Also, I'm a bit confused. I've looked at several VirtIO devices now
and have seen things like this:
#define VIRTIO_PCI_DEVICE_SIGNATURE SIGNATURE_32 ('V', 'P', 'C', 'I')
// ...
UINT32 Signature;
I'm quite confused because I can't seem to find this anywhere in the
VirtIO specification. The spec says nothing about signature values
anywhere, just the magic value of 0x74726976. So where does this come
from?
On 4/15/21, Ethin Probst via groups.io
<harlydavidsen=gmail....@groups.io> wrote:
> Hi Andrew,
>
> What would that protocol interface look like if we utilized your idea?
> With mine (though I need to add channel mapping as well), your
> workflow for playing a stereo sound from left to right would probably
> be something like this:
> 1) Encode the sound using a standard tool into a Wave PCM 16.
> 2) Place the Wave file in the Firmware Volume using a given UUID as
> the name. As simple as editing the platform FDF file.
> 3) Write some BDS code
> a) Lookup Wave file by UUID and read it into memory.
> b) Decode the audio file (audio devices will not do this decoding
> for you, you have to do that yourself).
> c) Call EFI_AUDIO_PROTOCOL.LoadBuffer(), passing in the sample rate
> of your audio, EFI_AUDIO_PROTOCOL_SAMPLE_FORMAT_S16 for signed 16-bit
> PCM audio, the channel mapping, the number of samples, and the samples
> themselves.
> d) call EFI_BOOT_SERVICES.CreateEvent()/EFI_BOOT_SERVICES.CreateEventEx()
> for a playback event to signal.
> e) call EFI_AUDIO_PROTOCOL.StartPlayback(), passing in the event you
> just created.
> The reason that LoadBuffer() takes so many parameters is because the
> device does not know the audio that your passing in. If I'm given an
> array of 16-bit audio samples, its impossible to know the parameters
> (sample rate, sample format, channel mapping, etc.) from that alone.
> Using your idea, though, my protocol could be greatly simplified.
> Forcing a particular channel mapping, sample rate and sample format on
> everyone would complicate application code. From an application point
> of view, one would, with that type of protocol, need to do the
> following:
> 1) Load an audio file in any audio file format from any storage mechanism.
> 2) Decode the audio file format to extract the samples and audio metadata.
> 3) Resample the (now decoded) audio samples and convert (quantize) the
> audio samples into signed 16-bit PCM audio.
> 4) forward the samples onto the EFI audio protocol.
> There is another option. (I'm happy we're discussing this now -- we
> can hammer out all the details now which will make a lot of things
> easier.) Since I'll most likely end up splitting the device-specific
> interfaces to different audio protocols, we could make a simple audio
> protocol that makes various assumptions about the audio samples your
> giving it (e.g.: sample rate, format, ...). This would just allow
> audio output and input in signed 16-bit PCM audio, as you've
> suggested, and would be a simple and easy to use interface. Something
> like:
> typedef struct EFI_SIMPLE_AUDIO_PROTOCOL {
> EFI_SIMPLE_AUDIO_PROTOCOL_RESET Reset;
> EFI_SIMPLE_AUDIO_PROTOCOL_START Start;
> EFI_SIMPLE_AUDIO_PROTOCOL_STOP Stop;
> } EFI_SIMPLE_AUDIO_PROTOCOL;
> This way, users and driver developers have a simple audio protocol
> they can implement if they like. It would assume signed 16-bit PCM
> audio and mono channel mappings at 44100 Hz. Then, we can have an
> advanced protocol for each device type (HDA, USB, VirtIO, ...) that
> exposes all the knobs for sample formats, sample rates, that kind of
> thing. Obviously, like the majority (if not all) UEFI protocols, these
> advanced protocols would be optional. I think, however, that the
> simple audio protocol should be a required protocol in all UEFI
> implementations. But that might not be possible. So would this simpler
> interface work as a starting point?
>
> On 4/15/21, Andrew Fish <af...@apple.com> wrote:
>>
>>
>>> On Apr 15, 2021, at 1:11 PM, Ethin Probst <harlydavid...@gmail.com>
>>> wrote:
>>>
>>>> Is there any necessity for audio input and output to be implemented
>>>> within the same protocol? Unlike a network device (which is
>>>> intrinsically bidirectional), it seems natural to conceptually separate
>>>> audio input from audio output.
>>>
>>> Nope, there isn't a necessity to make them in one, they can be
>>> separated into two.
>>>
>>>> The code controlling volume/mute may not have any access to the sample
>>>> buffer. The most natural implementation would seem to allow for a
>>>> platform to notice volume up/down keypresses and use those to control
>>>> the
>>>> overall system volume, without any knowledge of which samples (if any)
>>>> are currently being played by other code in the system.
