Re: [PATCH v4 00/16] mtd: spi-nor: add xSPI Octal DTR support

[<Tudor.Ambarus@xxxxxxxxxxxxx>]

Hi, Boris, Pratyush,

I stripped case 2/, we'll not treat it for now.

On Monday, May 11, 2020 12:27:12 PM EEST Boris Brezillon wrote:
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> On Mon, 11 May 2020 09:00:35 +0000
> 
> <Tudor.Ambarus@xxxxxxxxxxxxx> wrote:
> > Hi, Pratyush, Boris,
> > 
> > On Friday, April 24, 2020 9:43:54 PM EEST Pratyush Yadav wrote:
> > > This series adds support for octal DTR flashes in the spi-nor framework,
> > 
> > I'm still learning about this, but I can give you my 2 cents as of now, to
> > open the discussion. Enabling 2-2-2, 4-4-4, and 8-8-8 modes is dangerous
> > because the flash may not recover from unexpected resets. Entering one of
> > these modes can be:
> > 1/ volatile selectable, the device return to the 1-1-1 protocol after the
> > next power-on. I guess this is conditioned by the optional RESET pin, but
> > I'll have to check. Also the flash can return to the 1-1-1 mode using the
> > software reset or through writing to its Configuration Register, without
> > power-on or power- off.
> 
> My understanding is that there's no standard software reset procedure
> that guarantees no conflict with existing 1S commands, so even the
> software reset approach doesn't work here.
> 

The software reset procedure can't protect you from unexpected resets, but the 
hardware with its optional reset pin can. Pratyush to confirm.

cut

> 
> > Not recovering from unexpected resets is unacceptable. One should always
> > prefer option 1/ and condition the entering in 2-2-2, 4-4-4 and 8-8-8 with
> > the presence of the optional RESET pin.
> 
> Totally agree with you on that one, but we know what happens in
> practice...

What I proposed is to condition the entering in the state-full modes with the 
presence of the optional RESET pin. We would introduce an optional device tree 
property for the RESET pin. If hardware doesn't implement the optional RESET# 
signal, then we will not enter in the state-full modes.

Cheers,
ta