linux-kernel - Re: [PATCH v4 0/6] spi: spi-fsl-dspi: Target mode improvements

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Message-ID: <20250701152433.4m54y2rkjzfe4kag@skbuf>
Date: Tue, 1 Jul 2025 18:24:33 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Mark Brown <broonie@...nel.org>
Cc: James Clark <james.clark@...aro.org>,
	Vladimir Oltean <olteanv@...il.com>, Arnd Bergmann <arnd@...db.de>,
	Larisa Grigore <larisa.grigore@....com>,
	Frank Li <Frank.li@....com>, Christoph Hellwig <hch@....de>,
	linux-spi@...r.kernel.org, imx@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/6] spi: spi-fsl-dspi: Target mode improvements

On Tue, Jul 01, 2025 at 04:16:50PM +0100, Mark Brown wrote:
> On Tue, Jul 01, 2025 at 05:53:12PM +0300, Vladimir Oltean wrote:
> 
> > I suppose one could try using FIFO mode for transfers which request
> > timestamping and DMA for transfers which don't. I don't have an insight
> > into what impact that will have on the driver, but I suspect at the very
> > least one will have to transform "DSPI_DMA_MODE" into "dspi->dma_available"
> > and "dspi->dma_in_use", and reconfigure the SPI_RSER register (interrupt
> > routing, to DMA engine or to CPUs) at every transfer rather than at dspi_init().
> 
> > The question is whether you would be willing to see and maintain such
> > complexity increase, when AFAIK, the LS1028A FIFO mode passes its
> > requirements.
> 
> Switching between modes is incredibly common, usually between PIO (for
> very short transfers) and DMA, that's no problem.  Factoring in
> timestamping seems like a reasonable signal I guess, might trip someone
> who was trying to benchmark things up but probably not normal users.

Ah, ok, I vaguely remember something being discussed about can_dma()
on previous iterations of this patch, but in a different context.
Then that's an avenue to explore, I guess. Looking at that method's
prototype, I suppose dspi could simply return can_dma = false "if (xfer->ptp_sts)"
(timestamp requested), i.e. no core involvement in the decision process at all?
Sounds interesting, but I can't promise I'll follow up with patches.