its initial-SP and reset vector right at the start. In your bootloader when the first 8 bytes of the image are received, you hold them back and do not program that area until the rest of the image is written. By programming the reset vector last, if the programming is interrupted, that location will not be a valid address. The bootloader can validate it (or check if it is in the erase state) and if not valid/written, it can wait indefinitely for an update or simply reset to repeat the update polling. Be aware of a bit of an STM32 gotcha here though - most parts erase flash to "all-ones" (0xFF) state, some however (STM32Lxx parts) erase to "all-zeroes". Of course you couls simply check for 0x00000000 or 0xffffffff since neither would be a valid start address, or explicitly check the range.
Is it possible to write the stream of data directly from uart input to the flash memory area of each application? like this:
Yes, but remember that on STM32, normally the code is executing from the same flash memory you are trying to program and that the bus stalls during flash write and erase, such that if you are executing from flash, execution halts. For page erase, that can be several milliseconds - for parts with large pages, several hundred milliseconds even. That may mean that you fail to read characters on the UART if you are using polling or interrupt.
You can overcome this issue by protocol design. For example if you use a packet protocol where the last packet must be acknowledged before the next one is sent, you can use that as a flow control. Once you have collated a block of data to be written, you simply delay the acknowledgement of the last packet until after you have erased and/or written the data to flash. XMODEM-1K is a suitable protocol for that and despite its flaws its simplicity and support in common terminal emulator applications make it a good choice for this application.
Now all that said, you can increase the flash available to B1 by not buffering the image for B2 on B1 at all and simply implement a bi-directional pass-through such that the input on the UART of B1 is passed directly to the B1 RS-485 output, (surely also a UART so your port naming is ambiguous), and B1 RS-485 input passed directly to the UART output. That way B1 becomes "transparent" and the update tool will appear to be communicating directly with B2. That is perhaps far simpler and faster, and if the bootloader is "fail-safe" as described above, will still allow retries following interruption.
The pass-through function might be part of B1's application or a "mode" of the bootloader. The pass-through mode might be invoked by a particular boot command or you might have the application pass a "boot mode" command via the SRAM mechanism described earlier.
In either case ideally you would have identical bootloader code on both B1 and B2 for simplicity and flexibility. There is no reason why that should not be the case; they are both receiving the updates over UART.