Re: [PATCH v3 1/3] mtd: spi-nor: macronix: add support for Macronix octal dtr operation

[Pratyush Yadav <p.yadav@ti.com>]

On 04/05/21 01:31PM, zhengxunli@mxic.com.tw wrote:
> Hi Pratyush,
> 
> Thanks for your comment on this patch.
> 
> "Pratyush Yadav" <p.yadav@ti.com> wrote on 2021/04/27 上午 10:36:06:
> 
[...]
> > > +   if (!enable)
> > > +      spi_nor_spimem_setup_op(nor, &op, SNOR_PROTO_8_8_8_DTR);
> > 
> > When disabling, the op would be in 8D-8D-8D mode so having a data length 
> 
> > of 1 would be invalid. This is currently the case even in the patches 
> > that I sent for Micron and Cypress.
> > 
> > I am not sure what the correct fix for this is though. One option is to 
> > send the same byte twice, but I remember that on the Cypress flash the 
> > second byte over-writes the register at the next address. I'm not sure 
> > how Macronix flashes handle the second byte. Can you check what the 
> > behavior for your flash is when you write 2 bytes to the register?
> 
> I checked the behavior of Macronix and the second byte will overwrites the 
> register.

Yes, I see the same behaviour on Micron and Cypress flashes. Can your 
controller send a 1-byte write in 8D mode? I am curious if this is 
possible and how flashes react to it.

My theory is that even if you ask the controller to send 1 byte in 8D 
mode, it won't deassert the CS till the end of the cycle. This would 
result the flash in taking the default value of the lines as the second 
byte.

> Do we need to send the same bytes to resolve this error?

I think this is a design oversight by flash manufacturers. Having two 
registers at consecutive addresses is problematic in 8D-8D-8D mode. The 
only solution I see is to read the register at the next address, and set 
its value as the second byte in the transaction. This way its value will 
stay the same at the end of the transaction.

PS: If possible, please try to relay this issue to your hardware design 
team. Hopefully they can come up with a clever solution for future 
devices and make our lives easier ;-)

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.