Re: [RFC PATCH 0/2] New QuadSPI driver for Atmel SAMA5D2

[Piotr Bugalski]

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 <bugalski.piotr@xxxxxxxxx> wrote:
>
> > 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.
>
> > 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.

> >  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
>
> >  create mode 100644 drivers/spi/spi-atmel-qspi.c
>
> Thanks,
>
> Boris

Thank you for comments,
Piotr