Re: [PATCH 2/4] spi: spi-fsl-dspi: Use non-coherent memory for DMA

Vladimir Oltean
2025-06-12T14:23:11Z

On Thu, Jun 12, 2025 at 03:14:32PM +0100, James Clark wrote:
> > That's why I don't like the DMA mode in DSPI, it's still CPU-bound,
> > because the DMA buffers are very small (you can only provide one TX FIFO
> > worth of data per DMA transfer, rather than the whole buffer).
>
> Is that right? The FIFO size isn't used in any of the DMA codepaths, it
> looks like the whole DMA buffer is filled before initiating the transfer.
> And we increase the buffer to 4k in this patchset to fully use the existing
> allocation.

Uhm, yeah, no?

dspi_dma_xfer():

while (dspi->len) {
dspi->words_in_flight = dspi->len / dspi->oper_word_size;
if (dspi->words_in_flight > dspi->devtype_data->fifo_size)
dspi->words_in_flight = dspi->devtype_data->fifo_size;
dspi_next_xfer_dma_submit();
}