drivers: spi: Add support for half-duplex (3-wire) SPI

[ThreeEights]

Add support for half-duplex (3-wire) SPI operation using the Raspberry Pi Pico PIO. To allow control of the size of the driver, including half-duplex support is optional, under the control of Kconfig options.

The original PIO source code is also included as a reference.

[ThreeEights]

Drat - the tx/rx counts in the 3-wire logic were wrong. Corrected.

[soburi]

Is this file needed?

[ThreeEights]

Strictly speaking, no; at this time, this file is essentially documentation. We went back and forth on including this file. This is the original source for generating the embedded binary PIO code, but can't be used directly in builds without the ability to use pioasm in the build process. If it's the consensus of reviewers that it doesn't make sense to include this file now, I'll remove it.

[soburi]

This file should be deleted, as it is not possible to include unnecessary files other than documents.
It seems that the code is already embedded in the C source as a comment, but it might be better to write it in the form of the original code here.

[soburi]

@ThreeEights
Please remove this file.

[ThreeEights]

Removed.

[soburi]

This is just a matter of style and preference, but there are ifdefs all over the place, so it would be nice to see them put together. For example, how about changing the function to something like this?

static inline int spi_pico_pio_sm_put(PIO pio, uint sm, void *dataptr, size_t bits)
{
        switch(bits) {
		case 4:
			*((io_rw_32 *)&pio->txf[sm]) = *(uint32_t*)dataptr;
			return 4;
		case 2:
			*((io_rw_8 *)&pio->txf[sm]) = *(uint16_t*)dataptr;
			return 2;
		case 1:
			*((io_rw_8 *)&pio->txf[sm]) = *(uint8_t*)dataptr;
			return 1;
		default:
			return 0;
        }
}

[tbursztyka]

Unless one wants to save some ROM bytes, in this case the defines are useful. (unless, again, these defines disappear and become DTS properties)

[ThreeEights]

Condensing the ..._put and ..._get code, as @soburi suggested, is a good idea. That makes the source code a lot shorter and cleaner. I'll try to get that to work.

[ThreeEights]

Unfortunately, condensing the get and set functions won't work. The logic is different in the 4-wire and 3-wire cases, and it's necessary to split reading the data from the tx buffer and writing to the send FIFO: if there's data to send, it needs to use the content from the txbuf, but if there's no txbuf data, 0 has to be sent in order to clock the receive data. Similarly, the data in the receive FIFO has to be read out, even if there's no rxbuf, in order to sync the send and receive cycle with the spi_context.

The only changes I can see that might make sense would be to move the FIFO operations out of the get and put functions, eliminating those, which can reduce the number of places where compile-time checks are needed for the 2 and 4 byte transactions.

Would it make sense to remove those Kconfig options? Is that modest increase in overhead for the most common 8-bit case acceptable?

[ThreeEights]

@soburi, @tbursztyka, @yonsch - Can you please take a look at the latest changes and see if the issues are resolved? Thanks!

[Ablu]

@ThreeEights: Is this the last gap that is needed to use bluetooth/wifi on the Pico W? Or is there further glue code between the CYW43xxx driver and this required?

Do you happen to have device-trees for enabling the cyw43439 around? Then I would be happy to give it a try :).

[ThreeEights]

@Ablu - Alas, no: The current Infineon WiFi driver for the CYW43439 does not have SPI support. If you're interested, you can view my latest attempt (as of yesterday!) on my fork:
https://github.com/ThreeEights/zephyr on branch airoc-wifi-spi

[soburi]

@tbursztyka
May we have your opinion?

[ThreeEights]

@yonsch - Could you please take a look at this pull request? Thanks.