[PATCH v2 0/9] mtd: spi-nor: read while write support

Miquel Raynal miquel.raynal at bootlin.com
Thu Nov 10 07:55:04 PST 2022

Hello folks,

Here is the follow-up of the RFC trying to bring a little bit of
parallelism to support SPI-NOR Read While Write feature on parts
supporting it and featuring several banks.

I have received some hardware to make it work, so since the RFC, the
series has been updated to fix my mistakes, but the overall idea is the

There is nothing Macronix specific in the implementation, the operations
and opcodes are exactly the same as before. The only difference being:
we may consider the chip usable when it is in the busy state during a
write or an erase. Any chip with an internal split allowing to perform
parallel operations might possibly leverage the benefits of this

The first patches are just refactoring and preparation work, there is
almost no functional change, it's just a way to prepare the introduction
of the new locking mechanism and hopefully provide the cleanest and
simplest diff possible for this new feature. The actual change is all
contained in "mtd: spi-nor: Enhance locking to support reads while
writes". The logic is described in the commit log and copy/pasted here
for clarity:

The following constraints have to be taken into account:
1#: A single operation can be performed in a given bank.
2#: Only a single program or erase operation can happen on the entire chip.
3#: The I/O bus is unique and thus is the most constrained resource, all
    spi-nor operations requiring access to the spi bus (through the spi
    controller) must be serialized until the bus exchanges are over. So
    we must ensure a single operation can be "sent" at a time.
4#: Any other operation that would not be either a read or a write or an
    erase is considered requiring access to the full chip and cannot be
    parallelized, we then need to ensure the full chip is in the idle
    state when this occurs.

All these constraints can easily be managed with a proper locking model:
1#: Is protected by a per-bank mutex. Only a single operation can happen
    in a specific bank at any times. If the bank mutex is not available,
    the operation cannot start.
2#: Is handled by the pe_mode mutex which is acquired before any write
    or erase, and is released only at the very end of the
    operation. This way, no other destructive operation on the chip can
    start during this time frame.
3#: A device-wide mutex is introduced in order to capture and serialize
    bus accessed. This is the one being released "sooner" than before,
    because we only need to protect the chip against other SPI accesses
    during the I/O phase, which for the destructive operations is the
    beginning of the operation (when we send the command cycles and
    eventually the data), while the second part of the operation (the
    erase delay or the programmation delay) is when we can do something
    else with another bank.
4#: Is handled by the "generic" helpers which existed before, where
    basically all the locks are taken before the operation can start,
    and all locks are released once done.

As many devices still do not support this feature, the original lock is
also kept in a union: either the feature is available and we initialize
and use the new locks, or it is not and we keep using the previous

Here is now a benchmark with a Macronix MX25UW51245G with bank and RWW

     // Testing the two accesses in the same bank
     $ flash_speed -b0 -k0 -c10 -d /dev/mtd0
     testing read while write latency
     read while write took 51ms, read ended after 51ms

     // Testing the two accesses within different banks
     $ flash_speed -b0 -k4096 -c10 -d /dev/mtd0
     testing read while write latency
     read while write took 51ms, read ended after 20ms

Here is a branch with the mtd-utils patch bringing support for this
additional "-k" parameter (for the second block to use during RWW
testing), used to get the above result:


Miquel Raynal (9):
  mtd: spi-nor: Create macros to define chip IDs and geometries
  mtd: spi-nor: Introduce the concept of bank
  mtd: spi-nor: Add a macro to define more banks
  mtd: spi-nor: Reorder the preparation vs locking steps
  mtd: spi-nor: Separate preparation and locking
  mtd: spi-nor: Prepare the introduction of a new locking mechanism
  mtd: spi-nor: Add a RWW flag
  mtd: spi-nor: Enhance locking to support reads while writes
  mtd: spi-nor: macronix: Add support for mx25uw51245g with RWW

 drivers/mtd/spi-nor/core.c     | 224 +++++++++++++++++++++++++++++----
 drivers/mtd/spi-nor/core.h     |  61 +++++----
 drivers/mtd/spi-nor/macronix.c |   3 +
 include/linux/mtd/spi-nor.h    |  12 +-
 4 files changed, 250 insertions(+), 50 deletions(-)


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