Re: [PATCH v2 03/10] mtd: spi-nor: add DDR quad read support

[Huang Shijie <shijie8@xxxxxxxxx>]

On Wed, Jul 30, 2014 at 12:45:00AM -0700, Brian Norris wrote:
> 
> BTW, does your quadspi controller unconditionally support DDR, or is
> there any dependency on board/clock configuration? I'm just curious
> whether you need a DT binding to describe DDR support, or if (as long as
> the flash supports it, and we get the dummy cycles right) you can always
> use DDR QUAD.
The fsl-quadspi controller can support the DDR quad mode unconditionally.
In the other word, this controller is _designed_ for the DDR quad mode.

the driver does not needs a DT binding for the DDR. thanks.


> 
> > Please correct me if I am wrong. :)
> > 
> > > >   [1] add SPI_NOR_DDR_QUAD read mode.
> > > > 
> > > >   [2] add DDR Quad read opcodes:
> > > >        SPINOR_OP_READ_1_4_4_D / SPINOR_OP_READ4_1_4_4_D
> > > > 
> > > >   [3] add set_ddr_quad_mode() to initialize for the DDR quad read.
> > > >       Currently it only works for Spansion NOR.
> > > > 
> > > >   [3] about the dummy cycles.
> > > >       We set the dummy with 8 for DDR quad read by default.
> > > 
> > > Why? That seems wrong. You need to know for sure how many cycles should
> > > be used, not just guess a default.
> > 
> > Do you mean that if people do not set the DT node for dummy, we should
> > return an -EINVAL immediately?
> 
> Possibly. But I actually don't think this belongs in DT at all. See
> below.
> 
> > > >       The m25p80.c can not support the DDR quad read, but the SPI NOR controller
> > > >       can set the dummy value in its child DT node, and the SPI NOR framework
> > > >       can parse it out.
> > > 
> > > Why does the dummy value belong in device tree? I think this can be
> > > handled in software.

We can not handle it in the software way.
Why? since there are too many modes,and each mode needs different dummy
cycles. Even the same chips, different versions may uses different dummy
cycles in the same mode. ..

You can refer to the Spansion's  S25fs128s.

If you want to put these dummy cycles in the device ID, it will make you
crazy in the end. :)
> 
> Can you answer me this question?
> 
> > > You might, however, want a few other hardware
> > > description parameters in device tree to help you.
> > > 
> > > So I think spi-nor.c needs to know a few things:
> > > 
> > >  1. Does the hardware/driver support DDR clocking?
> > >  2. What granularity of dummy cycles are supported? So m25p80.c needs to
> > >     communicate that it only supports dummy cycles of multiples of 8,
> > >     and fsl-quadspi supports single clock cycles.
> > 
> > I think you can send patches for these features. I does not clear about:
> >   for what does the spi-nor needs to know the above things.
> 
> To properly abstract features between a driver and the spi-nor
> "library." For example, we need to make sure we don't try to use a
> command mode with 7 dummy cycles on m25p80.c; right now, each driver has
> to (implicitly) know the details of dummy cycles when selecting a 'mode'
> parameter for spi_nor_scan(). spi-nor.c should be selecting this for us,
> not forcing the driver to make the selection.
> 
> > > And spi-nor.c should be able to do the following:
> > > 
> > >  3. Set how many dummy cycles should be used.
> > where can we get the number of the cycles? 
> 
> This should be a property of the flash device, just like everything else
> we detect in spi-nor.c.
This is indeed a probably of the flash device.
and this is why I add a new DT binding file for the flash.

> 
> > >  4. Write to the configuration register, to choose a Latency Code
> > >     according to what the flash supports. I see modes that support 3, 6,
> > >     7, or 8. We'd probably just go for the fastest mode, which requires
> > >     8, right?
> > not right.
> > 
> > The DDR mode can not work if we set a wrong dummy cycles for the flash.
> > 
> > for some chips, the fastest mode may need 6 cycles, not 8. 
> 
> OK, but can spi-nor.c detect this instead of putting this in DT? e.g.,
> associate this with the device ID?

see the my comment above.
> 
> Or even better, we can support CFI detection for SPI NOR flash. I notice
> the datasheet for your Spansion flash [1] has an extensive set of CFI
> parameters defined, including the dummy cycle information. I think it
> might be more sustainable to try to support CFI [2] and SFDP [3] for
> detecting new flash, rather than adding new table entries ad-hoc.
I think different chip vendors may have different format to store the
info, just like the read-retry for nand chips.

do you want to add the code only available for Spansion?
What's more the code will be very long, i think.

Add a new DT binding file for the flash maybe is the simplest way.


> 
> > > So far, none of this seems to require a DT binding, unless there's
> > > something I'm missing about your fsl-quadspi controller.
> > > 
> > > > Test this patch for Spansion s25fl128s NOR flash.
> > > > 
> > > > Signed-off-by: Huang Shijie <b32955@xxxxxxxxxxxxx>
> > > > ---
> > > > +	/* DDR Quad/Quad/Dual-read mode takes precedence over fast/normal */
> > > > +	if (mode == SPI_NOR_DDR_QUAD && info->flags & SPI_NOR_DDR_QUAD_READ) {
> > > 
> > > Hmm, I think I should probably take another look at the design of
> > > spi-nor.c... Why does spi_nor_scan() take a single 'mode' argument? The
> > > driver should be communicating which (multiple) modes it supports, not
> > > selecting a single mode. spi-nor.c is the only one which knows what the
> > > *flash* supports, so it should be combining knowledge from the
> > > controller driver with its own knowledge of the flash.
> > 
> > It is okay for me to add multiples modes for the spi_nor_scan().
> > I added the single mode for spi_nor_scan is because that the fsl-quadspi
> > does not want to support the low speed modes. (Of course, the fsl-quadspi
> > controller does support the low speed modes.)
> 
> Right, fsl-quadspi only supports one mode. But m25p80.c is more
> flexible, and I think we might have broken some of it in the process
> (e.g., if the SPI controller supports single/dual/quad but the flash
> only supports single, then we might fail to probe).
> 
> I can take a look at this problem if you don't. I'd just recommend that
> we might take a step back on a few of these issues before blazing ahead
> with something irrevocable, like a DT binding.

I am glad you can spend some time on this issue. 


Could you please also read this patch ?:

http://lists.infradead.org/pipermail/linux-mtd/2014-May/054108.html

thanks
Huang Shijie
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