linux-kernel - Re: [PATCH 2/5] dma-mapping: Add devm_ interface for dma_map

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Date:	Tue, 20 May 2014 11:05:54 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Eli Billauer <eli.billauer@...il.com>
Cc:	gregkh@...uxfoundation.org, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] dma-mapping: Add devm_ interface for dma_map_single()

Hello,

On Tue, May 20, 2014 at 08:54:09AM +0300, Eli Billauer wrote:
> That seems OK to me, but the problem I'm concerned with is this: In
> devm_get_free_pages() it says
> 
>     devres = devres_alloc(devm_pages_release,
>                   sizeof(struct pages_devres), GFP_KERNEL);
>     if (unlikely(!devres)) {
>         free_pages(addr, order);
>         return 0;
>     }
> 
> What should I put instead of this "return 0" to conform with the current
> API?

Ah, okay.  No idea.

> And to make things even worse, on some architectures, dma_mapping_error()
> always returns success, regardless of dma_handle. So if we stick to the
> current API, the caller of devm_get_free_pages() will never know it failed
> on these architectures.
> 
> So my conclusion was that the caller must be aware that if
> devm_get_free_pages() returns zero it's an error, regardless of
> dma_mapping_error(). That breaks the API. Once it's broken, why not return
> zero on all possible errors?

What if 0 is a valid return for the architecture?  Isn't that the
whole point of dma_mapping_error() interface?  Maybe the right thing
to do is using a separate out argument for dma_handle?

	int dmam_map_single(blah blah, dma_addr_t *out_addr);

So that it can return -errno like other non-crazy functions?  We'll
probably ask somebody who knows this code better.

Thanks.

-- 
tejun
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