linux-kernel - Re: [PATCH] drivers: dma-coherent: use ioremap_wc() for DMA_MEMORY

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Date:	Fri, 4 Dec 2015 08:59:10 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Brian Starkey <brian.starkey@....com>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] drivers: dma-coherent: use ioremap_wc() for DMA_MEMORY_MAP

On Fri, Dec 4, 2015 at 2:50 AM, Catalin Marinas <catalin.marinas@....com> wrote:
> On Fri, Nov 20, 2015 at 02:20:26PM +0000, Brian Starkey wrote:
>> When the DMA_MEMORY_MAP flag is used, memory which can be accessed
>> directly should be returned, so use ioremap_wc() instead of ioremap().
>> Also, ensure that the correct memset operation is used in
>> dma_alloc_from_coherent() with respect to the region's flags.
>>
>> This fixes the below alignment fault on arm64, caused by invalid use
>> of memset() on Device memory.
>
> This is indeed affecting both arm32 and arm64 systems.
>
>> diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
>> index 55b8398..45358d0 100644
>> --- a/drivers/base/dma-coherent.c
>> +++ b/drivers/base/dma-coherent.c
>> @@ -31,7 +31,10 @@ static int dma_init_coherent_memory(phys_addr_t phys_addr, dma_addr_t device_add
>>       if (!size)
>>               goto out;
>>
>> -     mem_base = ioremap(phys_addr, size);
>> +     if (flags & DMA_MEMORY_MAP)
>> +             mem_base = ioremap_wc(phys_addr, size);
>> +     else
>> +             mem_base = ioremap(phys_addr, size);
>
> I wonder whether a memremap() approach for the DMA_MEMORY_MAP case would
> be better. This API was added recently by commit 92281dee825f ("arch:
> introduce memremap()"). It only supports write-back and write-through
> but we could add a MEMREMAP_WC flag for this case.

I originally included both MEMREMAP_WC and MEMREAMP_UC as potential
flags to this api, but ultimately decided against it.  The memremap()
api is meant for memory that is known to have no i/o side effects.  As
far as I can see WC and UC usages are a muddy mix of "sometimes
there's I/O side effects, but it depends by arch and driver".  In
other words we can't drop the "__iomem" annotation from WC and UC
mappings by default.
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