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

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Date:	Fri, 7 Aug 2015 11:05:46 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Feng Tang <feng.tang@...el.com>
Cc:	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>
Subject: Re: [PATCH v2] staging: ion: Add a default struct device for cma heap

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.

thanks,

greg k-h
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