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

[Michal Suchanek]

On 24 June 2016 at 00:14, 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.

If there was similar bit for write you could do 2-2-2 write but any
other option would be quite challenging.

Thanks

Michal