>>>
>>> Your assuming that the audio device your implementing the
>>> volume/muting has volume control and muting functionality within
>>> itself, then.
>>
>> Not really. We are assuming that audio hardware has a better
>> understanding
>> of how that system implements volume than some generic EFI Code that is
>> by
>> definition platform agnostic.
>>
>>> This may not be the case, and so we'd need to
>>> effectively simulate it within the driver, which isn't too hard to do.
>>> As an example, the VirtIO driver does not have a request type for
>>> muting or for volume control (this would, most likely, be within the
>>> VIRTIO_SND_R_PCM_SET_PARAMS request, see sec. 5.14.6.4.3). Therefore,
>>> either the driver would have to simulate the request or return
>>> EFI_UNSUPPORTED this instance.
>>>
>>
>> So this is an example of above since the audio hardware knows it is
>> routing
>> the sound output into another subsystem, and that subsystem controls the
>> volume. So the VirtIo Sound Driver know best how to bstract volume/mute
>> for
>> this platform.
>>
>>>> Consider also the point of view of the developer implementing a driver
>>>> for some other piece of audio hardware that happens not to support
>>>> precisely the same sample rates etc as VirtIO. It would be extremely
>>>> ugly to force all future hardware to pretend to have the same
>>>> capabilities as VirtIO just because the API was initially designed with
>>>> VirtIO in mind.
>>>
>>> Precisely, but the brilliance of VirtIO
>>
>> The brilliance of VirtIO is that it just needs to implement a generic
>> device
>> driver for a given operating system. In most cases these operating
>> systems
>> have sounds subsystems that manage sound and want fine granularity of
>> control on what is going on. So the drivers are implemented to maximizes
>> flexibility since the OS has lots of generic code that deals with sound,
>> and
>> even user configurable knobs to control audio. In our case that extra
>> layer
>> does not exist in EFI and the end user code just want to tell the driver
>> do
>> some simple things.
>>
>> Maybe it is easier to think about with an example. Lets say I want to play
>> a
>> cool sound on every boot. What would be the workflow to make the happen.
>> 1) Encode the sound using a standard tool into a Wave PCM 16.
>> 2) Place the Wave file in the Firmware Volume using a given UUID as the
>> name. As simple as editing the platform FDF file.
>> 3) Write some BDS code
>> a) Lookup Wave file by UUID and read it into memory.
>> b) Point the EFI Sound Protocol at the buffer with the Wave file
>> c) Tell the EFI Sound Protocol to play the sound.
>>
>> If you start adding in a lot of perimeters that work flow starts getting
>> really complicated really quickly.
>>
>>> is that the sample rate,
>>> sample format, &c., do not have to all be supported by a VirtIO
>>> device. Notice, also, how in my protocol proposal I noted that the
>>> sample rates, at least, were "recommended," not "required." Should a
>>> device not happen to support a sample rate or sample format, all it
>>> needs to do is return EFI_INVALID_PARAMETER. Section 5.14.6.2.1
>>> (VIRTIO_SND_R_JACK_GET_CONFIG) describes how a jack tells you what
>>> sample rates it supports, channel mappings, &c.
>>>
>>> I do understand how just using a predefined sample rate and sample
>>> format might be a good idea, and if that's the best way, then that's
>>> what we'll do. The protocol can always be revised at a later time if
>>> necessary. I apologize if my stance seems obstinate.
>>>
>>
>> I think if we add the version into the protocol and make sure we have a
>> separate load and play operation we could add a member to set the extra
>> perimeters if needed. There might also be some platform specific generic
>> tunables that might be interesting for a future member function.
>>
>> Thanks,
>>
>> Andrew Fish
>>
>>> Also, thank you, Laszlo, for your advice -- I hadn't considered that a
>>> network driver would be another good way of figuring out how async
>>> works in UEFI.
>>>
>>> On 4/15/21, Andrew Fish <af...@apple.com> wrote:
>>>>
>>>>
>>>>> On Apr 15, 2021, at 5:07 AM, Michael Brown <mc...@ipxe.org> wrote:
>>>>>
>>>>> On 15/04/2021 06:28, Ethin Probst wrote:
>>>>>> - I hoped to add recording in case we in future want to add
>>>>>> accessibility aids like speech recognition (that was one of the todo
>>>>>> tasks on the EDK2 tasks list)
>>>>>
>>>>> Is there any necessity for audio input and output to be implemented
>>>>> within
>>>>> the same protocol? Unlike a network device (which is intrinsically
>>>>> bidirectional), it seems natural to conceptually separate audio input
>>>>> from
>>>>> audio output.
