Re: [PATCH v2 1/4] dma-mapping: Add devm_ interface for dma_map_single()

[Shuah Khan]

On 06/01/2014 01:01 AM, Eli Billauer wrote:
> dmam_map_single() and dmam_unmap_single() are the managed counterparts
> for the respective dma_* functions.
>
> Note that dmam_map_single() returns a status value rather than the DMA handle.
> The DMA handle is passed to the caller through a pointer in the arguments.
>
> The reason for this API change is that dmam_map_single() allocates memory,
> which may fail even if the DMA mapping is successful. On the other hand,
> it seems like there's no platform-independent value for dma_handle that can
> be used to indicate an error.
>
> This leaves dmam_map_single() with no safe way to tell its caller that the
> memory allocation has failed, except for returning a status as an int. Trying
> to keep close to the non-devres API could also tempt programmers into using
> dma_mapping_error() on the dmam_* functions. This would work incorrectly on
> platforms, for which dma_mapping_error() ignores its argument, and always
> returns success.

I see the value of this interface in unmap case, this type of wrapper
can release dma buffers, drivers neglected to release leaving dangling
buffers.

However, driver writers should give it some thought before switching
from conventional dma_map_*() interfaces to these proposed managed
dma mapping interfaces. These new interfaces shouldn't be treated as
drop in replacements to dma_map_*() interfaces.

The reasons are:

1. This interface adds an overhead in allocation memory for devres to
   compared to other dma_map_* wrappers. Users need to be aware of that.
   This would be okay in the cases where a driver allocates several
   buffers at init time and uses them. However, several drivers allocate
   during run-time and release as soon as it is no longer needed. This
   overhead is going to be in the performance path.

2. It adds a failure case even when dma buffers are available. i.e if
   if devres alloc fails, it will return failure even if dma map could
   have succeeded. This is a new failure case for DMA-API.

   The reason dma_map_*() routines fail now is because there are no
   buffers available. Drivers handle this error as a retry case.

   Drivers using dmam_map_single() will have to handle the failure
   cases differently.

   Since the return values are different for dmam_map_*(), that is
   plus that these interfaces can't be drop in replacements to the
   dma_map_*() interfaces.

3. Similarly, it adds an overhead in releasing memory for devres to
   compared to other dma_unmap_* wrappers. Users need to be aware of
   that. This overhead is going to be in the performance path when
   drivers unmap buffers during run-time.

Adding Joerg Roedel to the thread.

-- Shuah

--
Shuah Khan
Senior Linux Kernel Developer - Open Source Group
Samsung Research America(Silicon Valley)
shuah.kh@xxxxxxxxxxx | (970) 672-0658
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