Re: [PATCH 1/1] mtd: spi-nor: add an alternative method to support memory >16MiB

[Brian Norris]

On Tue, Mar 22, 2016 at 12:27:32PM +0100, Cyrille Pitchen wrote:
> Le 21/03/2016 18:55, Brian Norris a écrit :
> > On Mon, Mar 21, 2016 at 06:16:32PM +0100, Cyrille Pitchen wrote:
> >> Le 21/03/2016 18:01, Brian Norris a écrit :
> >>> On Mon, Mar 21, 2016 at 11:33:49AM +0100, Cyrille Pitchen wrote:
> >>>> diff --git a/drivers/mtd/spi-nor/Kconfig b/drivers/mtd/spi-nor/Kconfig
> >>>> index d42c98e1f581..7fae36fc8526 100644
> >>>> --- a/drivers/mtd/spi-nor/Kconfig
> >>>> +++ b/drivers/mtd/spi-nor/Kconfig
> >>>> @@ -29,6 +29,18 @@ config MTD_SPI_NOR_USE_4K_SECTORS
> >>>>  	  Please note that some tools/drivers/filesystems may not work with
> >>>>  	  4096 B erase size (e.g. UBIFS requires 15 KiB as a minimum).
> >>>>  
> >>>> +config MTD_SPI_NOR_USE_4B_OPCODES
> >>>> +	bool "Use 4-byte address op codes"
> >>>> +	default n
> >>>> +	help
> >>>> +	  This is an alternative to the "Enter/Exit 4-byte mode" commands and
> >>>> +	  Base Address Register (BAR) updates to support flash size above 16MiB.
> >>>> +	  Using dedicated 4-byte address op codes for (Fast) Read, Page Program
> >>>> +	  and Sector Erase operations avoids changing the internal state of the
> >>>> +	  SPI NOR memory. Hence if a CPU reset occurs, early bootloaders can
> >>>> +	  still use regular 3-byte address op codes and read from the very first
> >>>> +	  16MiB of the flash memory.
> >>>> +
> >>>
> >>> Why does this need to be a Kconfig option? Can't it just as well be
> >>> supported by keeping entries in the ID table, to show which flash
> >>> support these opcodes and which don't? Kconfig is really a bad mechanism
> >>> for trying to configure your flash.
> >>
> >> Well, the only reason why I've chosen a Kconfig option is to be as safe as
> >> possible, if for some reason someone still wants to use the former
> >> implementation. Honestly I don't know why one would do so but I'm cautious
> >> so I also set "default n". This way no regression is introduced.
> > 
> > I think the main reason is that some manufacturers did not initially
> > support both methods. So we couldn't just say "all Micron" or "all
> > Macronix" should use these opcodes. Spansion was the only one to support
> > them consistently. See [1] for reference.
> > 
> > And actually, your Kconfig option is not "cautious", because you have no
> > guarantee that people will make intelligent choices here. So you're
> > making a Kconfig that if someone accidentally turns it on, their flash
> > might not work any more. That's much less safe than properly labelling
> > which flash supports which feature.
> > 
> >> If you think it's better to use a dedicated flag like SECT_4K or
> >> SPI_NOR_QUAD_READ in the spi_nor_ids[] table, I'm perfectly fine with it as
> >> well.
> > 
> > I think that would be better. (Really, an opt-out would be the least
> > work in the long-run I think, since IIRC there were only a few
> > early-generation flash that were both large enough to need 4 byte
> > addressing and didn't support the dedicated opcodes. But it'll be hard
> > to track those down now I think, so opt-in probably is most practical.)
> > 
> >> Just let me know your choice then I'll update my patch accordingly if needed.
> > 
> > Brian
> > 
> > [1]
> > commit 87c9511fba2bd069a35e1312587a29e112fc0cd6
> > Author: Brian Norris <computersforpeace@xxxxxxxxx>
> > Date:   Thu Apr 11 01:34:57 2013 -0700
> > 
> >     mtd: m25p80: utilize dedicated 4-byte addressing commands
> > 
> 
> Indeed, your commit message is very interesting since it points out some issue
> I already face: for my test I'm currently using a Macronix MX25L25673G
> (JEDEC ID C22019). This memory *does* support the dedicated 4byte address op
> codes.
> 
> Then if I look at the datasheet for the Macronix MX25L25635E, this older memory
> shares the exact same JEDEC ID, C22019 but *doesn't* support the 4byte address
> op codes.
> 
> None of these two memory types use ext id: the JEDEC ID is only 3byte long.
> Anyway, in QPI mode, the QPIID command (0xAF), which replaces the regular
> Read ID command (0x9f) only sends 3 bytes too.
> 
> The memory table inside the spi-nor framework is indexed by JEDEC ID (+ ext id)
> but we've just pointed out a case where it is not possible to create two
> different entries in this table, one for the 35E without the "4byte" flag, the
> other for 73G with the "4byte" flag.
> 
> So there is no mean for the software to discover at runtime what are the actual
> capabilities of the memory. Hence I think we should introduce a DT property
> which would clearly tell the spi-nor framework that we want to use the
> dedicated 4byte address op codes instead of entering the 4byte address mode
> (still the default behaviour since older memories only support this mode).

We don't absolutely need a new property. You could just support
"mxic,mx25l25673g" in the compatible property, and make sure that name
matching will override the ID matching. (Although, maybe this won't work
so well, since plenty of DT's erroneously have used "m25p80" even though
they are not "m25p80" flash...)

> So what about a new DT named "m25p,4byte-opcodes" so the naming is consistent
> with the existing "m25p,fast-read"?

(I never liked the fast-read property, FWIW. It wasn't necessary.)

If we're choosing new properties, let's not make them "m25p", since
that's neither a vendor prefix, nor does it reflect its driver use case.
(We would support it for non-m25p80-based drivers.)

"spi-nor," might be a better prefix.

> For DT guys, I just sum the initial issue up: entering the 4byte address mode
> as currently done changes the internal state of the SPI memory. If a SoC reset
> occurs, some early bootloaders not supporting this 4byte address mode expect
> to use only 3byte addresses for (Fast) Read operations => the boot is broken.
> Then using the dedicated 4byte address op codes fixes the issue as they don't
> change the internal memory state so early bootloader can still use 3byte
> address op codes.

Brian