linux-kernel - Re: [PATCH 2/4] mtd: rawnand: fsmc: Force to use 8 bits access when expected

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Message-ID: <20211117144715.731a9856@bootlin.com>
Date:   Wed, 17 Nov 2021 14:47:15 +0100
From:   Herve Codina <herve.codina@...tlin.com>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH 2/4] mtd: rawnand: fsmc: Force to use 8 bits access when
 expected

Hi,

On Fri, 12 Nov 2021 16:38:59 +0100
Miquel Raynal <miquel.raynal@...tlin.com> wrote:

> Hi Hervé,
> 
> herve.codina@...tlin.com wrote on Fri, 12 Nov 2021 15:38:53 +0100:
> 
> > Some data transfers are expected on 8 bits by the nand core.
> > The fsmc driver did not check this constraint and these transfers
> > can be done on 32 bits depending on buffer alignment and transfers
> > data size.
> > 
> > This patch ensures that these transfers will be 8bits transfers in
> > all cases.  
> 
> I believe there is a misunderstanding here: NAND buses -between the
> NAND controller and the NAND chip- are either 8-bit or 16-bit wide and
> the amount of bytes that you will retrieve per register read is not
> related to it.
> 
> When the controller supports 16-bit accesses, there are certain
> operations that must be performed using only the lowest 8 bits of the
> NAND bus, such as reading a status [1]. In this case, the controller
> must have a way to disable the 16-bit mode temporarily. See [2] and [3]
> for an example. Reading with readb() or readl() will IMHO not impact the
> amount of data lines used for the operation.
> 

Indeed, I misunderstood the force_8bit usage.
This patch is not needed and will be simply removed in v2 series.

Thanks,
Hervé