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

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Message-ID: <20150809084712.GA10856@shbuild888>
Date:	Sun, 9 Aug 2015 16:47:12 +0800
From:	Feng Tang <feng.tang@...el.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Laura Abbott <labbott@...hat.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 Sat, Aug 08, 2015 at 03:18:59PM -0700, Greg Kroah-Hartman wrote:
> 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.

Great! Hopefully this multiple cma heap support and other ion issue could
be addressed there.

Thanks,
Feng
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