linux-kernel - Re: RE: [EXT] Re: [PATCH 5/6] spi: imx: support dynamic burst length for ECSPI DMA mode

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Message-ID: <20251126-glistening-rampant-dragon-4ec9d3-mkl@pengutronix.de>
Date: Wed, 26 Nov 2025 13:52:20 +0100
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Carlos Song <carlos.song@....com>
Cc: "broonie@...nel.org" <broonie@...nel.org>, Frank Li <frank.li@....com>, 
	"s.hauer@...gutronix.de" <s.hauer@...gutronix.de>, "kernel@...gutronix.de" <kernel@...gutronix.de>, 
	"festevam@...il.com" <festevam@...il.com>, "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>, 
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RE: [EXT] Re: [PATCH 5/6] spi: imx: support dynamic burst length
 for ECSPI DMA mode

On 26.11.2025 12:29:35, Carlos Song wrote:
> > On 25.11.2025 18:06:17, Carlos Song wrote:
> > >  static int spi_imx_pio_transfer(struct spi_device *spi,
> > >  				struct spi_transfer *transfer)
> > >  {
> > > @@ -1780,9 +2112,14 @@ static int spi_imx_transfer_one(struct
> > spi_controller *controller,
> > >  	 * transfer, the SPI transfer has already been mapped, so we
> > >  	 * have to do the DMA transfer here.
> > >  	 */
> > > -	if (spi_imx->usedma)
> > > -		return spi_imx_dma_transfer(spi_imx, transfer);
> > > -
> > > +	if (spi_imx->usedma) {
> > > +		ret = spi_imx_dma_transfer(spi_imx, transfer);
> > > +		if (transfer->error & SPI_TRANS_FAIL_NO_START) {
> > > +			spi_imx->usedma = false;
> > > +			return spi_imx_pio_transfer(spi, transfer);
> > > +		}
> > > +		return ret;
> > > +	}
> >
> > Why do you do this? AFAICS the framework already does this for you see:
> > spi_transfer_one_message().
> >
> Hi,
>
> In frame work:
> 				if ((xfer->tx_sg_mapped || xfer->rx_sg_mapped) &&
> 				    (xfer->error & SPI_TRANS_FAIL_NO_START)) {
> 					__spi_unmap_msg(ctlr, msg);
> 					ctlr->fallback = true;
> 					xfer->error &= ~SPI_TRANS_FAIL_NO_START;
> 					goto fallback_pio;
> 				}
>
> It only will help do this for "framework finished DMA map case". But DMA dynamic burst feature is mapped in driver.
> So this condition never meet: (xfer->tx_sg_mapped || xfer->rx_sg_mapped) I think..

...and you remove "controller->can_dma" this deserves some comments in
the commit message. That you fool the framework, that you cannot do DMA,
and do all the mapping on your own.

If you can make it work with DMA_SLAVE_BUSWIDTH_1_BYTES then you can use
controller->prepare_message and controller->unprepare_message to do the
byte swapping. These functions are called before and after the DMA sync.

Marc

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