linux-kernel - Re: [PATCH v4 5/6] spi: spi-fsl-dspi: Increase DMA buffer size

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Message-ID: <aF70skVHeSRU3BTu@lizhi-Precision-Tower-5810>
Date: Fri, 27 Jun 2025 15:44:50 -0400
From: Frank Li <Frank.li@....com>
To: James Clark <james.clark@...aro.org>
Cc: Vladimir Oltean <olteanv@...il.com>, Mark Brown <broonie@...nel.org>,
	Vladimir Oltean <vladimir.oltean@....com>,
	Arnd Bergmann <arnd@...db.de>,
	Larisa Grigore <larisa.grigore@....com>,
	Christoph Hellwig <hch@....de>, linux-spi@...r.kernel.org,
	imx@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/6] spi: spi-fsl-dspi: Increase DMA buffer size

On Fri, Jun 27, 2025 at 11:21:41AM +0100, James Clark wrote:
> From: Larisa Grigore <larisa.grigore@....com>
>
> When the device is configured as a target, the host won't stop sending
> data while we're draining the buffer which leads to FIFO underflows
> and corruption.
>
> Increase the DMA buffer size to the maximum words that edma can
> transfer once to reduce the chance of this happening.
>
> While we're here, also change the buffer size for host mode back to a
> page as it was before commit a957499bd437 ("spi: spi-fsl-dspi: Fix
> bits-per-word acceleration in DMA mode"). dma_alloc_noncoherent()
> allocations are backed by a full page anyway, so we might as well use it
> all.
>
> Signed-off-by: Larisa Grigore <larisa.grigore@....com>
> Signed-off-by: James Clark <james.clark@...aro.org>
> ---
>  drivers/spi/spi-fsl-dspi.c | 42 ++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 38 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/spi/spi-fsl-dspi.c b/drivers/spi/spi-fsl-dspi.c
> index e7856f9c9440..46d3cae9efed 100644
> --- a/drivers/spi/spi-fsl-dspi.c
> +++ b/drivers/spi/spi-fsl-dspi.c
> @@ -493,6 +493,39 @@ static u32 dspi_pop_tx_pushr(struct fsl_dspi *dspi)
>  	return cmd << 16 | data;
>  }
>
> +static int dspi_dma_bufsize(struct fsl_dspi *dspi)
> +{
> +	if (spi_controller_is_target(dspi->ctlr)) {
> +		/*
> +		 * In target mode we have to be ready to receive the maximum
> +		 * that can possibly be transferred at once by EDMA without any
> +		 * FIFO underflows. This is CITER * SSIZE, where SSIZE is a max
> +		 * of 4 when transferring to a peripheral.
> +		 */
> +		return GENMASK(14, 0) * DMA_SLAVE_BUSWIDTH_4_BYTES;
> +	}
> +
> +	return PAGE_SIZE;
> +}
> +
> +static int dspi_dma_max_datawords(struct fsl_dspi *dspi)
> +{
> +	/*
> +	 * Transfers look like this so we always use a full DMA word regardless
> +	 * of SPI word size:
> +	 *
> +	 * 31              16 15                   0
> +	 * -----------------------------------------
> +	 * |   CONTROL WORD  |     16-bit DATA     |
> +	 * -----------------------------------------
> +	 * or
> +	 * -----------------------------------------
> +	 * |   CONTROL WORD  | UNUSED | 8-bit DATA |
> +	 * -----------------------------------------
> +	 */
> +	return dspi_dma_bufsize(dspi) / DMA_SLAVE_BUSWIDTH_4_BYTES;
> +}
> +
>  static int dspi_dma_transfer_size(struct fsl_dspi *dspi)
>  {
>  	return dspi->words_in_flight * DMA_SLAVE_BUSWIDTH_4_BYTES;
> @@ -608,6 +641,7 @@ static int dspi_next_xfer_dma_submit(struct fsl_dspi *dspi)
>  static void dspi_dma_xfer(struct fsl_dspi *dspi)
>  {
>  	struct spi_message *message = dspi->cur_msg;
> +	int max_words = dspi_dma_max_datawords(dspi);
>  	struct device *dev = &dspi->pdev->dev;
>
>  	/*
> @@ -619,8 +653,8 @@ static void dspi_dma_xfer(struct fsl_dspi *dspi)
>  		dspi_setup_accel(dspi);
>
>  		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;
> +		if (dspi->words_in_flight > max_words)
> +			dspi->words_in_flight = max_words;

you touch this line

		dspi->words_in_flight = min(dspi->words_in_flight, max_words);

Frank
>
>  		message->actual_length += dspi->words_in_flight *
>  					  dspi->oper_word_size;
> @@ -635,7 +669,7 @@ static void dspi_dma_xfer(struct fsl_dspi *dspi)
>
>  static int dspi_request_dma(struct fsl_dspi *dspi, phys_addr_t phy_addr)
>  {
> -	int dma_bufsize = dspi->devtype_data->fifo_size * 2;
> +	int dma_bufsize = dspi_dma_bufsize(dspi);
>  	struct device *dev = &dspi->pdev->dev;
>  	struct dma_slave_config cfg;
>  	struct fsl_dspi_dma *dma;
> @@ -719,7 +753,7 @@ static int dspi_request_dma(struct fsl_dspi *dspi, phys_addr_t phy_addr)
>
>  static void dspi_release_dma(struct fsl_dspi *dspi)
>  {
> -	int dma_bufsize = dspi->devtype_data->fifo_size * 2;
> +	int dma_bufsize = dspi_dma_bufsize(dspi);
>  	struct fsl_dspi_dma *dma = dspi->dma;
>
>  	if (!dma)
>
> --
> 2.34.1
>