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SubjectRe: [PATCH 3/7] spi: spi-mem: Add a driver for NXP FlexSPI controller
Hi Yogesh,

On Fri, 31 Aug 2018 16:00:00 +0530
Yogesh Gaur <yogeshnarayan.gaur@nxp.com> wrote:

> - Add a driver for NXP FlexSPI host controller
>
> (0) What is the FlexSPI controller?
> FlexSPI is a flexsible SPI host controller which supports two SPI
> channels and up to 4 external devices.
> Each channel supports Single/Dual/Quad/Octal mode data
> transfer (1/2/4/8 bidirectional data lines)
> i.e. FlexSPI acts as an interface to external devices,
> maximum 4, each with up to 8 bidirectional data lines.
>
> It uses new SPI memory interface of the SPI framework to issue flash
> memory operations to up to four connected flash chips (2 buses with
> 2 CS each).
> Chipselect for each flash can be selected as per address assigned in
> controller specific registers.
>
> FlexSPI controller is similar to the existing Freescale/NXP QuadSPI
> controller with advanced features.

Yep, I had a quick look at the code and they really look similar. Why
not extending the existing driver instead of creating a new one from
scratch?

>
> (1) The FlexSPI controller is driven by the LUT(Look-up Table)
> registers.
> The LUT registers are a look-up-table for sequences of instructions.
> A valid sequence consists of four LUT registers.
> Maximum 32 LUT sequences can be programmed simultaneously.
>
> (2) The definition of the LUT register shows below:
> ---------------------------------------------------
> | INSTR1 | PAD1 | OPRND1 | INSTR0 | PAD0 | OPRND0 |
> ---------------------------------------------------
>
> There are several types of INSTRx, such as:
> CMD : the SPI NOR command.
> ADDR : the address for the SPI NOR command.
> DUMMY : the dummy cycles needed by the SPI NOR command.
> ....
>
> There are several types of PADx, such as:
> PAD1 : use a singe I/O line.
> PAD2 : use two I/O lines.
> PAD4 : use quad I/O lines.
> PAD8 : use octal I/O lines.
> ....
>
> (3) LUTs are being created at run-time based on the commands passed
> from the spi-mem framework. Thus, using single LUT index.
>
> (4) Software triggered Flash read/write access by IP Bus.
>
> (5) Memory mapped read access by AHB Bus.

Do we really want to have this level of details in the commit message?
I mean, this is something you can document in the driver, but not here.

BTW, you might want to have a look at [1]. I think we can get rid of
the ->size field you're adding if this driver implements the dirmap
hooks.

>
> (6) Tested this driver with the mtd_debug and JFFS2 filesystem utility
> on NXP LX2160ARDB and LX2160AQDS targets.
> LX2160ARDB is having two NOR slave device connected on single bus A
> i.e. A0 and A1 (CS0 and CS1).
> LX2160AQDS is having two NOR slave device connected on separate buses
> one flash on A0 and second on B1 i.e. (CS0 and CS3).
> Verified this driver on following SPI NOR flashes:
> Micron, mt35xu512ab, [Read - 1 bit mode]
> Cypress, s25fl512s, [Read - 1/2/4 bit mode]

Ok, that's good to have in the commit message.

>
> - Add config option entry in 'spi-nor/Kconfig' for FlexSPI driver.

But this one is useless. If you add a new driver, you have no other
choice but to add a new entry in the Kconfig file.

>
> - Add entry in the 'spi-nor/Makefile'.
>

Ditto.

Before you re-send a new version, I'd like to understand why you think
you need to create a new driver, and I want you to try to implement the
dirmap hook and check if you can get rid of the ->size field when doing
that.

I also seem one extra benefit to having a single driver for both
FlexSPI and QuadSPI IPs: you'll help Frieder debug the last remaining
problems you reported on the new QuadSPI driver :-P.

Thanks,

Boris

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