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Message-ID: <adbbfc9c-5d21-4c8f-ba71-ae1103569037@arm.com>
Date: Mon, 16 Jun 2025 12:56:51 +0100
From: Robin Murphy <robin.murphy@....com>
To: James Clark <james.clark@...aro.org>, 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
Subject: Re: [PATCH v2 2/5] spi: spi-fsl-dspi: Use non-coherent memory for DMA
On 2025-06-13 10:28 am, James Clark wrote:
> Using coherent memory here isn't functionally necessary. Because the
> change to use non-coherent memory isn't overly complex and only a few
> synchronization points are required, we might as well do it while fixing
> up some other DMA issues.
If it doesn't need coherent memory then does the driver really need to
do its own bounce-buffering at all? Could you not simplify the whole lot
even more by getting rid of {tx,rx}_dma_buf altogether and relying on
the SPI core helpers to DMA-map the messages in-place?
Thanks,
Robin.
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: James Clark <james.clark@...aro.org>
> ---
> drivers/spi/spi-fsl-dspi.c | 55 +++++++++++++++++++++++++++++-----------------
> 1 file changed, 35 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/spi/spi-fsl-dspi.c b/drivers/spi/spi-fsl-dspi.c
> index 744dfc561db2..f19404e10c92 100644
> --- a/drivers/spi/spi-fsl-dspi.c
> +++ b/drivers/spi/spi-fsl-dspi.c
> @@ -379,6 +379,11 @@ static bool is_s32g_dspi(struct fsl_dspi *data)
> data->devtype_data == &devtype_data[S32G_TARGET];
> }
>
> +static int dspi_dma_transfer_size(struct fsl_dspi *dspi)
> +{
> + return dspi->words_in_flight * DMA_SLAVE_BUSWIDTH_4_BYTES;
> +}
> +
> static void dspi_native_host_to_dev(struct fsl_dspi *dspi, u32 *txdata)
> {
> switch (dspi->oper_word_size) {
> @@ -493,7 +498,10 @@ static void dspi_tx_dma_callback(void *arg)
> {
> struct fsl_dspi *dspi = arg;
> struct fsl_dspi_dma *dma = dspi->dma;
> + struct device *dev = &dspi->pdev->dev;
>
> + dma_sync_single_for_cpu(dev, dma->tx_dma_phys,
> + dspi_dma_transfer_size(dspi), DMA_TO_DEVICE);
> complete(&dma->cmd_tx_complete);
> }
>
> @@ -501,9 +509,13 @@ static void dspi_rx_dma_callback(void *arg)
> {
> struct fsl_dspi *dspi = arg;
> struct fsl_dspi_dma *dma = dspi->dma;
> + struct device *dev = &dspi->pdev->dev;
> int i;
>
> if (dspi->rx) {
> + dma_sync_single_for_cpu(dev, dma->rx_dma_phys,
> + dspi_dma_transfer_size(dspi),
> + DMA_FROM_DEVICE);
> for (i = 0; i < dspi->words_in_flight; i++)
> dspi_push_rx(dspi, dspi->dma->rx_dma_buf[i]);
> }
> @@ -513,6 +525,7 @@ static void dspi_rx_dma_callback(void *arg)
>
> static int dspi_next_xfer_dma_submit(struct fsl_dspi *dspi)
> {
> + size_t size = dspi_dma_transfer_size(dspi);
> struct device *dev = &dspi->pdev->dev;
> struct fsl_dspi_dma *dma = dspi->dma;
> int time_left;
> @@ -521,10 +534,9 @@ static int dspi_next_xfer_dma_submit(struct fsl_dspi *dspi)
> for (i = 0; i < dspi->words_in_flight; i++)
> dspi->dma->tx_dma_buf[i] = dspi_pop_tx_pushr(dspi);
>
