Re: [PATCH 3/3] spi: bcm2835: add module parameter to configure minimum length for dma

[kernel@xxxxxxxxxxxxxxxx]

> On 22.03.2019, at 13:36, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> 
> On Sun, Feb 24, 2019 at 04:23:11PM +0000, kernel@xxxxxxxxxxxxxxxx wrote:
>> +/* define dma min number of bytes to use in dma mode with value validation */
>> +static int dma_min_bytes_limit_set(const char *val,
>> +				   const struct kernel_param *kp)
>> +{
>> +	unsigned int v;
>> +
>> +	if (kstrtouint(val, 10, &v))
>> +		return -EINVAL;
>> +	/* value needs to be a multiple of 4 */
>> +	if (v % 4) {
>> +		pr_err("dma_min_bytes_limit needs to be a multiple of 4\n");
>> +		return -EINVAL;
>> +	}
> 
> Transfers don't need to be a multiple of 4 to be eligible for DMA,
> so this check can be dropped.

I definitely did not want to write a custom module argument parser
but if i remember correctly there is one limitation on the transmission path
where you would hit some inefficiencies in the DMA code when you run
transfers that are not a multiple of 4 - especially for short transfers.

That is why it was implemented.

But I can leave this precaution out.

Martin