On Thu, Aug 01, 2013 at 05:32:37AM +0800, Stephen Warren wrote:

On 07/30/2013 09:45 PM, Richard Zhao wrote:

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On Wed, Jul 31, 2013 at 12:46:42AM +0800, Stephen Warren wrote:

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On 07/29/2013 09:31 PM, Richard Zhao wrote:

quoted

On Sat, Jul 27, 2013 at 03:39:15AM +0800, Stephen Warren wrote:

quoted

On 07/23/2013 10:09 PM, Richard Zhao wrote:

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- driver: remove use of nvidia,dma-request-selector
	  use dma_request_slave_channel to request channel
- update binding doc

This patch needs to be amended so that the DMA channel is looked up
during probe() rather than open(), to guarantee that deferred probe
works. It's possible to hold off probe, but not possible AFAIK to hold
off opening the port.

How about avoid return -EPROBE_DEFER at open() time? It can be in
another patch.

I don't really understand what you mean. -EPROBE_DEFER is something that
only makes sense for probe() to return (or functions used by probe()).
It doesn't make sense to return it anywhere else, since probe deferral
is something that can only happen for probe().

What I meant is, it might not worth to move request channel to probe
only because we need to check error and return -EPROBE_DEFER.

If probe() doesn't attempt to validate that the/a DMA channel provider
exists, then there's no guarantee that the DMA provider has probed
before the DMA client driver either probes or executes any later code.

I'm not against defer probe, but dma_request_slave_channel might not be
the right function. Ideally dma channels are allocated at open() time
and dma controller loaded checking is at probe() time. So there's
better some function called check_dma_controller_loaded(), or we could
also use initcalls to let dma driver probe earlier.

This patch intend to be simple to only change the binding.

So, you could quite easily end up with a situation where the serial
driver's open fails because the DMA provider has not yet probed. This is
exactly what deferred probe was intended to avoid. This reason alone is
enough to make it worth fixing this.

quoted

dma_request_slave_channel is better to be called as late as possible to
make better dynamic use of dma channels.

probe() could validate that the provider exists in a way that doesn't
permanently hold ownership of the DMA channel. For example, request it
then immediately release it, in the absence of a dedicated "validate my
DMA providers exist" API.

It's ugly but may work. The current driver didn't check dma controller
loaded at probe time. Let's leave the task to another patch?

quoted

Actually, I searched the kernel code, most drivers don't return EPROBE_DEFER
when dma_request_slave_channel failed. dma_request_slave_channel can
fail in many cases, and dma device not probed is only one case.

That sounds like a bug. If a DMA channel /is/ specified, it should be
possible to acquire it. If no DMA channel is specified, and if the
device can do PIO, then it's reasonable to continue.

Idealy when dma_request_slave_channel failed at open time, it can also
move to PIO.

Thanks
Richard