[PATCH 3/8] spi: davinci: limit the transfer size if DMA enabled

[Peter Ujfalusi]

On 02/13/2017 07:59 AM, Sekhar Nori wrote:
> + Peter
>
> On Friday 10 February 2017 08:59 PM, Frode Isaksen wrote:
>> Limit the transfer size to 20 scatter/gather pages if
>> DMA is enabled.
>> The eDMA DMA engine is limited to 20 SG entries in one DMA
>> transaction. If this number is exceeded, DMA receive fails.
>> This error occurs with large vmalloc'ed buffers.
>
> This needs more explanation because there is support available in edma
> driver for long SG lists by breaking them down into transfers using 20
> PaRAM entries at a time. If thats not working for you, that needs
> further debug.

While breaking down long SG list works in the eDMA driver, it is not w/o 
it's limitation. It might take (to) long to re-configure the paRAM slots 
for the next batch. We see missed events with MMC/SD in case of long SG 
list, so I would not be surprised if this is not causing issues in other 
places...

>
>>
>> Signed-off-by: Frode Isaksen <fisaksen at baylibre.com>
>
> Thanks,
> Sekhar
>
>> ---
>>  drivers/spi/spi-davinci.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/spi/spi-davinci.c b/drivers/spi/spi-davinci.c
>> index b7b2da1..f1b46f6 100644
>> --- a/drivers/spi/spi-davinci.c
>> +++ b/drivers/spi/spi-davinci.c
>> @@ -485,6 +485,16 @@ static bool davinci_spi_can_dma(struct spi_master *master,
>>  	return __davinci_spi_can_dma(spi);
>>  }
>>
>> +static size_t davinci_spi_max_transfer_size(struct spi_device *spi)
>> +{
>> +	/*
>> +	 * The eDMA DMA engine is limited to 20 SG entries in one DMA
>> +	 * transaction. If this number is exceeded, DMA receive fails.
>> +	 * An extra SG entry is needed when the buffer is not page aligned.
>> +	 */
>> +	return (__davinci_spi_can_dma(spi)) ? 19 * PAGE_SIZE : SIZE_MAX;
>> +}
>> +
>>  static int davinci_spi_check_error(struct davinci_spi *dspi, int int_status)
>>  {
>>  	struct device *sdev = dspi->bitbang.master->dev.parent;
>> @@ -1010,6 +1020,7 @@ static int davinci_spi_probe(struct platform_device *pdev)
>>  	master->setup = davinci_spi_setup;
>>  	master->cleanup = davinci_spi_cleanup;
>>  	master->can_dma = davinci_spi_can_dma;
>> +	master->max_transfer_size = davinci_spi_max_transfer_size;
>>
>>  	dspi->bitbang.chipselect = davinci_spi_chipselect;
>>  	dspi->bitbang.setup_transfer = davinci_spi_setup_transfer;
>>
>

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