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

[Piotr Bugalski <bugalski.piotr@gmail.com>]

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@gmail.com> 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