Re: [PATCH v4 5/9] spi: spi-mem: Mark dummy transfers by setting dummy_data bit

[Sowjanya Komatineni <skomatineni@nvidia.com>]

On 12/18/20 12:44 PM, Mark Brown wrote:
> On Fri, Dec 18, 2020 at 08:41:02PM +0000, Mark Brown wrote:
>> On Sat, Dec 19, 2020 at 12:49:38AM +0530, Pratyush Yadav wrote:
>>> Anyway, if the SPI maintainers think this is worth it, I won't object.
>> This gets kind of circular, for me it's a question of if there's some
>> meaningful benefit from using the feature vs the cost to support it and
>> from the sounds of it we don't have numbers on the benefits from using
>> it at present.
> ...although I do have to say looking at the implementation that the cost
> seems low, it's just a flag set on an existing transfer.  The only issue
> is if we'd get more win from coalesing the entire transaction (or entire
> transmit) into a single transfer that could be DMAed and/or requires
> fewer trips through the stack which does make it seem like an unclear
> tradeoff from the point of view of client drivers

Using HW dummy cycles save extra software cycle of transfer which 
involves transfer setup register writes, writing dummy bytes to TX FIFO, 
interrupt processing.

Implementation wise it just a single bit field added to spi_transfer and 
on Tegra controller driver programming dummy cycles with prior transfer 
and skipping sw dummy transfer which is actually not complex.

 From quick check, I see HW dummy cycles transfer of 128KB shows 18 Mb/s 
while SW transfer of bytes shows 17.3 MB/s on average.

When back-to-back read commands are executed using HW dummy cycles will 
definitely save cycles.