[PATCH v2 2/5] Documentation: mtd: add a DT property to set the number of dummy cycles

[Cyrille Pitchen]

Hi Marek,

Le 22/07/2015 15:43, Marek Vasut a écrit :
> On Wednesday, July 22, 2015 at 03:17:07 PM, Cyrille Pitchen wrote:
>> Depending on the SPI clock frequency, the Fast Read op code and the
>> Single/Dual Data Rate mode, the number of dummy cycles can be tuned to
>> improve transfer speed.
>> The actual number of dummy cycles is specific for each memory model and is
>> provided by the manufacturer thanks to the memory datasheet.
>>
>> Signed-off-by: Cyrille Pitchen <cyrille.pitchen at atmel.com>
>> ---
>>  Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
>> b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt index
>> 2bee68103b01..4387567d8024 100644
>> --- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
>> +++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
>> @@ -19,6 +19,11 @@ Optional properties:
>>                     all chips and support for it can not be detected at
>> runtime. Refer to your chips' datasheet to check if this is supported by
>> your chip.
>> +- m25p,num-dummy-cycles : Set the number of dummy cycles for Fast Read
>> commands. +                          Depending on the manufacturer
>> additional dedicated +                          commands are sent to the
>> flash memory so the +                          controller and the memory
>> can agree on the number of +                          dummy cycles to use.
> 
> Can't you just try negotiating this value at probe time, starting with some
> high value and see how low you can get with the negotiations ? This way, you'd 
> be able to effectively auto-detect this value at probe-time.
> 
> I might be wrong though :)
> 
I don't know whether it would be reliable enough. It is the exact same idea as
for the latency code used by Spansion QSPI memories. Micron memories allow to
skip the step of converting the number of dummy cycles into a latency code, you
directly program the right number of dummy cycles into a Micron specific
register, the Volatile Configuration Register.

However for both manufacturers the number of dummy cycles to use during Fast 
Read commands is given though tables found into the memory datasheet. The
number of dummy cycles depends on the Fast Read command, the SPI bus clock
frequency and the Single/Dual Data Rate mode.

It should be confirmed by Quad SPI memory manufacturers but since the number of
dummy cycles depends on the bus clock frequency, I guess the values provided by
the datasheets are recommendations. I think a too low value should not be so
easy to detect. For a given frequency one Fast Read command may succeed whereas
the same command with the very same number of dummy cycles might fail on the
next try. To be honest, I'm not sure about the memory behavior in limit
conditions so maybe the command will always succeed or always fail.

Also we can't be sure the read data are valid if we don't write them first. So
we would have to save the original data to restore them at the end of the
probing. Writing data at each probe would also reduce the memory lifetime. We
should also be aware of the bad blocks, which is more a job for upper layers.

It would be interesting to have some feedbacks from Micron, Spansion or other
QSPI memory manufacturer :)

> Best regards,
> Marek Vasut
> 
Best regards,

Cyrille