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

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Date:	Sat, 17 May 2014 15:19:21 +0300
From:	Eli Billauer <eli.billauer@...il.com>
To:	Tejun Heo <tj@...nel.org>
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 Tejun,

On 17/05/14 00:08, Tejun Heo wrote:
>
> Don't we wanna map the underlying operation - dma_map_single_attrs() -
> instead?
>    

I'll resubmit this patch promptly, with a follow-up patch for the diff 
to implement dmam_map_single_attrs() instead. Plus a define-statement 
for dmam_map_single(). I can't test the case of a non-NULL value for 
@attrs however.

>    
>> +	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.
* It seems like a lot of dma_mapping_error() implementations always 
return no-error, since the DMA mapping can't fail on specific 
architectures. If callers use dma_mapping_error(), the possible 
devres_alloc() failure will be missed.

By the way, where I've seen dma_mapping_error() doing something, it 
checks for dma_handle == 0.

Submitting updated patches for the DMA mapping part soon.

Regards,
    Eli

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