Re: [PATCH 7/8] mtd: spi-nor: factor out write_enable() for erase commands

[Brian Norris <computersforpeace@gmail.com>]

On Thu, Sep 11, 2014 at 12:12:37AM +0800, Huang Shijie wrote:
> On Wed, Sep 10, 2014 at 12:47:08AM -0700, Brian Norris wrote:
> > On Wed, Sep 10, 2014 at 11:20:21PM +0800, Huang Shijie wrote:
> > > On Wed, Sep 10, 2014 at 12:05:37AM -0700, Brian Norris wrote:
> > > > On Tue, Aug 12, 2014 at 08:59:12AM +0800, Huang Shijie wrote:
> > > you miss a write_enable for each sector's erase. 
> > 
> > But is that necessary? I thought 'write-enabled' was retained across
> > operations, so why would you have to perform it before each sector's
> > erase?
> The legacy code did so.
> 
> > 
> > Or do you have a flash datasheet which says you must send WREN before
> > each sector erase?
> See the belowing from Spansion Nor S25fl129:
> 
> "
> The Sector Erase (SE) command sets all bits at all addresses within a
> specified sector to a logic 1. A WREN
> command is required prior to writing the SE command.
> "
> 
> It does not tell we send a WREN for each sector erase, but i am not sure if we can remove it.

OK, well I guess I can rework this to retain the original behavior. That
was in fact, an oversight (thanks for catching), although I'm not
convinced it's actually necessary.

(Edit: I think you may be right that WREN is necessary before every
erased command. From a Micron N25Q256 datasheet:

  The write enable latch bit must be set before every PROGRAM, ERASE,
  WRITE, ENTER 4-BYTE ADDRESS MODE, and EXIT 4-BYTE ADDRESS MODE
  command.
  
But I don't recall seeing any ill effects last time I tested this on my
Micron SPI flash, so I'm still mildly confused.)

Brian