linux-kernel - Re: [PATCH] dmaengine: device must have at least one channel

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Date:	Mon, 8 Aug 2016 13:47:54 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	linaro-kernel@...ts.linaro.org, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: device must have at least one channel

On Fri, Aug 05, 2016 at 09:30:50AM -0700, Viresh Kumar wrote:
> On 03-08-16, 21:34, Greg Kroah-Hartman wrote:

> > To follow up on this, as you found out, this patch breaks working
> > hardware, so please don't apply it anywhere.  It needs more debugging to
> > figure out exactly what is going on with some very strange hardware that
> > seems to be relying on some strange code...
> 
> Thanks for your email Greg, and I know what the issue is.
> 
> So, the dmaengine core actually allows registering a device without any channels
> and the dma driver can add channels later (directly to the dma-device channel
> list), for example, from the of-xlate callback once someone requests for a
> channel.

I do not think that is good idea

> Though its not the right way of doing things, but it still works. The only thing
> which breaks in that case is sysfs interface to dmaengine as the channels aren't
> registered as devices and so they aren't visible in userspace, as that code is
> *only* executed while the dma-device is registered.
> 
> I think its dmaengine core's decision to keep supporting such drivers or not and
> so I would like Vinod to comment on that..

My view would be to ensure that controllers have channels before they
register. If they dont, then please register later.

> But if we do want to support such drivers, then the core must be updated to fix
> the memleak I reported here, as idr_ref is allocated and not used at all (also
> dma-id allocated for it).
> 
> Or provide another API to properly and fully add channels at a later point of
> time.

And looking at drivers, I am not seeing anyone who needs such a support, so
am not inclined to support such a feature atm.

-- 
~Vinod