[PATCH v2 5/6] spi: davinci: do not use DMA if transfer length is less than 16

[Frode Isaksen]

On 17/02/2017 17:37, Arnd Bergmann wrote:
> On Fri, Feb 17, 2017 at 11:38 AM, Frode Isaksen <fisaksen at baylibre.com> wrote:
>> Higher bitrate and lower CPU load if using PIO in this case.
>>
>> Signed-off-by: Frode Isaksen <fisaksen at baylibre.com>
>> ---
>>  drivers/spi/spi-davinci.c | 13 +++++++------
>>  1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/spi/spi-davinci.c b/drivers/spi/spi-davinci.c
>> index b69a370..826bff1 100644
>> --- a/drivers/spi/spi-davinci.c
>> +++ b/drivers/spi/spi-davinci.c
>> @@ -110,6 +110,8 @@
>>  #define SPIDEF         0x4c
>>  #define SPIFMT0                0x50
>>
>> +#define DMA_MIN_BYTES  16
> 16 seems low as the cutoff. Have you found this experimentally and
> tested that for e.g. 32
> bytes there is actually an advantage in using DMA?
Yes, I have tested for different sizes. Actually 32 bytes is better @30MHz, but if the bit rate goes down, DMA wins over PIO. Do you prefer 32 bytes ?

Frode
>
>     Arnd