linux-kernel - Re: [PATCH 3/7] spi: spi-mem: Add a driver for NXP FlexSPI controller

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Message-ID: <20180904165842.774ed960@bbrezillon>
Date:   Tue, 4 Sep 2018 16:58:42 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Yogesh Gaur <yogeshnarayan.gaur@....com>
Cc:     linux-mtd@...ts.infradead.org, marek.vasut@...il.com,
        linux-spi@...r.kernel.org, devicetree@...r.kernel.org,
        robh@...nel.org, mark.rutland@....com, shawnguo@...nel.org,
        linux-arm-kernel@...ts.infradead.org, computersforpeace@...il.com,
        frieder.schrempf@...eet.de, linux-kernel@...r.kernel.org
Subject: Re: [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@....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