linux-kernel - Re: [PATCH v2] staging: ion: Add a default struct device for cma heap

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Date:	Sat, 8 Aug 2015 15:18:59 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Laura Abbott <labbott@...hat.com>
Cc:	Feng Tang <feng.tang@...el.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	John Stultz <john.stultz@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [PATCH v2] staging: ion: Add a default struct device for cma heap

On Fri, Aug 07, 2015 at 04:09:12PM -0700, Laura Abbott wrote:
> On 08/07/2015 11:05 AM, Greg Kroah-Hartman wrote:
> >On Fri, Aug 07, 2015 at 11:50:04PM +0800, Feng Tang wrote:
> >>On Fri, Aug 07, 2015 at 04:48:28PM +0200, Michal Nazarewicz wrote:
> >>>On Fri, Aug 07 2015, Feng Tang wrote:
> >>>>As I described above, the dummy struct device is only needed for
> >>>>dma request, its lifetime is align with the cma_heap itself.
> >>>
> >>>Again, this is from perspective of someone who is unfamiliar with ION,
> >>>but perhaps a viable solution is to bypass DMA API and just call
> >>>cma_alloc directly?
> >>
> >>For ion cma heap, the buffer allocation func ion_cma_allocate() will
> >>call dma_alloc_coherent(dev, ...). And dma_alloc_coherent() is
> >>implemented by each architeture(arm/m68k/x86 etc), and many Arch's
> >>implementation doesn't use cma, but use alloc_pages() like APIs.
> >>So I'm afraid we can't direcly call cma_alloc directly here.
> >
> >Ick.  But using a "fake" struct device here, for no real reason,
> >makes me very nervous that you are going to hit a codepath somewhere
> >that assumes this is a "real" struct device and tries to do something
> >with it (dev_printk(), look up what bus it is on, change the name of it,
> >etc.)  Trying to fake out the subsystem in this manner is a sign that
> >something is really wrong here.
> >
> >Please either make this a real device, or fix up the api to not need
> >this type of thing.
> >
> 
> I think this issue represents one of the many current issues with Ion.
> When the void * == struct dev was added, everything was working off of
> board files. We now have devicetree which makes the device association
> even more awkward to pull off. Every vendor out there is doing something
> different right now so the assertion in the commit text about 'normal'
> is not true; existing code has managed to work with the (not super great)
> API.
> 
> There is going to be an Ion session at Plumbers in a few weeks. I'd like
> to propose holding off on merging anything until after plumbers when
> there can be some more discussion about what would be a reasonable API,
> taking into consideration the points brought up in this patch series.

Sounds like a good idea.  I'll be at that talk as well.

thanks,

greg k-h
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