Add DMA APIs for count modes

[kilograham]

There is no ability to configure the Count Modes field of the DMA`s CHx_TRANS_COUNT register for RP2350 - its processing is missing in the SDK API version 2.0.0.

From RP2350 datasheet:

File "~\.pico-sdk\sdk\2.0.0\src\rp2_common\hardware_dma\include\hardware\dma.h"
Actual code:

static inline void dma_channel_set_trans_count(uint channel, uint32_t trans_count, bool trigger) {
    if (!trigger) {
        dma_channel_hw_addr(channel)->transfer_count = trans_count;
    } else {
        dma_channel_hw_addr(channel)->al1_transfer_count_trig = trans_count;
    }
}

Expected code example:

static inline void dma_channel_set_trans_count(uint channel, uint32_t mode, uint32_t trans_count, bool trigger) {
    assert(mode == DMA_CH0_TRANS_COUNT_MODE_VALUE_NORMAL || mode == DMA_CH0_TRANS_COUNT_MODE_VALUE_TRIGGER_SELF || mode == DMA_CH0_TRANS_COUNT_MODE_VALUE_ENDLESS);
    assert(trans_count < (1 << DMA_CH0_TRANS_COUNT_MODE_LSB));
    if (!trigger) {
        dma_channel_hw_addr(channel)->transfer_count = trans_count;
    } else {
        dma_channel_hw_addr(channel)->al1_transfer_count_trig = trans_count;
    }
    dma_channel_hw_addr(channel)->transfer_count |= mode << DMA_CH0_TRANS_COUNT_MODE_LSB;
}

[TheLostLambda]

Also running into this issue — is this working on the development branch yet?

Or alternatively, is this something that still needs help?

[TheLostLambda]

For what it's worth, this didn't work for me outside of the SDK:

 dma_channel_hw_addr(channel)->transfer_count |= mode << DMA_CH0_TRANS_COUNT_MODE_LSB;

I found that reading dma_channel_hw_addr(channel)->transfer_count (or dma_hw->ch[dma_chan].transfer_count in my code) always returned zero until after the channel was started / triggered, at which point updating the transfer count had no effect. I needed to set it before dma_channel_start(), but I couldn't read it until after dma_channel_start().

To avoid this issue, I ended up with something that just directly wrote the final value to transfer_count:

dma_channel_hw_addr(channel)->transfer_count = trans_count | mode << DMA_CH0_TRANS_COUNT_MODE_LSB;

[TheLostLambda]

Happy to write a PR for this, but it would be good to know if people think we should update dma_channel_configure too, since that is another common route for setting transfer_count?