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

[Marek Vasut]

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.

> Thanks
> 
> Michal
> 


-- 
Best regards,
Marek Vasut