Re: [PATCH 2/4] spi: spi-fsl-dspi: Use non-coherent memory for DMA

James Clark
2025-06-10T15:46:40Z

On 10/06/2025 4:15 pm, Frank Li wrote:
> On Mon, Jun 09, 2025 at 04:32:39PM +0100, 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.
>
> Any beanfit by use on-coherent memory here?
>
> Frank
>

Presumably less cache maintenance traffic?

Thanks
James

>>
>> Suggested-by: Arnd Bergmann <arnd@arndb.de>
>> Signed-off-by: James Clark <james.clark@linaro.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 386a17871e79..567632042f8f 100644
>> --- a/drivers/spi/spi-fsl-dspi.c
>> +++ b/drivers/spi/spi-fsl-dspi.c
>> @@ -247,6 +247,11 @@ struct fsl_dspi {
>> void (*dev_to_host)(struct fsl_dspi *dspi, u32 rxdata);
>> };
>>
>> +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) {
>> @@ -361,7 +366,10 @@ 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);
>> }
>>
>> @@ -369,9 +377,13 @@ 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]);
>> }
>> @@ -381,6 +393,7 @@ 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;
>> @@ -389,10 +402,9 @@ 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) {
>> @@ -407,10 +419,10 @@ 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) {
>> @@ -512,17 +524,17 @@ 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;
>> @@ -557,11 +569,12 @@ 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:
>> @@ -582,14 +595,16 @@ 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);
>> }
>> }
>>
>> --
>> 2.34.1
>>