Hi Boris,

Thank you very much for quick response.

On Wed, 20 Jun 2018, Boris Brezillon wrote:

Hi Piotr,

On Mon, 18 Jun 2018 18:21:22 +0200
Piotr Bugalski [off-list ref] wrote:

quoted

Hello,

Atmel SAMA5D2 is equipped with two QSPI interfaces. These interfaces can
work as in SPI-compatible mode or use two / four lines to improve
communication speed. At the moment there is QSPI driver strongly tied to
NOR-flash memory and MTD subsystem.
Intention of this change is to provide new driver which will not be tied
to MTD and allows using QSPI with NAND-flash memory or other peripherals
New spi-mem API provides abstraction layer which can disconnect QSPI
from MTD. This driver doesn't support regular SPI interface, it should
be used with spi-mem interface only.

Glad to see that people are starting to convert their SPI NOR
controller drivers to the SPI mem approach.

quoted

Unfortunately SAMA5D2 hardware by default supports only NOR-flash
memory. It allows 24- and 32-bit addressing while NAND-flash requires
16-bit long. To workaround hardware limitation driver is a bit more
complicated.

Request to spi-mem contains three fiels: opcode (command), address,
dummy bytes. SAMA5D2 QSPI hardware supports opcode, address, dummy and
option byte where address field can only be 24- or 32- bytes long.
Handling 8-bits long addresses is done using option field. For 16-bits
address behaviour depends of number of requested dummy bits. If there
are 8 or more dummy cycles, address is shifted and sent with first dummy
byte. Otherwise opcode is disabled and first byte of address contains
command opcode (works only if opcode and address use the same buswidth).
The limitation is when 16-bit address is used without enough dummy
cycles and opcode is using different buswidth than address. Other modes
are supported with described workaround.

It looks like hardware has some limitation in performance. The same issue
exists in current QSPI driver (MTD/nor-flash) and soft-pack (bare-metal
library from Atmel). Without using DMA read speed is much worse than
maximum bandwidth (efficiency 30-40%). Any help with performance
improvement is highly welcome, especially for NAND-flash memories which
offers higher capacity than NOR-flash used with previous driver.

Best Regards,
Piotr

Piotr Bugalski (2):
  spi: Add QuadSPI driver for Atmel SAMA5D2
  dt-bindings: spi: QuadSPI driver for Atmel SAMA5D2 documentation

 .../devicetree/bindings/spi/spi_atmel-qspi.txt     |  41 ++
 drivers/spi/Kconfig                                |   9 +
 drivers/spi/Makefile                               |   1 +
 drivers/spi/spi-atmel-qspi.c                       | 480 +++++++++++++++++++++

I'd like a solution where we remove the old driver. I definitely don't
want to have both in parallel. Did you test the new driver with a SPI
NOR to check if it still works correctly? If you did, then I'd suggest
that you add a patch updating defconfigs where the SPI_ATMEL_QUADSPI is
selected and another patch removing the old driver.

I misunderstood a bit your idea. My main concern was NAND-flash with
QSPI interface and I expected original nor-flash driver to stay
untouched - at least now. However idea of replacement older driver
with spi-mem approach makes a lot of sense to me.
I'll test nor-flash with new driver first, it should work but I prefer
to be sure. In next version I'll follow your suggestion and replace
old nor-flash driver.

quoted

 4 files changed, 531 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/spi/spi_atmel-qspi.txt

This should be a simple mv from
Documentation/devicetree/bindings/mtd/atmel-quadspi.txt to
Documentation/devicetree/bindings/spi/spi-atmel-qspi.txt

quoted

 create mode 100644 drivers/spi/spi-atmel-qspi.c

Thanks,

Boris

Thank you for comments,
Piotr