Re: [PATCH 7/9] mtd: m25p80: add support of dual and quad spi protocols to all commands

[Michal Suchanek]

On 24 June 2016 at 00:50, Marek Vasut <marex@xxxxxxx> wrote:
> On 06/24/2016 12:43 AM, Michal Suchanek wrote:
>> On 24 June 2016 at 00:14, Marek Vasut <marex@xxxxxxx> wrote:
>>> On 06/23/2016 11:58 PM, Michal Suchanek wrote:
>>>> On 23 June 2016 at 22:46, Marek Vasut <marex@xxxxxxx> wrote:
>>>>> On 06/23/2016 10:35 PM, Michal Suchanek wrote:
>>>>>> Hello,
>>>>>
>>>>> Hi,
>>>>>
>>>>>> this patch is kind of awesome.
>>>>>>
>>>>>> I have a few practical concerns however.
>>>>>>
>>>>>> On 20 June 2016 at 18:50, Cyrille Pitchen <cyrille.pitchen@xxxxxxxxx> wrote:
>>>>>>> Before this patch, m25p80_read() supported few SPI protocols:
>>>>>>> - regular SPI 1-1-1
>>>>>>> - SPI Dual Output 1-1-2
>>>>>>> - SPI Quad Output 1-1-4
>>>>>>> On the other hand, all other m25p80_*() hooks only supported SPI 1-1-1.
>>>>>>
>>>>>> Under typical use my estimate is that huge majority of data is
>>>>>> transferred in _read() seconded by _write().
>>>>>>
>>>>>> As I understand it the n-n-n means how many bits you transfer in
>>>>>> parallel when sending command-address-data.
>>>>>>
>>>>>> In _read() the command and data overhead is negligible when you can
>>>>>> read kilobytes at once. So difference between 1-1-4 and 4-4-4 is not
>>>>>> meaningful performance-wise. Are there flash chips that support one
>>>>>> but not the other?
>>>>>
>>>>> That's quite unlikely.
>>>>>
>>>>>> For _write() the benefits are even harder to assess.
>>>>>
>>>>> The page program usually works on 256B pages, so the math is rather easy.
>>>>>
>>>>>> You can
>>>>>> presumably write at n-n-4 or n-n-2 if your controller and flash
>>>>>> supports it transferring the page faster. And then spend possibly
>>>>>> large amount of time waiting for the flash to get ready again. If the
>>>>>> programming time is fixed transferring the page faster may or may not
>>>>>> have benefits. It may at least free the bus for other devices to use.
>>>>>>
>>>>>> The _reg_ stuff is probably negligible altogether,
>>>>>>
>>>>>> Lastly the faster transfers of address bytes seem to be achieved with
>>>>>> increasingly longer command codes given how much the maximum command
>>>>>> length increased. So even in a page write where the address is a few %
>>>>>> of the transfer the benefit of these extra modes is dubious.
>>>>>>
>>>>>> Overall I wonder how much it is worthwhile to complicate the code to
>>>>>> get all these modes in every single function.
>>>>>
>>>>> In my opinion, 1-1-x makes sense as it is supported by most flashes,
>>>>> while n-m-x where n,m>1 does not make sense as it often requires some
>>>>> stateful change to non-volatile register with little gain.
>>>>>
>>>>
>>>> There is actually one thing that x-x-x modes make easier. If I were to
>>>> implement dual mode switch on my SPI master controller it would be
>>>> probably set for whole message and would not change mid-transfer.
>>>
>>
>>>
>>>> Still you can probably simulate x-x-x with 1-1-x by scattering the
>>>> 1-1-x command bits across more bytes.
>>>
>>> That's not how you usually implement it. It's quite often a shift register.
>>>
>>
>> Checking the manual there is a bit in a register that switches the
>> master controller to dual mode receive (only). So the master
>> controller can do 1-1-2 read (only). I don't use that feature because
>> afaict there is no code in m25p80 which does the switch and as pointed
>> out the reg_read commands are done in 1-1-1.
>
> I don't think I understand. Are you talking about some specific
> controller now ?
>

Yes, the sunxi spi controller can do dual read according to manual but
not sure if anyone has tired that, At least the A31 variant can.

Thanks

Michal