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Question about `bios_flash_offset` in `targets/kosagi_fomu.py` #241

Closed JosephBushagour closed 3 years ago

JosephBushagour commented 3 years ago

In the kosagi_fomu.py file the --bios-flash-offset argument has a default of 0x60000 but the flash function disregards its bios_flash_offset argument and always places the bios at 0x0020000. I would have thought this was a bug but using the file I can successfully boot into bios while leaving the bios_flash_offset value at 0x60000. I could be missing something, why does this work? https://github.com/litex-hub/litex-boards/blob/4b48f15265c902dc2931809444b5eb803023d0b5/litex_boards/targets/kosagi_fomu.py#L120-L161

mithro commented 3 years ago

@xobs / @umarcor - Any idea?

xobs commented 3 years ago

I'm not sure, but I suspect it's because the bios is built out of bram, and isn't XIP from SPI flash or anywhere like that.

enjoy-digital commented 3 years ago

@JosephBushagour It indeed seems the flash function and flash offset support was not completely finished, which I just did with https://github.com/litex-hub/litex-boards/commit/a455713e0c75538832d01fc35bf50a4e1d4a2820.

0x60000 - 0x20000 = 0x40000 which I suppose is the offset added by Foboot (for the DFU bitstream + firmware) which i now made explicit. @xobs can you confirm this?

xobs commented 3 years ago

With Foboot, it doesn't matter /where/ the BIOS goes since:

  1. RISC-V code is mostly PIC within a 4 MB window, and
  2. BRAM can shift anywhere since it's not actually reading from flash

The program that it jumps to is usually located at offset 0x4_0000 from the start of flash, or address 0x2004_0000 in memory-mapped RAM (SPI is mapped XIP at `0x2000_0000). See https://github.com/im-tomu/foboot/blob/master/doc/FLASHLAYOUT.md for the flash layout -- I wish we had memory maps generated as part of the documentation.

I'm not sure what the situation is with this command, since I've never used this to prepare an image. However, if you're loading a bitstream from SPI flash, then indeed it will be located on the flash chip at offset 0x4_0000.

enjoy-digital commented 3 years ago

Thanks @xobs, I indeed wanted to have confirmation of the offset introduced by the DFU programmer (ie that a write at 0 is physically localed at 0x4_0000 in the SPI Flash).

In this target, we are relying on DFU for the flashing and then using XiP for the BIOS, so we just need to handle this offset which is now the case.

enjoy-digital commented 3 years ago

@JosephBushagour: I think we can now close this since fixed (while doing the fix I also tested it with different --bios_flash_offset values). Please re-open if you have troubles.