[RFC 03/10] mtd: spi-nor: Configure read latency for read commands

[Prabhakar Kushwaha]

Hi Cyrille,

> -----Original Message-----
> From: Cyrille Pitchen [mailto:cyrille.pitchen at wedev4u.fr]
> Sent: Wednesday, December 06, 2017 4:16 PM
> To: Prabhakar Kushwaha <prabhakar.kushwaha at nxp.com>; linux-
> mtd at lists.infradead.org
> Cc: boris.brezillon at free-electrons.com; computersforpeace at gmail.com;
> dedekind1 at gmail.com
> Subject: Re: [RFC 03/10] mtd: spi-nor: Configure read latency for read commands
> 
> Hi Prabhakar,
> 
> Le 06/12/2017 à 09:15, Prabhakar Kushwaha a écrit :
> > All read commands have read latency (dummy cycle). It defines a
> > period between the end of address or mode and the beginning of
> > read data returning to the host. Flashes have default read latency.
> > This default read latency may not match with the selected latency.
> >
> 
> The read latency is either hard-coded (0 mode cycles and 8 wait-states)
> for Fast Read 1-1-z operations or read from the Basic Flash Parameter
> Table programmed as one mandatory table of the JEDEC JESD126B
> specification when SFDP tables are supported by the memory parts.
> 
> In all cases, those values are factory settings. I don't understand why
> there would be a mismatch between the default read latency and the
> selected latency if any boot-loader or flash programming tool has ever
> changed the memory settings.
> 
> Could you please explain in which case you found a mismatch ?
> 
If we only talk about 1-1-z than yes. The Default value is always "0/8". 

This mismatch start to come when we see 1-2-2 or 1-4-4 read protocols.
For eg. S25FL128S/S25FL256S default dummy latency is 00. Which is 4 for  1-2-2 or 1-4-4.

I saw your review comments of using SFDP for 1-2-2 and 1-4-4. 
Let me explore this path if we really require this patch..

Thanks for guidance!!

--pk