> + dma_sync_single_for_device(dev, dma->tx_dma_phys, size, DMA_TO_DEVICE);
> dma->tx_desc = dmaengine_prep_slave_single(dma->chan_tx,
> - dma->tx_dma_phys,
> - dspi->words_in_flight *
> - DMA_SLAVE_BUSWIDTH_4_BYTES,
> + dma->tx_dma_phys, size,
> DMA_MEM_TO_DEV,
> DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
> if (!dma->tx_desc) {
> @@ -539,10 +551,10 @@ static int dspi_next_xfer_dma_submit(struct fsl_dspi *dspi)
> return -EINVAL;
> }
>
> + dma_sync_single_for_device(dev, dma->rx_dma_phys, size,
> + DMA_FROM_DEVICE);
> dma->rx_desc = dmaengine_prep_slave_single(dma->chan_rx,
> - dma->rx_dma_phys,
> - dspi->words_in_flight *
> - DMA_SLAVE_BUSWIDTH_4_BYTES,
> + dma->rx_dma_phys, size,
> DMA_DEV_TO_MEM,
> DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
> if (!dma->rx_desc) {
> @@ -644,17 +656,17 @@ static int dspi_request_dma(struct fsl_dspi *dspi, phys_addr_t phy_addr)
> goto err_tx_channel;
> }
>
> - dma->tx_dma_buf = dma_alloc_coherent(dma->chan_tx->device->dev,
> - dma_bufsize, &dma->tx_dma_phys,
> - GFP_KERNEL);
> + dma->tx_dma_buf = dma_alloc_noncoherent(dma->chan_tx->device->dev,
> + dma_bufsize, &dma->tx_dma_phys,
> + DMA_TO_DEVICE, GFP_KERNEL);
> if (!dma->tx_dma_buf) {
> ret = -ENOMEM;
> goto err_tx_dma_buf;
> }
>
> - dma->rx_dma_buf = dma_alloc_coherent(dma->chan_rx->device->dev,
> - dma_bufsize, &dma->rx_dma_phys,
> - GFP_KERNEL);
> + dma->rx_dma_buf = dma_alloc_noncoherent(dma->chan_rx->device->dev,
> + dma_bufsize, &dma->rx_dma_phys,
> + DMA_FROM_DEVICE, GFP_KERNEL);
> if (!dma->rx_dma_buf) {
> ret = -ENOMEM;
> goto err_rx_dma_buf;
> @@ -689,11 +701,12 @@ static int dspi_request_dma(struct fsl_dspi *dspi, phys_addr_t phy_addr)
> return 0;
>
> err_slave_config:
> - dma_free_coherent(dma->chan_rx->device->dev,
> - dma_bufsize, dma->rx_dma_buf, dma->rx_dma_phys);
> + dma_free_noncoherent(dma->chan_rx->device->dev, dma_bufsize,
> + dma->rx_dma_buf, dma->rx_dma_phys,
> + DMA_FROM_DEVICE);
> err_rx_dma_buf:
> - dma_free_coherent(dma->chan_tx->device->dev,
> - dma_bufsize, dma->tx_dma_buf, dma->tx_dma_phys);
> + dma_free_noncoherent(dma->chan_tx->device->dev, dma_bufsize,
> + dma->tx_dma_buf, dma->tx_dma_phys, DMA_TO_DEVICE);
> err_tx_dma_buf:
> dma_release_channel(dma->chan_tx);
> err_tx_channel:
> @@ -714,14 +727,16 @@ static void dspi_release_dma(struct fsl_dspi *dspi)
> return;
>
> if (dma->chan_tx) {
> - dma_free_coherent(dma->chan_tx->device->dev, dma_bufsize,
> - dma->tx_dma_buf, dma->tx_dma_phys);
> + dma_free_noncoherent(dma->chan_tx->device->dev, dma_bufsize,
> + dma->tx_dma_buf, dma->tx_dma_phys,
> + DMA_TO_DEVICE);
> dma_release_channel(dma->chan_tx);
> }
>
> if (dma->chan_rx) {
> - dma_free_coherent(dma->chan_rx->device->dev, dma_bufsize,
> - dma->rx_dma_buf, dma->rx_dma_phys);
> + dma_free_noncoherent(dma->chan_rx->device->dev, dma_bufsize,
> + dma->rx_dma_buf, dma->rx_dma_phys,
> + DMA_FROM_DEVICE);
> dma_release_channel(dma->chan_rx);
> }
> }
>
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