linux-kernel - [PATCH v2 3/5] spi: spi-fsl-dspi: Increase DMA buffer size

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Message-Id: <20250613-james-nxp-spi-dma-v2-3-017eecf24aab@linaro.org>
Date: Fri, 13 Jun 2025 10:28:58 +0100
From: James Clark <james.clark@...aro.org>
To: Vladimir Oltean <olteanv@...il.com>, Mark Brown <broonie@...nel.org>
Cc: Vladimir Oltean <vladimir.oltean@....com>, 
 Arnd Bergmann <arnd@...db.de>, Larisa Grigore <larisa.grigore@....com>, 
 Frank Li <Frank.li@....com>, linux-spi@...r.kernel.org, imx@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, James Clark <james.clark@...aro.org>
Subject: [PATCH v2 3/5] spi: spi-fsl-dspi: Increase DMA buffer size

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 f19404e10c92..48c2ebefcd4a 100644
--- a/drivers/spi/spi-fsl-dspi.c
+++ b/drivers/spi/spi-fsl-dspi.c
@@ -384,6 +384,39 @@ static int dspi_dma_transfer_size(struct fsl_dspi *dspi)
 	return dspi->words_in_flight * DMA_SLAVE_BUSWIDTH_4_BYTES;
 }
 
+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 void dspi_native_host_to_dev(struct fsl_dspi *dspi, u32 *txdata)
 {
 	switch (dspi->oper_word_size) {
@@ -606,6 +639,7 @@ static void dspi_setup_accel(struct fsl_dspi *dspi);
 static int 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;
 	int ret = 0;
 
@@ -618,8 +652,8 @@ static int 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;
 
 		message->actual_length += dspi->words_in_flight *
 					  dspi->oper_word_size;
@@ -636,7 +670,7 @@ static int 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;
@@ -720,7 +754,7 @@ 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