linux-kernel - Re: [PATCH v11 00/10] spi: Add support for stacked/parallel memories

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Date: Tue, 19 Dec 2023 09:12:21 +0100
From: Miquel Raynal <miquel.raynal@...tlin.com>
To: Richard Weinberger <richard@....at>
Cc: Amit Kumar Mahapatra <amit.kumar-mahapatra@....com>, Mark Brown
 <broonie@...nel.org>, Tudor Ambarus <tudor.ambarus@...aro.org>, pratyush
 <pratyush@...nel.org>, Vignesh Raghavendra <vigneshr@...com>,
 sbinding@...nsource.cirrus.com, Lee Jones <lee@...nel.org>, james schulman
 <james.schulman@...rus.com>, david rhodes <david.rhodes@...rus.com>,
 rf@...nsource.cirrus.com, Jaroslav Kysela <perex@...ex.cz>, tiwai@...e.com,
 linux-spi <linux-spi@...r.kernel.org>, linux-kernel
 <linux-kernel@...r.kernel.org>, Michael Walle <michael@...le.cc>, linux-mtd
 <linux-mtd@...ts.infradead.org>, Nicolas Ferre
 <nicolas.ferre@...rochip.com>, Alexandre Belloni
 <alexandre.belloni@...tlin.com>, Claudiu Beznea <claudiu.beznea@...on.dev>,
 michal simek <michal.simek@....com>, linux-arm-kernel
 <linux-arm-kernel@...ts.infradead.org>, alsa-devel
 <alsa-devel@...a-project.org>, patches@...nsource.cirrus.com,
 linux-sound@...r.kernel.org, git@....com, amitrkcian2002@...il.com
Subject: Re: [PATCH v11 00/10] spi: Add support for stacked/parallel
 memories

Hi Richard,

richard@....at wrote on Mon, 18 Dec 2023 23:10:20 +0100 (CET):

> ----- Ursprüngliche Mail -----
> > Von: "Amit Kumar Mahapatra" <amit.kumar-mahapatra@....com>  
> 
> > This patch series updated the spi-nor, spi core and the AMD-Xilinx GQSPI
> > driver to add stacked and parallel memories support.  
> 
> I wish the series had a real cover letter which explains the big picture
> in more detail.
> 
> What I didn't really get so far, is it really necessary to support multiple
> chip selects within a single mtd?
> You changes introduce hard to maintain changes into the spi-nor/mtd core code
> which alert me.
> Why can't we have one mtd for each cs and, if needed, combine them later?
> We have drivers such as mtdconcat for reasons.

The Xilinx SPI controller is a bit convoluted, there are two ways to
address the bits in a memory:
* Either your extend the memory range with the second chip "on
  top" of the first (which would typically be a mtd-concat use case)
* Or you use the two chips in parallel and you store the even bits
  in one device (let's say cs0) and the odd bits in the other (cs1).
  Extending mtd-concat for this might be another solution, I don't know
  how feasible it would be.

Maybe these bindings will help understanding the logic:
e2edd1b64f1c ("spi: dt-bindings: Describe stacked/parallel memories modes")
eba5368503b4 ("spi: dt-bindings: Add an example with two stacked flashes")

However I agree the changes will likely be hard to maintain given the
complexity brought with such a different controller.

Thanks,
Miquèl