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

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Date:	Mon, 8 Aug 2016 08:24:49 -0700
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Vinod Koul <vinod.koul@...el.com>
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 08-08-16, 13:47, Vinod Koul wrote:
> 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.

Right and so I believe that applying this patch is the right thing to do..

-- 
viresh