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

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Message-ID: <20151207164714.GH7083@e104818-lin.cambridge.arm.com>
Date:	Mon, 7 Dec 2015 16:47:15 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Brian Starkey <brian.starkey@....com>,
	"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>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] drivers: dma-coherent: use ioremap_wc() for
 DMA_MEMORY_MAP

On Mon, Dec 07, 2015 at 08:19:27AM -0800, Dan Williams wrote:
> On Mon, Dec 7, 2015 at 5:28 AM, Brian Starkey <brian.starkey@....com> wrote:
> > On Fri, Dec 04, 2015 at 05:15:54PM +0000, Catalin Marinas wrote:
> >>
> >> On Fri, Dec 04, 2015 at 08:59:10AM -0800, Dan Williams wrote:
> >>>
> >>> 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.
> >
> >
> > The DMA_MEMORY_MAP flag is pretty much a statement of "no side-
> > effects", so as Catalin says it would fit OK here. That said, if it's
> > not possible to deprecate ioremap_wc() in the same way as
> > ioremap_cache() then I wonder if there's even much benefit in adding
> > it to memremap().
> 
> I don't see a problem adding a _WC option to memremap.
> 
> The only difference is that it can't replace ioremap_wc.  I.e. unlike
> _WB, and _WT case where ioremap_cache and ioremap_wt are now
> deprecated we'd have ioremap_wc continuing to live alongside
> memremap(..., MEMREMAP_WC).

I think that's fine. The difference is that memory returned by
ioremap_wc() should (in theory) only be accessed with I/O accessors
while the range returned by memremap(MEMREMAP_WC) will be directly
accessible.

-- 
Catalin
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