linux-kernel - Re: [Linux-stm32] [ v3 2/3] mtd: rawnand: stm32_fmc2: add STM32 FMC2 NAND flash controller driver

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Date:   Fri, 7 Dec 2018 13:47:24 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Benjamin Gaignard <benjamin.gaignard@...com>
Cc:     christophe.kerello@...com, Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Richard Weinberger <richard@....at>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Vasut <marek.vasut@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        linux-mtd@...ts.infradead.org,
        Miquèl Raynal <miquel.raynal@...tlin.com>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [Linux-stm32] [ v3 2/3] mtd: rawnand: stm32_fmc2: add STM32 FMC2
 NAND flash controller driver

On Fri, Dec 7, 2018 at 11:22 AM Benjamin GAIGNARD
<benjamin.gaignard@...com> wrote:
> On 12/7/18 10:06 AM, Linus Walleij wrote:
> > Hi Christophe,
> >
> > On Thu, Nov 29, 2018 at 5:42 PM Christophe Kerello
> > <christophe.kerello@...com> wrote:
> >
> >> +/* FMC2 Controller Registers */
> >> +#define FMC2_BCR1                      0x0
> >> +#define FMC2_PCR                       0x80
> > (...)
> >> +/* Register: FMC2_BCR1 */
> >> +#define FMC2_BCR1_FMC2EN               BIT(31)
> > Well this looks like an especially clever register map and a specific choice
> > of bit 31 in the fist register to activate FMC2. Registers 0x04 thru
> > 0x7c are completely unused save for one bit.
> >
> > It's almost like this is the good old FSMC integrated in parallel with FMC2,
> > so that if you don't set bit 31, this becomes something that can be used
> > with drivers/mtd/nand/raw/fsmc_nand.c, and FMC2 mode is activated
> > by setting this bit, activating all the new registers.
> >
> > It wouldn't surprise me given how HW designers like to work.
> >
> > Is this the case?
>
> No, it is the same story than for stmfx driver, it looks to be the same
> from registers
>
> point of view but internal hardware block design is completely different.

I'm not saying FMC2 is the same, just that it seems they have
duct-taped both IP-blocks (FSMC and FMC2) together at some point.

It just looks so extremely odd to leave all registers below 0x80 unused
except for 0x0 where a single bit is used.

Maybe there is no FSMC there, but it sure looks like the hardware
engineers planned for old+new FSM[C] block coexistence in the
same address space.

Yours,
Linus Walleij