Am 2021-04-29 20:41, schrieb Pratyush Yadav:
On 29/04/21 06:23PM, Michael Walle wrote:
I've had a look at the LS1028A FlexSPI calibration feature. The
reference manual is very sparse on details, though. What you need to
do there is to program a special read command sequence (the whole
controller is made of these lookup table entries, where you can
have a short sequence of operations for read/write/program and so
on). Therefore, for data learning you'll take the read operation
and insert a LEARN op in between and read a specific data pattern.
Then the hardware will automatically figure out the correct sample
phase for the read data pins.
Unfortunately, it does not mention how often you have to do it. It
might be the case that is has to be calibrated more than once.
I haven't read the datasheet, I wonder how long this calibration takes.
If it is too long then the overhead might not even be worth the extra
read throughput. Especially when using a file system on top which
generally don't do very large reads in one go.
I was just thinking of compensating a possible temperature drift. You
wouldn't have to do it on every read.
There is a second mode, where it is actually done on every read. But
that will be used where the flash supports a read preamble, where
dummy bytes after the read opcode are replaced by a calibration pattern.
If the pattern has the same length as the dummy bytes there is no
penalty.
IIRC the controller just supports a pattern of max 32 bits.
Oh I forgot to mention, this doesn't need to be repeated. I guess
the hardware already captures all possible phases (there are only
16) and compares each one with a predefined pattern.
Anyway, when the do_calibration() is called the controller can save the
calibration op and use it later as needed. It knows when an exec_op()
will result in a read since it has access to the whole op.
I'm just mentioning this so it won't be lost. If needed, it can
be added later.
Am 2021-03-24 09:08, schrieb Pratyush Yadav:
> On 24/03/21 12:07AM, Michael Walle wrote:
> > Am 2021-03-11 20:12, schrieb Pratyush Yadav:
> > > Some controllers like the Cadence OSPI controller need to perform a
> > > calibration sequence to operate at high clock speeds. This calibration
> > > should happen after the flash is fully initialized otherwise the
> > > calibration might happen in a different SPI mode from the one the flash
> > > is finally set to. Add a hook that can be used to tell the controller
> > > when the flash is ready for calibration. Whether calibration is needed
> > > depends on the controller.
> > >
> > > Signed-off-by: Pratyush Yadav <p.yadav@xxxxxx>
> > > ---
> > > drivers/spi/spi-mem.c | 12 ++++++++++++
> > > include/linux/spi/spi-mem.h | 8 ++++++++
> > > 2 files changed, 20 insertions(+)
> > >
> > > diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
> > > index dc713b0c3c4d..e2f05ad3f4dc 100644
> > > --- a/drivers/spi/spi-mem.c
> > > +++ b/drivers/spi/spi-mem.c
> > > @@ -464,6 +464,18 @@ int spi_mem_adjust_op_size(struct spi_mem *mem,
> > > struct spi_mem_op *op)
> > > }
> > > EXPORT_SYMBOL_GPL(spi_mem_adjust_op_size);
> > >
> > > +int spi_mem_do_calibration(struct spi_mem *mem, struct spi_mem_op *op)
> > > +{
> > > + struct spi_controller *ctlr = mem->spi->controller;
> > > +
> > > + if (!ctlr->mem_ops || !ctlr->mem_ops->do_calibration)
> > > + return -EOPNOTSUPP;
> > > +
> > > + ctlr->mem_ops->do_calibration(mem, op);
> >
> > Can't a calibration fail?
>
> It can. If it does, the controller falls back to lower speed transfers.
> There is not much the upper layer can do about this. That's why it is
> not informed whether it succeeded or not.
Ok, if needed, that should be an easy change.
op is there to decide if we need a calibration at all, correct?
Yes. It can also be used to choose which calibration algorithm to use.
For example on the Cadence controller, there are different algorithms
for 8S and 8D operations.
What if there are different factors, like frequency? For example
on the LS1028A its just a matter of the SCK frequency. It seems
that this parameter is tailored to the OPHY.
As of now there is no way in SPI MEM to tell the controller the
expected
speed of the operation. AFAIK most controllers get the speed via device
tree. So in the current case, the controller already knows the speed it
should run at, and can decide if calibration is needed or not.
But if operation speed is eventually added to SPI MEM, I would assume
it
would be part of struct spi_mem_op. The op passed in would have this
information filled, and the controller can use that information to
decide if it needs to perform the calibration or not.
I am all for making this API flexible, but with very few controllers
supporting this feature in the wild, it is difficult to predict all the
information that might be needed. In the current state, I think the API
provides a fair bit of information to the controller about how a read
operation would look like.
Sure, and its also quite hard to review without any other hardware
which supports that ;) I was thinking about letting the spi driver
call into spi-mem to retrieve the information it needs instead of
having that second argument. Anyway, if we need any changes in the
future this isn't set in stone.
-michael
