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

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Date:	Mon, 19 May 2014 16:17:12 -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, Eli.

On Sat, May 17, 2014 at 03:19:21PM +0300, Eli Billauer wrote:
> >>+	if (dma_mapping_error(dev, dma_handle)) {
> >>+		devres_free(dr);
> >>+		return 0;
> >Can't we just keep returning dma_handle?  Even if that means invoking
> >->mapping_error() twice?  It's yucky to have subtly different error
> >return especially because in most cases it won't fail.
> Yucky it is indeed. There are however two problems with keeping the existing
> API:
> 
> * What to do if devres_alloc() fails. How do I signal back an error? The
> only way I can think of is returning zero. But if the caller should know
> that zero means failure, I've already broken the API. I might as well return
> zero for any kind of failure.

What can't it just do the following?

	if (dma_mapping_error(dev, dma_handle)) {
		devres_free(dr);
		return dma_handle;
	}

The caller would have to invoke dma_mapping_error() again but is that
a problem?

Thanks.

-- 
tejun
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