[alsa-devel] [PATCH 1/4] ALSA: control: return payload length for TLV operation

[Takashi Sakamoto]

Hi,

On Sep 2 2016 22:09, Takashi Iwai wrote:
> On Fri, 02 Sep 2016 13:30:33 +0200,
> Takashi Sakamoto wrote:
>> On Sep 1 2016 00:40, Takashi Iwai wrote:
>>> Which application do you have in mind?
>>
>> At first, I did never suggest that 'TLV feature should handle byte
>> stream' or 'TLV feature should be extended to somewhere'. I just
>> addressed that 'Now, TLV feature is not used only for transmission of
>> pure threshold information between applications/drivers, but it's also
>> used for I/O operation. Unfortunately, protocol of TLV packet payload is
>> not kept anymore.'
>>
>> It's not my intension to discuss about this patchset in a point of 'byte
>> stream'. It comes from your bias, and the other developers are led to
>> the wrong direction, sigh. I really hope them to read my comment in this
>> patchset carefully and compare them to actual code implementations
>> in driver/core/library and applications[1][2][3].
> 
> Well, without the particular purpose explained, it's hard to
> understand why your patchset is required.  That was the failure.
> The implementation details come after the design.

Read my comment in the first patch, again.
http://mailman.alsa-project.org/pipermail/alsa-devel/2016-August/112245.html

I wrote 'This is inconvenient to applications'. It shows that I take
care of ALSA applications in this patch.

>> Again, I have few interests about actual implementation of TLV feature
>> in driver side and for what purposes applications are developed to
>> utilize TLV feature. They're free for developers in each area now, and
>> for future. What we should do is how to assist their activity via the
>> design of APIs. This is my place in this patchset. It's not my intension
>> to request extensions of TLV feature.
> 
> But your patchset changes the call patterns largely.  This annoyed me,
> and supposedly most people, too.  If the TLV feature isn't needed to
> be extended, the necessary change shouldn't be too intrusive, either.

Surprisingly, the annoying planted a bias to you and lost calmness in
your brain, away from the core concept of this patchset.

Well, to me, it's really normal for I/O APIs to return processed bytes
to applications. This is a reason that I explain like that.

Of course, I don't say I can always write comments to fully explains
patch features so that everyone can get them. But I believe that my
comments describes an item:
 - I/O operation recent supported in TLV feature ignores
   the protocol of TLV packet.
 - Then, applications have disadvantages when calling TLV ioctl in a
   point of operated bytes.

That's all of my concerns. Most of you interpret the item as what
convenient to them.

>>> Applications would access via either alsa-lib or tinyalsa.  And these
>> libraries do already care about how to access via TLV.
>>
>> Without enough care due to implementation in kernel land.
>>
>> As I already cleared, current TLV feature has a difficulty not to return
>> the length of actually handled bytes[4]. Correspondingly, APIs in these
>> libraries have defections, as APIs for I/O operation or transmission of
>> arbitrary data, because applications cannot get to know the actual
>> length of handled data.
> 
> It's TLV, so the actual length is always encoded in the block (in
> tlv[1]).  What's missing...?

I've already addressed two cases that 'struct snd_ctl_tlv.tlv[1]'
doesn't have actual length of data:
http://mailman.alsa-project.org/pipermail/alsa-devel/2016-August/112398.html

>> There's no way for user space to get appropriate length in advance, this
>> brings contradiction for content of given buffer. For example, in ALSA
>> SoC part, the length is trimmed according to a parameter of driver
>> instance, implicitly[5]. As a result, users are beguiled. They requests
>> a certain length of buffer to be handled via TLV feature. Even if they
>> receive success, a _part_ of buffer is actually handled. This is not
>> good for software stacks in a point of abstraction layer of hardware.
>> There's no transparency independent from hardwares. Application
>> developers is forced to consider about device driver's codes which are
>> not open via APIs.
>>
>> I already wrote patchset for alternative TLV APIs to alsa-lib:
>> https://github.com/takaswie/alsa-lib/tree/new-tlv-api
>>
>> In the remote branch, you can see new APIs at commit 5f13de, which
>> allows applications to receive the length of actually handled data:
>> https://github.com/takaswie/alsa-lib/commit/5f13deacfc65d26d6acbb066da0f2c35538f7497
>>
>>
>>> What makes better what, practically seen?
>>
>> Already described in this message.
>>
>> If you're still against to this patchset, I'm OK to cancel this discussion.
>>
>> But in this case, involuntarily, when my friends are going to use TLV
>> feature for their ALSA applications (I hope it's unlikely), I'll
>> recommend them not to use it, because it certainly confuses application
>> developers and bring them to particular-hardware-specific something with
>> the lack of transparency. That's a waste of their expensive time.
> 
> The usage of TLV for this type of communication has *NEVER* been
> recommended.  That's why I wrote it was an "(ab)usage".  It was
> accepted just because the proposed usage fitted with the current TLV
> access pattern.  If it were for more generic purpose, it wouldn't have
> been accepted from the beginning.
> 
> The extended API is merely to pass a large blob over the control API.
> That's the only purpose, and wouldn't be more than that.

You missed my intention, again. Please be in user space, then consider
about TLV ioctls and the state of buffers, length. TLV feature supports
read, write and command operations.

I'm sorry but it's time to go to bed. I got tired because it's the last
day of daily work in a week...


Thanks

Takashi Sakamoto