Re: [RFC PATCH v2 16/22] sound: usb: Introduce QC USB SND offloading support

[Pierre-Louis Bossart]

On 1/25/23 21:14, Wesley Cheng wrote:
> Several Qualcomm SoCs have a dedicated audio DSP, which has the ability to
> support USB sound devices.  This vendor driver will implement the required
> handshaking with the DSP, in order to pass along required resources that
> will be utilized by the DSP's USB SW.  The communication channel used for
> this handshaking will be using the QMI protocol.  Required resources
> include:
> - Allocated secondary event ring address
> - EP transfer ring address
> - Interrupter number
> 
> The above information will allow for the audio DSP to execute USB transfers
> over the USB bus.  It will also be able to support devices that have an
> implicit feedback and sync endpoint as well.  Offloading these data
> transfers will allow the main/applications processor to enter lower CPU
> power modes, and sustain a longer duration in those modes.
> 
> Audio offloading is initiated with the following sequence:
> 1. Userspace configures to route audio playback to USB backend and starts
> playback on the platform soundcard.

what happens if the DSP driver is probed after the USB one?
Or vice-versa?

Userspace needs to be notified of what is detected at the kernel level,
I don't see how we can assume a specific route is always present.


> +config QC_USB_AUDIO_OFFLOAD
> +	tristate "Qualcomm Audio Offload driver"

USB Audio Offload


> +struct uaudio_dev {
> +	struct usb_device *udev;
> +	/* audio control interface */
> +	struct usb_host_interface *ctrl_intf;
> +	unsigned int card_num;
> +	unsigned int usb_core_id;
> +	atomic_t in_use;
> +	struct kref kref;
> +	wait_queue_head_t disconnect_wq;
> +
> +	/* interface specific */
> +	int num_intf;
> +	struct intf_info *info;
> +	struct snd_usb_audio *chip;
> +};
> +
> +static struct uaudio_dev uadev[SNDRV_CARDS];

I don't follow what this array is? Does this shadow all possible cards,
even non-USB ones?

> +static struct uaudio_qmi_dev *uaudio_qdev;
> +static struct uaudio_qmi_svc *uaudio_svc;
> +static DEFINE_MUTEX(qdev_mutex);

> +/**
> + * disable_audio_stream() - disable usb snd endpoints
> + * @subs: usb substream
> + *
> + * Closes the USB SND endpoints associated with the current audio stream
> + * used.  This will decrement the USB SND endpoint opened reference count.
> + *
> + */
> +static void disable_audio_stream(struct snd_usb_substream *subs)
> +{
> +	struct snd_usb_audio *chip = subs->stream->chip;
> +
> +	if (subs->data_endpoint || subs->sync_endpoint) {
> +		close_endpoints(chip, subs);
> +
> +		mutex_lock(&chip->mutex);
> +		subs->cur_audiofmt = NULL;
> +		mutex_unlock(&chip->mutex);

can you explain why the format selection is protected by a mutex? I
don't quite get what level of concurrency might happen here?

