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

[Vignesh Raghavendra <vigneshr@ti.com>]

On 12/05/20 11:46 am, Tudor.Ambarus@microchip.com wrote:
> 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:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the
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>>
>> On Mon, 11 May 2020 09:00:35 +0000
>>
>> <Tudor.Ambarus@microchip.com> 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.
>

Are you asking for dedicated SW controllable reset line or just an
indication from DT that OSPI reset line is connected to board level
soft/hard reset lines?

Mandating SW controllable RESET line is bit of a stretch IMO... Board
design may not allow wasting dedicated pin due to lack of GPIOs perhaps..

For eg.: TI EVM has OSPI reset line connected to board level reset out.
This ensures any soft/warm/hard CPU reset will trigger OSPI Flash reset,
but there is no SW control that allows OSPI flash alone to be reset.
Isn't such a reset mechanism sufficient?

Regards
Vignesh