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

Vladimir Oltean
2025-06-12T14:32:02Z

On Thu, Jun 12, 2025 at 03:28:37PM +0100, James Clark wrote:
>
>
> On 12/06/2025 3:23 pm, Vladimir Oltean wrote:
> > 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();
> > }
>
> Right but that's before the change in this patchset to use the whole page
> that was allocated, hence the next bit:
>
> > And we increase the buffer to 4k in this patchset to fully use the
> existing allocation.
>
> We were allocating for the size of the FIFO (multiplied by two to hold the
> control words), but dma_alloc_coherent() will be backed by a whole page
> anyway, even if you only ask for a few bytes.
>
> After changing that to make use of the full allocation the FIFO length is no
> longer involved.

Ok, I haven't walked through patch 3 yet, I didn't realize it would be
changing that. I will want to test it on LS1028A.