> +	}
> +
> +	snd_usb_autosuspend(chip);
> +}
> +
> +/**
> + * enable_audio_stream() - enable usb snd endpoints
> + * @subs: usb substream
> + * @pcm_format: pcm format requested
> + * @channels: number of channels
> + * @cur_rate: sample rate
> + * @datainterval: interval
> + *
> + * Opens all USB SND endpoints used for the data interface.  This will increment
> + * the USB SND endpoint's opened count.  Requests to keep the interface resumed
> + * until the audio stream is stopped.  Will issue the USB set interface control
> + * message to enable the data interface.
> + *
> + */
> +static int enable_audio_stream(struct snd_usb_substream *subs,
> +				snd_pcm_format_t pcm_format,
> +				unsigned int channels, unsigned int cur_rate,
> +				int datainterval)
> +{
> +	struct snd_usb_audio *chip = subs->stream->chip;
> +	struct snd_pcm_hw_params params;
> +	const struct audioformat *fmt;
> +	int ret;
> +
> +	_snd_pcm_hw_params_any(&params);
> +	_snd_pcm_hw_param_set(&params, SNDRV_PCM_HW_PARAM_FORMAT,
> +			(__force int) pcm_format, 0);
> +	_snd_pcm_hw_param_set(&params, SNDRV_PCM_HW_PARAM_CHANNELS,
> +			channels, 0);
> +	_snd_pcm_hw_param_set(&params, SNDRV_PCM_HW_PARAM_RATE,
> +			cur_rate, 0);
> +
> +	pm_runtime_barrier(&chip->intf[0]->dev);
> +	snd_usb_autoresume(chip);
> +
> +	fmt = find_format(&subs->fmt_list, pcm_format, cur_rate,
> +			channels, datainterval, subs);
> +	if (!fmt) {
> +		dev_err(uaudio_qdev->dev,
> +			"cannot find format: format = %#x, rate = %d, ch = %d\n",
> +			pcm_format, cur_rate, channels);
> +		return -EINVAL;
> +	}
> +
> +	if (atomic_read(&chip->shutdown)) {
> +		dev_err(uaudio_qdev->dev, "chip already shutdown\n");
> +		ret = -ENODEV;
> +	} else {
> +		if (subs->data_endpoint)
> +			close_endpoints(chip, subs);
> +
> +		subs->data_endpoint = snd_usb_endpoint_open(chip, fmt,
> +				&params, false);
> +		if (!subs->data_endpoint) {
> +			dev_err(uaudio_qdev->dev, "failed to open data endpoint\n");
> +			return -EINVAL;
> +		}
> +
> +		if (fmt->sync_ep) {
> +			subs->sync_endpoint = snd_usb_endpoint_open(chip,
> +					fmt, &params, true);
> +			if (!subs->sync_endpoint) {
> +				dev_err(uaudio_qdev->dev,
> +					"failed to open sync endpoint\n");
> +				return -EINVAL;
> +			}
> +
> +			subs->data_endpoint->sync_source = subs->sync_endpoint;
> +		}
> +
> +		mutex_lock(&chip->mutex);
> +		subs->cur_audiofmt = fmt;
> +		mutex_unlock(&chip->mutex);
> +
> +		if (subs->sync_endpoint) {
> +			ret = snd_usb_endpoint_prepare(chip, subs->sync_endpoint);
> +			if (ret < 0)
> +				return ret;
> +		}
> +
> +		ret = snd_usb_endpoint_prepare(chip, subs->data_endpoint);
> +		if (ret < 0)
> +			return ret;

what happens in those two error cases? Should the format selected above
remain set even though the prepare failed?

> +
> +		dev_dbg(uaudio_qdev->dev,
> +			"selected %s iface:%d altsetting:%d datainterval:%dus\n",
> +			subs->direction ? "capture" : "playback",
> +			fmt->iface, fmt->altsetting,
> +			(1 << fmt->datainterval) *
> +			(subs->dev->speed >= USB_SPEED_HIGH ?
> +			BUS_INTERVAL_HIGHSPEED_AND_ABOVE :
> +			BUS_INTERVAL_FULL_SPEED));
> +	}
> +
> +	return 0;
> +}

<snip>

> diff --git a/sound/usb/qcom/usb_audio_qmi_v01.c b/sound/usb/qcom/usb_audio_qmi_v01.c
> new file mode 100644
> index 000000000000..95ae434f0a41
> --- /dev/null
> +++ b/sound/usb/qcom/usb_audio_qmi_v01.c
> @@ -0,0 +1,892 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2022 Qualcomm Innovation Center, Inc. All rights reserved.
> + */
> +
> +#include <linux/soc/qcom/qmi.h>
> +
> +#include "usb_audio_qmi_v01.h"
> +
> +static struct qmi_elem_info mem_info_v01_ei[] = {
> +	{
> +		.data_type	= QMI_UNSIGNED_8_BYTE,
> +		.elem_len	= 1,
> +		.elem_size	= sizeof(u64),
> +		.array_type	= NO_ARRAY,
> +		.tlv_type	= 0,
> +		.offset		= offsetof(struct mem_info_v01, va),
> +	},
> +	{
> +		.data_type	= QMI_UNSIGNED_8_BYTE,
> +		.elem_len	= 1,
> +		.elem_size	= sizeof(u64),
> +		.array_type	= NO_ARRAY,
> +		.tlv_type	= 0,
> +		.offset		= offsetof(struct mem_info_v01, pa),
> +	},
> +	{
> +		.data_type	= QMI_UNSIGNED_4_BYTE,
> +		.elem_len	= 1,
> +		.elem_size	= sizeof(u32),
> +		.array_type	= NO_ARRAY,
> +		.tlv_type	= 0,
> +		.offset		= offsetof(struct mem_info_v01, size),
> +	},
> +	{
> +		.data_type	= QMI_EOTI,
> +		.array_type	= NO_ARRAY,
> +		.tlv_type	= QMI_COMMON_TLV_TYPE,
> +	},

<snip>

> +	{
> +		.data_type	= QMI_EOTI,
> +		.array_type	= NO_ARRAY,
> +		.tlv_type	= QMI_COMMON_TLV_TYPE,
> +	},
> +};

Are those dozens of descriptors needed? They look mostly the same, not
sure how anyone could review this.