[PATCH v3 15/17] mtd: spi-nor-core: Perform a Soft Reset on boot

[Pratyush Yadav]

On 13/05/20 03:26PM, Jagan Teki wrote:
> On Wed, May 13, 2020 at 2:24 PM Pratyush Yadav <p.yadav at ti.com> wrote:
> >
> > On 13/05/20 12:17PM, Jagan Teki wrote:
> > > On Mon, Mar 30, 2020 at 9:16 PM Pratyush Yadav <p.yadav at ti.com> wrote:
> > > >
> > > > When the flash is handed to us in a stateful mode like 8D-8D-8D, it is
> > > > difficult to detect the mode the flash is in. One option is to read SFDP
> > > > in all modes and see which one gives the correct "SFDP" signature, but
> > > > not all flashes support SFDP in 8D-8D-8D mode.
> > > >
> > > > Further, even if you detect the mode of the flash via SFDP, you still
> > > > have the problem of actually reading the ID. The Read ID command is not
> > > > standardized across flash vendors. Flashes can have different dummy
> > > > cycles needed for reading the ID. Some flashes even expect a 4-byte
> > > > dummy address with the Read ID command. All this information cannot be
> > > > obtained from the SFDP table.
> > > >
> > > > So, perform a Software Reset sequence before reading the ID and
> > > > initializing the flash. A Soft Reset will bring back the flash in its
> > > > default protocol mode assuming no non-volatile configuration was set.
> > > > This will let us detect the flash even if ROM hands it to us in Octal
> > > > DTR mode.
> > > >
> > > > To accommodate cases where there is more than one flash on a board, and
> > > > only one of them needs a soft reset, failure to reset is not made fatal,
> > > > and we still try to read ID if possible.
> > > >
> > > > Signed-off-by: Pratyush Yadav <p.yadav at ti.com>
> > > > ---
> > > >  drivers/mtd/spi/Kconfig        | 11 +++++++++++
> > > >  drivers/mtd/spi/spi-nor-core.c | 27 +++++++++++++++++++++++++++
> > > >  2 files changed, 38 insertions(+)
> > > >
> > > > diff --git a/drivers/mtd/spi/spi-nor-core.c 
> > > > b/drivers/mtd/spi/spi-nor-core.c
> > > > index 52b3775035..3af2c6afa9 100644
> > > > --- a/drivers/mtd/spi/spi-nor-core.c
> > > > +++ b/drivers/mtd/spi/spi-nor-core.c
> > > > @@ -2922,6 +2922,33 @@ int spi_nor_scan(struct spi_nor *nor)
> > > >
> > > >         nor->setup = spi_nor_default_setup;
> > > >
> > > > +#if defined(CONFIG_SPI_FLASH_SOFT_RESET)
> > > > +       /*
> > > > +        * When the flash is handed to us in a stateful mode like 8D-8D-8D, it
> > > > +        * is difficult to detect the mode the flash is in. One option is to
> > > > +        * read SFDP in all modes and see which one gives the correct "SFDP"
> > > > +        * signature, but not all flashes support SFDP in 8D-8D-8D mode.
> > > > +        *
> > > > +        * Further, even if you detect the mode of the flash via SFDP, you
> > > > +        * still have the problem of actually reading the ID. The Read ID
> > > > +        * command is not standardized across flash vendors. Flashes can have
> > > > +        * different dummy cycles needed for reading the ID. Some flashes even
> > > > +        * expect a 4-byte dummy address with the Read ID command. All this
> > > > +        * information cannot be obtained from the SFDP table.
> > > > +        *
> > > > +        * So, perform a Software Reset sequence before reading the ID and
> > > > +        * initializing the flash. A Soft Reset will bring back the flash in
> > > > +        * its default protocol mode assuming no non-volatile configuration was
> > > > +        * set. This will let us detect the flash even if ROM hands it to us in
> > > > +        * Octal DTR mode.
> > > > +        *
> > > > +        * To accommodate cases where there is more than one flash on a board,
> > > > +        * and only one of them needs a soft reset, failure to reset is not
> > > > +        * made fatal, and we still try to read ID if possible.
> > > > +        */
> > > > +       spi_nor_soft_reset(nor);
> > > > +#endif /* CONFIG_SPI_FLASH_SOFT_RESET */
> > >
> > > Does it specific to a particular flash chip? I believe add flash chip
> > > fixups would make more sense instead of having it like this.
> >
> > To run a flash fixup, we need to know which flash it is. But if ROM
> > hands us the flash in 8D mode, we can't reliably detect the ID of the
> > flash since the Read ID command differs from one flash to another. For
> > example, on the Cypress S28 flash family, the Read ID command in 8D mode
> > expects 4 dummy address bytes and 3 dummy cycles. But on the Micron
> > MT35XU flashes, Read ID in 8D needs 0 address bytes and 8 dummy cycles.
> >
> > This works around the issue by issuing a soft reset command so even if
> > the flash is in 8D mode, it gets switched back to 1S mode so we can
> > detect it.
> 
> I got this from the commit message, thanks. Point I'm trying to
> understand here about can't we detect soft reset of associate flash
> via some bits in SFDP? if yes then we can dynamically process the soft
> reset on associated flash like quad enable does?

If the device supports the Read SFDP command in 8D mode (it is optional 
as per JESD251), then we can detect soft reset support via BFPT DWORD 16 
bits 13:8.

If we go that path, we would need to take the below steps before we can 
proceed with the probe:

- Issue Read SFDP in 8D mode and see if we get the "SFDP" signature 
  correctly. If we don't, exit.

- Read BFPT in 8D mode, and see if Soft Reset is supported.

- If it is, issue a soft reset and go on as usual. If it isn't, error 
  out, and the probe fails.

The problem is, I'm not sure if controllers will be okay doing this on a 
flash that is in 1S mode. Say the controller expects the DQS line to 
toggle when the flash is sending data. Now, we tell the controller we 
will read 16 bytes (SFDP header), but since the flash is in 1S mode, it 
sees some other command and might not send out any data at all. In this 
case, will the controller hang, waiting forever for the flash to toggle 
DQS?

FWIW, the controller on our board (Cadence QSPI) doesn't hang. But I'm 
not sure how other controllers would react. That's why I went with the 
more minimal option. The soft reset sequence doesn't expect to receive 
any data. It only sends it.

If you think no contoller will trip over this, then I don't mind 
discovering Soft Reset capability via BFPT.

-- 
Regards,
Pratyush Yadav
Texas Instruments India