linux-kernel - Re: [PATCH] dmaengine: pl330: use subsys

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Date:	Fri, 17 Oct 2014 13:15:55 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Vinod Koul <vinod.koul@...el.com>
CC:	Ray Jui <rjui@...adcom.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Scott Branden <sbranden@...adcom.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: pl330: use subsys_initcall

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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