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Date:   Fri, 12 Mar 2021 23:44:49 +0530
From:   Pratyush Yadav <p.yadav@ti.com>
To:     <Tudor.Ambarus@microchip.com>
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        <lokeshvutla@ti.com>
Subject: Re: [RFC PATCH 0/6] spi: Add OSPI PHY calibration support for
 spi-cadence-quadspi
Message-ID: <20210312181447.dlecnw2oed7jtxe7@ti.com>
References: <20210311191216.7363-1-p.yadav@ti.com>
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On 12/03/21 11:23AM, Tudor.Ambarus@microchip.com wrote:
> On 3/12/21 12:10 PM, Pratyush Yadav wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > 
> > On 12/03/21 09:09AM, Tudor.Ambarus@microchip.com wrote:
> >> On 3/11/21 9:12 PM, Pratyush Yadav wrote:
> >>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> >>>
> >>> Hi,
> >>>
> >>> This series adds support for OSPI PHY calibration on the Cadence OSPI
> >>> controller. This calibration procedure is needed to allow high clock
> >>> speeds in 8D-8D-8D mode. The procedure reads some pre-determined pattern
> >>> data from the flash and runs a sequence of test reads to find out the
> >>> optimal delays for high speed transfer. More details on the calibration
> >>> procedure in patch 5/6.
> >>
> >> Can the calibration sequence be avoided if the controller is informed
> >> about the frequency on which the flash operates?
> 
> s/frequency/maximum supported frequency by the flash

Again, the max frequency does not matter. If it is fast enough PHY 
calibration is needed to make sure lines are sampled at the correct 
time.

> 
> > 
> > Maybe I don't understand this correctly, but there should not be any
> > frequency on which the flash operates. The controller drives the SPI
> > clock so the frequency is decided by the controller. Sure, there is a
> > max supported frequency for the flash but the controller can run it
> > slower than that if it wishes. The flash has no say in that.
> > 
> > Anyway, the exact frequency at which the flash is running is not it is
> > looking for. More details below.
> 
> I thought about choosing at the controller side:
> min(max_frequency_controller, max_frequency_flash)
> 
> And there is also the need of changing the frequency on which an op
> runs, like the READ SFDP cmd, for which it is recommended to be run at
> 50 MHz, but maybe this is another topic, let's see.

Right. This is not directly related to the need for having the 
calibration.

Right now calibration is run only after the flash is fully initialized, 
so there should not be any SFDP commands from that point on. But if we 
do want to be conservative about it, a field can be added in spi_mem_op 
that mentions the maximum speed for the op so the controller can decide 
accordingly.

> 
> > 
> >>
> >> Can you add more details about the optimal delays? Are we talking about
> >> flash's AC characteristics? Is the calibration still needed if the upper
> >> layer informs the QSPI controller about the needed delays?
> > 
> > There is usually a delay from when the flash drives the data line (IOW,
> > puts a data bit on it) and when the signal reaches the controller. This
> > delay can vary by the flash, board, silicon characteristics,
> > temperature, etc.
> 
> I wonder whether the delay advertised by the flash matters the most, while
> all the other are negligible.

The delay advertised by the flash does matter. Specifically, the clock 
to data output delay. But the IO delay in the host (the delay from the 
pin to the internal FIFO) matters equally as much. Both are accounted 
for by the tuning.

> When I talk about delay, I'm thinking for example at the delay required
> between two consecutive transfers without removing the chip select, or about
> the minimum delay needed between the activation or the deactivation of the
> chip select. These are all described by the flash. Does your controller have
> such fields in its registers, to set such delays? If yes, is the calibration 
> sequence still needed if all the delays are set correctly?

The CS related delays are indeed accounted for by a register in the 
controller. But this is not the delay the calibration is concerned with. 
The delays the calibration is concerned with are the clock edge to data 
transition delay (TX delay), and the DS edge to data transition delay 
(RX delay). The two are totally unrelated.

