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

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Date:	Wed, 23 Jul 2014 14:56:11 -0700
From:	Laura Abbott <lauraa@...eaurora.org>
To:	Catalin Marinas <catalin.marinas@....com>
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 7/23/2014 3:45 AM, Catalin Marinas wrote:
> On Wed, Jul 23, 2014 at 02:35:06AM +0100, Laura Abbott wrote:
>> --- a/arch/arm/mm/dma-mapping.c
>> +++ b/arch/arm/mm/dma-mapping.c
>> @@ -298,37 +298,19 @@ static void *
>>  __dma_alloc_remap(struct page *page, size_t size, gfp_t gfp, pgprot_t prot,
>>  	const void *caller)
>>  {
>> -	struct vm_struct *area;
>> -	unsigned long addr;
>> -
>>  	/*
>>  	 * DMA allocation can be mapped to user space, so lets
>>  	 * set VM_USERMAP flags too.
>>  	 */
>> -	area = get_vm_area_caller(size, VM_ARM_DMA_CONSISTENT | VM_USERMAP,
>> -				  caller);
>> -	if (!area)
>> -		return NULL;
>> -	addr = (unsigned long)area->addr;
>> -	area->phys_addr = __pfn_to_phys(page_to_pfn(page));
>> -
>> -	if (ioremap_page_range(addr, addr + size, area->phys_addr, prot)) {
>> -		vunmap((void *)addr);
>> -		return NULL;
>> -	}
>> -	return (void *)addr;
>> +	return dma_common_contiguous_remap(page, size,
>> +			VM_ARM_DMA_CONSISTENT | VM_USERMAP,
>> +			prot, caller);
> 
> I think we still need at least a comment in the commit log since the arm
> code is moving from ioremap_page_range() to map_vm_area(). There is a
> slight performance penalty with the addition of a kmalloc() on this
> path.
> 
> Or even better (IMO), see below.
> 
>> --- 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.

Thanks,
Laura

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