Changing SPI clock is unexpectedly expensive

[ProfFan]

Changing the SPI frequency essentially goes to

esp-hal/esp-hal/src/spi/master.rs

Line 2576 in a20054c

self.setup(frequency, clocks);

which is a loop-based search. However in situations like multi-slave SPI bus with different clocks, this will need to be executed many times.

Don't know what is the best way, could

1. add forced inlining to the logic
2. add a shortcut when the clock speed is multiples of clk_spi_mst/32 as this is the most common scenario
3. add a static cache for computed values?

XREF: rust-embedded/embedded-hal#605

[Dominaezzz]

3. add a static cache for computed values?

I think this the answer.
I imagine something like this.

let clock_setting = Spi::calculate_clock_settings(&clocks, frequency);

// User can store clock_setting object per device.

spi.set_frequency(clock_setting);

EDIT: This will also be very handy for setting up configurable segmented transfers.

[ProfFan]

Bumping this issue. SPI sharing is a very common use case and this can be easily supported by introducing a SpiClock struct that has From<Hz> and let the SpiBus::Config struct store this instead of the Hz value.

[bugadani]

I don't think this is as easy as you think, the peripherals can have multiple clock sources. Since we intend to support configuring clock sources, we'll have to somehow tie these config options together, while at the same time remain backward compatible when we introduce the clock source configuration.

cc #1668