Re: [PATCH v2] i2c: i2c-mxs: Use DMA mode even for small transfers

[Alexandre Belloni <alexandre.belloni@free-electrons.com>]

Hi Marek,

On 03/07/2013 06:37, Marek Vasut wrote:
> 
> I'm attaching a patch. Alex, please give it a go and see if it fixes your issue. 
> It is _VERY_ ugly.
>

Quite ugly ;)

It indeed seems to fix the issue.

> The basic idea behind the the patch is that, as (attempted to be) explained 
> above, subsequent writes to DATA register in PIO mode cause constant generation 
> of clock on the bus and therefore a very long transfer of zero data. This 
> confuses the I2C peripherals of course.
> 
> The patch implements clock stretching for PIO writes (maybe we need this for 
> reads too) by making the controller blast out only 4 (or less) bytes of data in 
> each write into the DATA register. To prevent interruption of the transfer 
> between writes into the DATA register, the SCK is held low using the 
> RETAIN_CLOCK bit.
> 
> But (!) here comes the caveat. The PIO was introduced to speed up small 
> transfers. Introducing clock stretching into PIO mode operation might completely 
> remove this advantage. This has to be measured again.
> 

And now, PIO mode is slower than DMA...


-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com