> 
> When I hear about "board delays", I think about the impedance of the lines,
> which should correspond to the impedance of the Flash's IOs (which depends on
> the frequency on which the flash runs). A mechanism to choose the best
> frequency and impedance level can be added.

Board delays in this case are caused by the length of the wires/lines. 
Even if the lines are perfectly impedance matched to the flash's IOs, 
there will be a small time delay from when the data signal is launched 
by the flash and when it is received by the device. This causes a small 
but noticeable difference in the timing and consequently the final 
calibration values. For example, this is observed when comparing J721E 
EVM (evaluation module) and SVB (silicon validation board) platforms.

> 
> Flashes have an interval of temperature on which they are guaranteed to
> work (I would expect in the same conditions). Information about temperature
> ranges and associated delays (if measured?) can be passed too.

This would not be sufficient for placing TX and RX delays unless we have 
a perfect model for both the flash and the host device IO delays. Such a 
model would have to account for variations in timing caused by 
variations in the manufacturing process, voltage, and temperature. This 
is not practically feasible.

> 
> Cheers,
> ta
> > 
> > At lower speeds (25 MHz for example) this delay is not a problem because
> > the clock period is longer so there is much more time to sample the data
> > line. It is very likely the controller will sample at a time when the
> > data line is valid. At high speeds (166 MHz for example), especially in
> > DDR mode, this delay starts to play a larger role because the time to
> > sample the data line is much smaller. Now unless the delay is accounted
> > for, it is possible that the controller samples the data line too late
> > or too early and sees invalid data.
> > 
> > These delays depend on physical characteristics so it is not possible
> > for any upper layer to inform the controller about it. How will they
> > even know what the required delay is?
> > 
> > In summary, no, there is no way an upper layer can inform the controller
> > about this delay.
> > 
> >>
> >> Cheers,
> >> ta
> >>
> >>>
> >>> The main problem here is telling the controller where to find the
> >>> pattern and how to read it. This RFC uses nvmem cells which point to a
> >>> fixed partition containing the data to do the reads. It depends on [0]
> >>> and [1].
> >>>
> >>> The obvious problem with this is it won't work when the partitions are
> >>> defined via command line. I don't see any good way to add nvmem cells to
> >>> command line partitions. I would like some help or ideas here. We don't
> >>> necessarily have to use nvmem either. Any way that can cleanly and
> >>> consistently let the controller find out where the pattern is stored is
> >>> good.
> >>>
> >>> The dts patch depends on [2].
> >>>
> >>> Tested on TI's J721E EVM.
> >>>
> >>> [0] https://patchwork.ozlabs.org/project/linux-mtd/patch/20210302190012.1255-1-zajec5@gmail.com/
> >>> [1] https://patchwork.ozlabs.org/project/linux-mtd/patch/20210308011853.19360-1-ansuelsmth@gmail.com/
> >>> [2] https://patchwork.kernel.org/project/linux-arm-kernel/patch/20210305153926.3479-2-p.yadav@ti.com/
> >>>
> >>> Pratyush Yadav (6):
> >>>   spi: spi-mem: Tell controller when device is ready for calibration
> >>>   mtd: spi-nor: core: consolidate read op creation
> >>>   mtd: spi-nor: core: run calibration when initialization is done
> >>>   spi: cadence-qspi: Use PHY for DAC reads if possible
> >>>   spi: cadence-qspi: Tune PHY to allow running at higher frequencies
> >>>   arm64: dts: ti: k3-j721e-som-p0: Enable PHY calibration
> >>>
> >>>  arch/arm64/boot/dts/ti/k3-j721e-som-p0.dtsi |  55 ++
> >>>  drivers/mtd/spi-nor/core.c                  |  74 +-
> >>>  drivers/spi/spi-cadence-quadspi.c           | 820 +++++++++++++++++++-
> >>>  drivers/spi/spi-mem.c                       |  12 +
> >>>  include/linux/spi/spi-mem.h                 |   8 +
> >>>  5 files changed, 916 insertions(+), 53 deletions(-)
> >>>

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.