Re: [RFC PATCH 3/6] spi: spi-s3c64xx: Add coherent buffers for DMA transfers

["Łukasz Czerwiński" <l.czerwinski@samsung.com>]

On 09/09/2013 04:45 PM, Mark Brown wrote:
> On Mon, Sep 09, 2013 at 04:09:23PM +0200, Lukasz Czerwinski wrote:
>> The spi-s3c64xx currently doesn't support transfers from non-contiguous
>> client buffers.
>>
>> This patch adds two coherent buffers which allow transfers from
>> non-contiguous client buffers without extra coherent memory allocation
>> in the client driver.
>>
>> Buffer size is hardcoded to 16kB for Tx/Rx. Client drivers shouldn't
>> exceed that value.
>
> This seems like a very low limit to have, consider things like firmware
> downloads for example.  It seems reasonable to have a preallocated small
> buffer but there should be some fallback for larger transfer sizes.
>
I have tested my modification with different buffer sizes with S5C73M3 
driver (355560 B upload). I obtained following upload times:

16kB buffer:
	- 83.972 ms
	- 79.196 ms
	- 79.432 ms
128kB buffer:
	- 74.449 ms
	- 80.719 ms
	- 75.599 ms

For 256kB I obtained similar results as in 128kB case. Normally 
non-interrupted SPI transfer should take 56ms (50MHz). Performance loss 
is approximately about 6% between 16kB and 128KB buffer. I my opinion 
there are no need to use bigger buffers.

I propose add extra module parameter which allows to change buffer size.

> I also didn't notice any checks in the code for the length of transfers
> so this will corrupt memory if a client driver tries to transfer more
> than the preallocated buffer as things stand.
>
Right, I will add that.

Thanks
Lukasz