>>>>>
>>>>>> - Muting and volume control could easily be added by just replacing
>>>>>> the sample buffer with silence and by multiplying all the samples by
>>>>>> a
>>>>>> percentage.
>>>>>
>>>>> The code controlling volume/mute may not have any access to the sample
>>>>> buffer. The most natural implementation would seem to allow for a
>>>>> platform to notice volume up/down keypresses and use those to control
>>>>> the
>>>>> overall system volume, without any knowledge of which samples (if any)
>>>>> are
>>>>> currently being played by other code in the system.
>>>>>
>>>>
>>>> I’ve also thought of adding NVRAM variable that would let setup, UEFI
>>>> Shell,
>>>> or even the OS set the current volume, and Mute. This how it would be
>>>> consumed concept is why I proposed mute and volume being separate APIs.
>>>> The
>>>> volume up/down API in addition to fixed percentage might be overkill,
>>>> but
>>>> it
>>>> does allow a non liner mapping to the volume up/down keys. You would be
>>>> surprised how picky audiophiles can be and it seems they like to file
>>>> Bugzillas.
>>>>
>>>>>> - Finally, the reason I used enumerations for specifying parameters
>>>>>> like sample rate and stuff was that I was looking at this protocol
>>>>>> from a general UEFI applications point of view. VirtIO supports all
>>>>>> of
>>>>>> the sample configurations listed in my gist, and it seems reasonable
>>>>>> to allow the application to control those parameters instead of
>>>>>> forcing a particular parameter configuration onto the developer.
>>>>>
>>>>> Consider also the point of view of the developer implementing a driver
>>>>> for
>>>>> some other piece of audio hardware that happens not to support
>>>>> precisely
>>>>> the same sample rates etc as VirtIO. It would be extremely ugly to
>>>>> force
>>>>> all future hardware to pretend to have the same capabilities as VirtIO
>>>>> just because the API was initially designed with VirtIO in mind.
>>>>>
>>>>> As a developer on the other side of the API, writing code to play
>>>>> sound
>>>>> files on an arbitrary unknown platform, I would prefer to simply
>>>>> consume
>>>>> as simple as possible an abstraction of an audio output protocol and
>>>>> not
>>>>> have to care about what hardware is actually implementing it.
>>>>>
>>>>
>>>> It may make sense to have an API to load the buffer/stream and other
>>>> APIs
>>>> to
>>>> play or pause. This could allow an optional API to configure how the
>>>> stream
>>>> is played back. If we add a version to the Protocol that would at least
>>>> future proof us.
>>>>
>>>> We did get feedback that it is very common to speed up the auto
>>>> playback
>>>> rates for accessibility. I’m not sure if that is practical with a
>>>> simple
>>>> PCM
>>>> 16 wave file with the firmware audio implementation. I guess that is
>>>> something we could investigate.
>>>>
>>>> In terms of maybe adding text to speech there is an open source project
>>>> that
>>>> conceptually we could port to EFI. It would likely be a binary that
>>>> would
>>>> have to live on the EFI System Partition due to size. I was thinking
>>>> that
>>>> words per minute could be part of that API and it would produce a PCM
>>>> 16
>>>> wave file that the audio protocol we are discussing could play.
>>>>
>>>>> Both of these argue in favour of defining a very simple API that
>>>>> expresses
>>>>> only a common baseline capability that is plausibly implementable for
>>>>> every piece of audio hardware ever made.
>>>>>
>>>>> Coupled with the relatively minimalistic requirements for boot-time
>>>>> audio,
>>>>> I'd probably suggest supporting only a single format for audio data,
>>>>> with
>>>>> a fixed sample rate (and possibly only mono output).
>>>>>
>>>>
>>>> In my world the folks that work for Jony asked for a stereo boot bong
>>>> to
>>>> transition from left to right :). This is not the CODEC you are looking
>>>> for
>>>> was our response…. I also did not mention that some languages are right
>>>> to
>>>> left, as the only thing worse than one complex thing is two complex
>>>> things
>>>> to implement.
>>>>
>>>>> As always: perfection is achieved, not when there is nothing more to
>>>>> add,
>>>>> but when there is nothing left to take away. :)
>>>>>
>>>>
>>>> "Simplicity is the ultimate sophistication”
>>>>
>>>> Thanks,
>>>>
>>>> Andrew Fish
>>>>
>>>>> Thanks,
>>>>>
>>>>> Michael
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Signed,
>>> Ethin D. Probst
>>
>>
>
>
> --
> Signed,
> Ethin D. Probst
>
>
>
>
>
>
--
Signed,
Ethin D. Probst
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