Re: [PATCH] spi: Replace `dummy.nbytes` with `dummy.ncycles`

[Chuanhong Guo]

Hi!

On Thu, Mar 9, 2023 at 10:04 PM Michael Walle <michael@xxxxxxxx> wrote:
>
> Am 2023-03-09 14:54, schrieb Tudor Ambarus:
> > On 09.03.2023 15:33, Michael Walle wrote:
> >>>>> The controllers that can talk in dummy ncycles don't need the
> >>>>> dummy.{buswidth, dtr} fields.
> >>>>>
> >>>>> The controllers that can't talk in dummy cycles, but only on a
> >>>>> "byte"
> >>>>> boundary need both buswidth and dtr fields. Assume a flash needs 32
> >>>>> dummy cycles for an op on 8D-8D-8D mode. If the controller does not
> >>>>> have
> >>>>> the buswidth and dtr info, it can't convert the dummy ncycles to
> >>>>> nbytes.
> >>>>> If he knows only that buswidth is 8, it will convert ncycles to 4
> >>>>> bytes.
> >>>>> If dtr is also specified it converts ncycles to 2 bytes.
> >>>>
> >>>> No they don't need it. Lets take your semper flash and assume it
> >>>> needs
> >>>> 12 latency cycles. SPI-NOR will set ncycles to 12 *regardless of the
> >>>> mode
> >>>> or dtr setting*. The controller then knows we need 12 clock cycles.
> >>>> It has
> >>>> then to figure out how that can be achieved. E.g. if it can only do
> >>>> the
> >>>> "old" byte programming and is in quad mode, good for it. It will
> >>>> send 6
> >>>> dummy bytes, which will result in 12 dummy clock cycles, because 1
> >>>> byte
> >>>> takes two clock cycles in quad SDR mode. If its in octal mode, send
> >>>> 12
> >>>> bytes. If its in dual mode, send 3 bytes. Obiously, it cannot be in
> >>>> single bit mode, because it cannot send 1.5 bytes..
> >>>>
> >>>
> >>> You miss the fact that you can have 1-1-4. What buswidth do you use
> >>> for dummy, the address buswidth or the data buswidth?
> >>
> >> Doesn't matter, does it? The driver is free to chose, 1, 4, or
> >> anything
> >> else. You don't sample any data during the dummy phase.
> >> To answer your question: single for instruction, single for address,
> >> whatever you choose for dummy as long as there are ncycles space
> >> between
> >> address and data, and quad for data.
> >
> > Huh? How does the controller chose, based on what?
>
> Based on its own capabilities. It can choose either way. In the end
> what matters is how many clock cycles there are between the address
> and data phase. And you only need to convey that information to the
> SPI controller - your new ncycles.

It does matter. Controller may be designed to actively drive
MOSI/WP/HOLD during single-spi dummy cycles and WP/HOLD
during dual-spi dummy cycles. If the pin modes between the
controller and device mismatched, worst case scenario
the chip may get fried.

-- 
Regards,
Chuanhong Guo