Re: [PATCH 2/5] i2c: sh_mobile: add DMA support

[Vinod Koul <vinod.koul@intel.com>]

On Mon, Dec 15, 2014 at 09:31:01AM +0100, Geert Uytterhoeven wrote:
> Please let me summarize...
Thanks :)

> During probe of a DMA client driver, the DMA engine driver may not be available,
> causing dma_request_slave_channel*() to return -EPROBE-DEFER.
> There are actually two different reasons that the DMA engine driver may not
> be available:
>   1. The DMA engine driver hasn't been initialized yet, due to probe order.
>      This is more likely to happen with i2c client drivers, as they
> are initialized
>      from subsys_initcall() instead of module_init() (E.g. I never saw
> it with the
>      spi-rspi driver).
>      => The DMA client driver wants to return -EPROBE_DEFER too, and
>        retry later.
>   2. The DMA engine driver is not included in the kernel build.
>      => The DMA client driver wants to fall back to PIO.
> 
> Now, how to distinguish between the two cases above?
Quite right, this is a good question. Today we cannot distinguish between the
two. Should we improve the deferred probe to tell us when the init
is complete and all the modules have been initialized? If we ever have such
a mechanism to check then we know no modules are to be inserted then we can
fall back to PIO mode. Without that we should use some timeout counter to
fall back on, say try requesting 5 times and give up and move to PIO after that

> Currently, e.g. i2c-sh_mobile always returns -EPROBE_DEFER, never falling
> back to PIO, breaking case 2. While e.g. spi-rspi always falls back to PIO,
> which is suboptimal in case 1 (but I never encountered that case with spi).
> 
> Solutions under consideration:
>   1. Wolfram posted a patch to make i2c-sh_mobile fall back to PIO, and retry
>      DMA initialization in every request, so it will switch to DMA
> when it becomes
>      available. But this is suboptimal, as it adds overhead to every
> request (and
>      DMA may never become available in case 2).
> 
>   2. Delay i2c initialization, by moving from subsys_initcall() to
> module_init(),
>       in the hope the i2c client driver will be initialized after the
> DMA engine.
>      This is being discussed in the thread you quoted above.
> 
> I hope this explains the problem well.
Yes and it has nothing to do with channels being exhausted, the problem I
assumed earlier!!

-- 
~Vinod