Re: [PATCH] dmaengine: pl330: use subsys_initcall
From: Lars-Peter Clausen
Date: Fri Oct 17 2014 - 07:16:08 EST
On 10/17/2014 09:35 AM, Vinod Koul wrote:
On Fri, Oct 17, 2014 at 09:45:45AM +0200, Lars-Peter Clausen wrote:
On 10/17/2014 02:48 AM, Ray Jui wrote:
As part of subsystem that many slave drivers depend on, it's more
appropriate for the pl330 DMA driver to be initialized at
subsys_initcall than device_initcall
Well, we do have -EPROBE_DEFER these days to handle these kinds of
dependencies so we no longer have to these kinds of manual init
reordering tricks.
How ould that work?
Consider for example SPI and dmanegine. SPI driver got probed, then to start
a transaction requested a channel... while dmaengine driver is still getting
probed/not probed yet. So SPI driver didnt get a channel.
Ideally the SPI driver requests the channel in probe function and if the DMA
controller is not yet probed returns EPROBE_DEFER. If the SPI driver
requests the channel in the transfer handler it needs to deal with being
able to fall back to non DMA transfers anyway so this shouldn't be a problem.
But in any case fiddling around with the init sequences is just a quick hack
and might makes the problem less likely to appear in some cases, but there
is no guarantee that it works. And I think the proper solution at the moment
is to use probe deferral.
Other subsystems have seen patches which moved drivers from using
subsys_initcall to device_initcall/module_..._driver/ with the reasoning
that this is no longer necessary because of EPROBE_DEFER. So I don't think
we should be doing the exact opposite in DMA framework. Also if we'd apply
this patch it won't take to long until somebody suggest going back to
module_platform_driver() instead of subsys_initcall.
- Lars
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