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

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Message-ID: <65622175-1c5a-4228-831b-6e2f8c05a9f0@linaro.org>
Date: Mon, 30 Jun 2025 09:59:36 +0100
From: James Clark <james.clark@...aro.org>
To: Frank Li <Frank.li@....com>
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 27/06/2025 8:44 pm, Frank Li wrote:
> 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

Ack

>>
>>   		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
>>