[Skiboot] [PATCH 3/5] libflash/mbox-flash: Update to V2 of the protocol

[Benjamin Herrenschmidt]

On Mon, 2017-04-24 at 19:14 +1000, Cyril Bur wrote:
> +       if (mbox_flash->version == 1) {
> +               msg_put_u16(msg, 0, pos >> mbox_flash->shift);
> +               msg_put_u32(msg, 2, len);
> +       } else {
> +               uint64_t window_pos = (pos - mbox_flash->write.cur_pos);
> +               uint64_t window_len = len + (window_pos & mbox_flash_mask(mbox_flash));
> +
> +               msg_put_u16(msg, 0, window_pos >> mbox_flash->shift);
> +               msg_put_u16(msg, 2, ALIGN_UP(window_len, 1 << mbox_flash->shift) >> mbox_flash->shift);
> +       }
> +

Those alignments are gross and I think not quite right.

Let's say your block size is 4k. window_pos is 3k and window_len is 2k

Intuitively, you see that you need to mark both block 0 and 1 dirty we
straddle two blocks.

However, your code will align window_len to 4k, which shifted means 1
so you'll only mark 1 block.

What you need to do (and it's cleaner than this constant shifting
around) is to calculate the start and end (not len)

Then align them.

Then you can do end-start

So in my example, start becomes aligned to 0, end which is 5k becomes
aligned to 8k, so your substraction now gives you 2 blocks.

Cheers,
Ben.