Re: [PATCH 2/3] ALSA: core: modify .ack callback to take arguments for updating appl ptr

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 5/19/17 1:27 AM, Takashi Iwai wrote:
> On Fri, 19 May 2017 05:57:05 +0200,
> Takashi Sakamoto wrote:
>>
>> Hi,
>>
>> On May 16 2017 10:01, Subhransu S. Prusty wrote:
>>> From: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
>>>
>>> When appl_ptr is updated let low-level driver know, e.g.  to let the
>>> low-level driver/hardware pre-fetch data opportunistically.
>>>
>>> The existing .ack callback is extended with new attribute argument, to
>>> support this capability. Legacy driver subscribe to SND_PCM_ACK_LEGACY and
>>> doesn't process ack if it is not set. SND_PCM_ACK_APP_PTR can be used to
>>> process the application ptr update in the driver like in the skylake
>>> driver which can use this to inform position of appl pointer to host DMA
>>> controller. The skylake driver to process the SND_PCM_ACK_APP_PTR will be
>>> submitted with a separate patch set.
>>>
>>> In the ALSA core, this capability is independent from the NO_REWIND
>>> hardware flag. The low-level driver may however tie both options and only
>>> use the updated appl_ptr when rewinds are disabled due to hardware
>>> limitations.
>>>
>>> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
>>> Signed-off-by: Jaikrishna Nemallapudi <jaikrishnax.nemallapudi@intel.com>
>>> Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
>>> ---
>>>   include/sound/pcm-indirect.h  |  4 ++--
>>>   include/sound/pcm.h           |  8 +++++++-
>>>   sound/core/pcm_lib.c          |  6 ++++--
>>>   sound/core/pcm_native.c       | 24 +++++++++++++++++++++++-
>>>   sound/mips/hal2.c             | 14 +++++++++++---
>>>   sound/pci/cs46xx/cs46xx_lib.c | 18 ++++++++++++++----
>>>   sound/pci/emu10k1/emupcm.c    |  8 ++++++--
>>>   sound/pci/rme32.c             | 15 ++++++++++++---
>>>   8 files changed, 79 insertions(+), 18 deletions(-)
>>
>> I think it better to take care of
>> 'drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c' as
>> well. This is a driver for sound functionality of Raspberry PI 1/zero
>> and some people may still get influences from this work.
>>
>> $ git grep -A20 'struct snd_pcm_ops' v4.12-rc1 | grep \\.ack
>> v4.12-rc1:drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c-...
>> ...
>
> Yep, we need to cover all codes if we really change the PCM ops.

The reason precisely why I preferred to avoid mucking with the existing 
.ack(). we have a risk of introducing problems for legacy and staging, 
not to mention out-of-tree.

>
>
> Takashi
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