Re: [PATCH 1/3] mtd: m25p80: utilize dedicated 4-byte addressing commands

[Marek Vasut <marex@denx.de>]

Dear Brian Norris,

> On Sun, Mar 10, 2013 at 4:18 AM, Marek Vasut <marex@denx.de> wrote:
> >> Traditionally, the command set used by SPI flash only supported a 3-byte
> >> address. However, large SPI flash (>= 32MB, or 256Mbit) require 4 bytes
> >> to address the entire flash. Most manufacturers have supplied a mode
> >> switch (via a "bank register writer", or a "enable 4-byte mode"
> >> command), which tells the flash to expect 4 address cycles from now on,
> >> instead of 3. This mode remains until power is cut, the reset line is
> >> triggered (on packages where present), or a command is sent to reset the
> >> flash or to reset the 3-byte addressing mode.
> >> 
> >> As an alternative, some flash manufacturers have developed a new command
> >> set that accept a full 4-byte address. They can be used orthogonally to
> >> any of the modes; that is, they can be used when the flash is in either
> >> 3-byte or 4-byte address mode.
> >> 
> >> Now, there are a number of reasons why the "stateful" 4-byte address
> >> mode switch may not be acceptable. For instance, some SoC's perform a
> >> dumb boot sequence in which they only send 3-byte read commands to the
> >> flash. However, if an unexpected reset occurs, the flash chip cannot be
> >> guaranteed to return to its 3-byte mode. Thus, the SoC controller and
> >> flash will not understand each other. (One might consider hooking up the
> >> aforementioned reset pin to the system reset line so that any system
> >> reset will reset the flash to 3-byte mode, but some packages do not
> >> provide this pin. And in some other packages, one must choose between
> >> having a reset pin and having enough pins for 4-output QSPI support.
> >> It is an error prone process choosing a flash that will support a
> >> hardware reset pin!)
> >> 
> >> This patch provides support for the new stateless command set, so that
> >> we can avoid the problems that come with a stateful addressing mode
> >> change. The flash can be left in "3-byte mode" while still accessing the
> >> entire flash.
> >> 
> >> Note that Spansion supports this command set on all its large flash
> >> (e.g, S25FL512S), and Macronix has begun supporting this command set on
> >> some new flash (e.g., MX25L25635F). For the moment, I don't know how to
> >> differentiate the Macronix that don't support this command set (e.g.,
> >> MX25L25635E) from those that do, so this patch only supports Spansion.
> >> 
> >> Signed-off-by: Brian Norris <computersforpeace@gmail.com>
> > 
> > Looks reasonable
> > 
> > Acked-by: Marek Vasut <marex@denx.de>
> > 
> > What system/CPU do you observe these issue on just out of curiosity?
> 
> A Broadcom 74xx series SoC.

Oh ok. The bootrom programmers really do a poor job :(

Best regards,
Marek Vasut