linux-kernel - Re: [PATCHv4 3/5] common: dma-mapping: Introduce common remapping functions

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Message-ID: <20140724135238.GC13371@arm.com>
Date:	Thu, 24 Jul 2014 14:52:38 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Laura Abbott <lauraa@...eaurora.org>
Cc:	Will Deacon <Will.Deacon@....com>,
	David Riley <davidriley@...omium.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ritesh Harjain <ritesh.harjani@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Thierry Reding <thierry.reding@...il.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCHv4 3/5] common: dma-mapping: Introduce common remapping
 functions

On Wed, Jul 23, 2014 at 10:56:11PM +0100, Laura Abbott wrote:
> On 7/23/2014 3:45 AM, Catalin Marinas wrote:
> > On Wed, Jul 23, 2014 at 02:35:06AM +0100, Laura Abbott wrote:
> >> --- a/drivers/base/dma-mapping.c
> >> +++ b/drivers/base/dma-mapping.c
> > [...]
> >> +void *dma_common_contiguous_remap(struct page *page, size_t size,
> >> +			unsigned long vm_flags,
> >> +			pgprot_t prot, const void *caller)
> >> +{
> >> +	int i;
> >> +	struct page **pages;
> >> +	void *ptr;
> >> +
> >> +	pages = kmalloc(sizeof(struct page *) << get_order(size), GFP_KERNEL);
> >> +	if (!pages)
> >> +		return NULL;
> >> +
> >> +	for (i = 0; i < (size >> PAGE_SHIFT); i++)
> >> +		pages[i] = page + i;
> >> +
> >> +	ptr = dma_common_pages_remap(pages, size, vm_flags, prot, caller);
> >> +
> >> +	kfree(pages);
> >> +
> >> +	return ptr;
> >> +}
> > 
> > You could avoid the dma_common_page_remap() here (and kmalloc) and
> > simply use ioremap_page_range(). We know that
> > dma_common_contiguous_remap() is only called with contiguous physical
> > range, so ioremap_page_range() is suitable. It also makes it a
> > non-functional change for arch/arm.
> 
> My original thought with using map_vm_area vs. ioremap_page_range was
> that ioremap_page_range is really intended for mapping io devices and
> the like into the kernel virtual address space. map_vm_area is designed
> to handle pages of kernel managed memory. Perhaps it's too nit-picky
> a distinction though.

I think you are right. We had a discussion in the past about using
ioremap on valid RAM addresses and decided not to allow this. This would
be similar with the ioremap_page_range() here.

>From my perspective, you can leave the code as is (wouldn't be any
functional change for arm64 since it was using vmap() already). But
please add a comment in the commit log about this change.

-- 
Catalin
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