linux-kernel - Re: [PATCH 0/3] spi-nor: Add Octal SPI support

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Date:   Fri, 12 Oct 2018 10:52:42 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Vignesh R <vigneshr@...com>
Cc:     Marek Vasut <marek.vasut@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Yogesh Gaur <yogeshnarayan.gaur@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Linux ARM Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>
Subject: Re: [PATCH 0/3] spi-nor: Add Octal SPI support

Hi Vignesh,

On Mon, 8 Oct 2018 21:06:02 +0530
Vignesh R <vigneshr@...com> wrote:

> Hi Boris,
> 
> Sorry I missed this mail.
> 
> On Thursday 04 October 2018 04:47 PM, Boris Brezillon wrote:
> > On Thu, 4 Oct 2018 16:05:36 +0530
> > Vignesh R <vigneshr@...com> wrote:
> >   
> >>>>
> >>>>  .../devicetree/bindings/mtd/cadence-quadspi.txt       |  1 +
> >>>>  drivers/mtd/spi-nor/cadence-quadspi.c                 |  9 +++++++++    
> >>>
> >>> On a slightly different topic, do you plan to convert the Cadence
> >>> driver to spi-mem? And if you don't, is it because you don't have time
> >>> or because some features are missing in spi-mem (I remember you
> >>> mentioned a few things back when you were reviewing the spi-mem series)?
> >>>     
> >>
> >> I do not have plans to convert cadence QSPI driver to spi-mem yet,
> >> mainly due to lack of time. Also, not sure if original author Marek and
> >> other altera people are okay with that.
> >>
> >> I see couple of issues in the way of conversion:
> >> 1. I would wait to know what direction would direct mapping APIs[1] go
> >> before starting spi-mem conversion for Cadence QSPI driver. Else, we
> >> have may to re write again if direct mapping APIs are merged.  
> > 
> > I'd suggest reviewing the proposal I posted so that you can influence
> > the design of this new API ;-).
> >   
> 
> I did take a look and proposal seems fine. Will try to prototype and
> test cadence QSPI driver with these. Thanks for the patches!

That's great news! Let me know how it goes, and don't hesitate to ask
if you have any questions.

> 
> 
> >> 2. New Cadence OSPI IP has an integrated PHY to support high throughput
> >> OSPI flashes operating up 200MHz in Octal DDR mode. In order to work
> >> with such flashes, PHY DLLs need to be calibrated. Highly simplified
> >> calibration sequence is as below(See [2] for actual sequence):
> >> -Read flash ID at low speed and store it.
> >> -Enable PHY and set DLLs to a defined initial value
> >> -Increment RX DLL value
> >> -Read flash ID and check for correctness of data read
> >> -repeat above two steps until a band of passing values is obtained for
> >> RX DLL where flash ID is correctly read.
> >> -DLL needs to set to middle of the passing band.  
> > 
> > Is the Read ID operation hardcoded or do you just use it as a way to
> > trigger predictable transfers on the IO bus?
> >   
> 
> Just a way to trigger predictable data reads.

Good.


> 
> >>
> >> I am trying to figure out how to fit this into the spi-mem framework as
> >> controller would to need to store READ ID opcode and expected JEDEC ID
> >> before starting calibration sequence.  
> > 
> > I think this should be split in 2:
> > 
> > - the SPI NOR framework passing the operation to use to do the
> >   calibration (here a READ ID)
> > - the SPI controller framework replaying the same operation with
> >   different DLL configs until it finds the best match
> > 
> > So, it would basically be added as a new hook:
> > 
> > 	int (*calibrate)(struct spi_mem *mem,
> > 			 const struct spi_mem_op *tmpl);
> > 
> > and a new function provided by the spi-mem API
> > 
> > int spi_mem_calibrate(struct spi_mem *mem,
> > 		      const struct spi_mem_op *tmpl);
> > 
> > and calibration outcome would be somehow attached to the spi_mem
> > object.
> > 
> > This way we stay memory agnostic but still provide the necessary blocks
> > at the spi-mem level to do such callibrations.
> > 
> > Would that work?
> >   
> 
> That would work and hopefully is not intrusive to spi-mem framework.
> 

Okay. Don't hesitate to post a proposal along those lines and I'll try
to review it.

Thanks,

Boris