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

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Message-ID: <20250612143157.bu4vayvhieohdtbu@skbuf>
Date: Thu, 12 Jun 2025 17:31:57 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: James Clark <james.clark@...aro.org>
Cc: Arnd Bergmann <arnd@...db.de>, Frank Li <Frank.li@....com>,
	Vladimir Oltean <olteanv@...il.com>,
	Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
	imx@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] spi: spi-fsl-dspi: Use non-coherent memory for DMA

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.