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 <[email protected]> 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
