linux-kernel - Re: [PATCH 2/3] mtd: spi-nor: Add a driver for the VIA/WonderMedia serial flash controller

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Message-ID: <CABjd4YyRScBgDbi8Sk0D3vxcmLF8+YBetUdkfhrS_4Y7M+gS1g@mail.gmail.com>
Date: Mon, 12 May 2025 21:50:26 +0400
From: Alexey Charkov <alchark@...il.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Mark Brown <broonie@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Tudor Ambarus <tudor.ambarus@...aro.org>, Michael Walle <mwalle@...nel.org>, 
	Miquel Raynal <miquel.raynal@...tlin.com>, Richard Weinberger <richard@....at>, 
	Vignesh Raghavendra <vigneshr@...com>, Krzysztof Kozlowski <krzk@...nel.org>, linux-spi@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mtd@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/3] mtd: spi-nor: Add a driver for the VIA/WonderMedia
 serial flash controller

Hi Pratyush,

On Mon, May 12, 2025 at 1:20 PM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> Hi Alexey,
>
> On Sat, May 10 2025, Alexey Charkov wrote:
>
> > The controller is tailored to SPI NOR flash memory and abstracts away all
> > SPI communications behind a small set of MMIO registers and a physical
> > memory mapping of the actual chip contents.
> >
> > It doesn't expose chip probing functions beyond reading the ID though, so
> > use lower level chip opcodes via the "programmable command mode" of the
> > controller and the kernel's SPI NOR framework to avoid hard-coding chip
> > parameters for each ID the way the vendor kernel does it.
> >
> > Currently tested on a WonderMedia WM8950 SoC with a Macronix MX25L4005A
> > flash chip (APC Rock board), but should work on all VIA/WonderMedia SoCs
> >
> > Signed-off-by: Alexey Charkov <alchark@...il.com>
> > ---
> >  MAINTAINERS                                  |   1 +
> >  drivers/mtd/spi-nor/controllers/Kconfig      |  14 +
> >  drivers/mtd/spi-nor/controllers/Makefile     |   1 +
> >  drivers/mtd/spi-nor/controllers/wmt-sflash.c | 525 +++++++++++++++++++++++++++
>
> Drivers in drivers/mtd/spi-nor/controllers/ are deprecated, and we want
> to eventually get rid of the API. The expected way is for drivers to use
> SPI MEM (drivers/spi/spi-mem.c). SPI MEM drivers are usually more
> general and not tailored specifically to SPI NOR flashes, so it might be
> a bit tricky to write drivers for specialized hardware with it. But I
> think the drivers/spi/spi-intel.c driver is written for similar kind of
> hardware so it should be possible.

Oops. I've had a look at spi-mem, and it seems like it's not a
particularly fitting abstraction for this controller.

>From what I understood, spi-mem primarily expects to be talking SPI
opcodes to the controller, and for the controller/driver to bring
their own chip probing routines. This controller on the other hand
abstracts the opcodes away, and wants someone to tell it what its
flash chip can do (the controller itself can only get a chip ID in
"normal" mode, and it needs to somehow know the chip size and
standard/fast read capability of the chip). So pretty much the
opposite, huh.

In the end, I only need something like spi_nor_detect() and can do the
rest directly on top of the MTD framework without touching any SPI
opcodes after the detection is done. Is there any other non-deprecated
framework that can provide something like this? Maybe physmap? It
looks even older than SPI NOR though :)

Best regards,
Alexey