commit 72cbf34381e856a4deb04a57e35bdc4b9f3e368a
author Eunchan Kim <eunchan@opentitan.org> Wed Apr 27 17:28:17 2022 -0700
committer Eli Kim <github@elikim.dev> Mon May 02 08:59:33 2022 -0700
tree 66fffd00b08d31ca62d9e7f42d21c6879980d387
parent 0bef721484db950fdb9c898e9a0b5f4ba8599ac3

[spi_device] Let readsram sends Sram Req

To make mailbox crossing logic simpler, let SRAM request from readsram
go out to DPSRAM even the operation is not permitted (readbuffer while
in PassThrough)

In order to block the SRAM request, the logic needs:

1. readcmd to hold fifo pop while not in mailbox as FIFO is empty
2. strb in readsram should be reset prior to move to StActive. When the
   address crosses the mailbox (readbuf -> mailbox), the strb is always
   0. strb in the current version latches `current_address_i` when first
   SRAM request sends out to DPSRAM and is incremented at every byte
   being pushed to FIFO.
3. `addr_in_mailbox` should be latched then be used to block the FIFO
   pop. at the last byte of the readbuf, the addr_in_mailbox is already
   high as it sees `addr_d` to check the mailbox region.

So, to make logic simpler, I decided to let request out and sends the
Readbuf data while not in mailbox (in Passthrough mode). The logic will
be blocked by `mailbox_assumed_o` signal at the spi_device top.

To do this, `mailbox_assumed_o` is also revised to set at every byte
while in the mailbox, and to clear if not in the mailbox region.

Signed-off-by: Eunchan Kim <eunchan@opentitan.org>