mysteries_intel: Add a section for software vs hardware sequencing

This attempts to explain software sequencing, hardware sequencing,
and the "Opaque flash chip".

Change-Id: I2445e926aad96060f26d0bc55dd7642c1a404296
Signed-off-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/42485
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/Documentation/mysteries_intel.txt b/Documentation/mysteries_intel.txt
index 60ad21a76..088abb877 100644
--- a/Documentation/mysteries_intel.txt
+++ b/Documentation/mysteries_intel.txt
@@ -5,6 +5,48 @@
  A0h), so we have no clue if or where it is on ICH8. Out current policy is to
  not touch it at all and assume/hope it is 0.
 
+= Software Sequencing vs. Hardware Sequencing and the "Opaque flash chip" =
+Software sequencing and hardware sequencing are two methods used to interface
+with the SPI controller on Intel platforms. They can be selected using either
+ich_spi_mode=swseq or ich_spi_mode=hwseq programmer parameters. Flashrom will
+attempt to automatically detect which mode to use.
+
+Software sequencing is the traditional method whereby software running on the
+CPU handles most of the logic needed to interact with the flash chip. This
+offers good flexibility since the user can utilize any opcode available in the
+OPMENU registers, and OPMENU can be left unlocked or on coreboot-supported
+platforms the owner of the system may program it for their needs before locking
+it. Advanced or non-standard features of a chip such as write protection and
+OTP may therefore be directly utilized by software.
+
+Hardware sequencing is a newer method (since around 2011) whereby most of the
+logic for interacting with the SPI flash chip is contained within the SPI
+controller itself and software such as flashrom may only select a few operations
+chosen by Intel via the Flash Cycle (FCYCLE) field. The chip must conform to
+specifications from Intel for each chipset/PCH. The specs are given in the
+"SPI Programming Guide" application note. See [SPI_PROG] cited at the bottom of
+this document for an example.
+
+Hardware sequencing simplifies things from a software perspective since the
+software is guaranteed some minimal level of support and doesn't even need to
+know the chip's ID or opcodes; it just needs to tell the SPI controller to
+perform a type of transaction such as "read", "4k block erase", etc. Hence when
+using hardware sequencing one will see "Opaque flash chip" as the chip's
+description since software might not be able to identify the chip. The SPI
+controller can combine multiple physical flash chips to logically appear as a
+single large flash device, and in such cases it would not make sense for
+flashrom to try to identify the chip.
+
+In many non-Intel systems the software has full control of a generic SPI
+controller where the software controls the SPI signals and also constructs the
+data payload including pre-op (e.g. write enable latch), opcode, address, and
+data. Intel SPI flash controllers are purpose-built for flash chip access and
+the software does not control the hardware directly. This makes Intel SPI
+controllers less flexible from a software standpoint, however there are some
+benefits such as guaranteed atomicity and multi-master arbitration needed for
+modern Intel platforms where the CPU and various microprocessors can share the
+same flash chip.
+
 = SMM BIOS Write Protection =
 Sometimes a hardware vendor will enable "SMM BIOS Write Protect" (SMM_BWP)
 in the firmware during boot time. The bits that control SMM_BWP are in the