ALSA: snd-usb: add some documentation
Document the new streaming code and some of the functions so that contributers can catch up easier. Signed-off-by: Daniel Mack <zonque@gmail.com> Signed-off-by: Takashi Iwai <tiwai@suse.de>
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Daniel Mack
Takashi Iwai
Родитель
c75a8a7ae5
Коммит
94c27215bc
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@ -35,6 +35,32 @@
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#define EP_FLAG_ACTIVATED 0
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#define EP_FLAG_RUNNING 1
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/*
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* snd_usb_endpoint is a model that abstracts everything related to an
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* USB endpoint and its streaming.
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*
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* There are functions to activate and deactivate the streaming URBs and
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* optinal callbacks to let the pcm logic handle the actual content of the
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* packets for playback and record. Thus, the bus streaming and the audio
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* handlers are fully decoupled.
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*
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* There are two different types of endpoints in for audio applications.
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*
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* SND_USB_ENDPOINT_TYPE_DATA handles full audio data payload for both
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* inbound and outbound traffic.
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*
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* SND_USB_ENDPOINT_TYPE_SYNC are for inbound traffic only and expect the
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* payload to carry Q16.16 formatted sync information (3 or 4 bytes).
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*
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* Each endpoint has to be configured (by calling
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* snd_usb_endpoint_set_params()) before it can be used.
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*
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* The model incorporates a reference counting, so that multiple users
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* can call snd_usb_endpoint_start() and snd_usb_endpoint_stop(), and
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* only the first user will effectively start the URBs, and only the last
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* one will tear them down again.
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*/
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/*
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* convert a sampling rate into our full speed format (fs/1000 in Q16.16)
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* this will overflow at approx 524 kHz
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@ -91,6 +117,14 @@ static const char *usb_error_string(int err)
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}
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}
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/**
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* snd_usb_endpoint_implicit_feedback_sink: Report endpoint usage type
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*
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* @ep: The endpoint
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*
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* Determine whether an endpoint is driven by an implicit feedback
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* data endpoint source.
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*/
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int snd_usb_endpoint_implict_feedback_sink(struct snd_usb_endpoint *ep)
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{
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return ep->sync_master &&
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@ -99,7 +133,13 @@ int snd_usb_endpoint_implict_feedback_sink(struct snd_usb_endpoint *ep)
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usb_pipeout(ep->pipe);
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}
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/* determine the number of frames in the next packet */
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/*
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* For streaming based on information derived from sync endpoints,
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* prepare_outbound_urb_sizes() will call next_packet_size() to
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* determine the number of samples to be sent in the next packet.
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*
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* For implicit feedback, next_packet_size() is unused.
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*/
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static int next_packet_size(struct snd_usb_endpoint *ep)
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{
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unsigned long flags;
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@ -237,6 +277,19 @@ static inline void prepare_inbound_urb(struct snd_usb_endpoint *ep,
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}
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}
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/*
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* Send output urbs that have been prepared previously. Urbs are dequeued
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* from ep->ready_playback_urbs and in case there there aren't any available
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* or there are no packets that have been prepared, this function does
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* nothing.
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*
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* The reason why the functionality of sending and preparing urbs is separated
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* is that host controllers don't guarantee an ordering in returing inbound
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* and outbound packets to their submitters.
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*
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* This function is only used for implicit feedback endpoints. For endpoints
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* driven by sync endpoints, urbs are submitted from their completion handler.
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*/
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static void queue_pending_output_urbs(struct snd_usb_endpoint *ep)
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{
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while (test_bit(EP_FLAG_RUNNING, &ep->flags)) {
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@ -270,6 +323,7 @@ static void queue_pending_output_urbs(struct snd_usb_endpoint *ep)
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for (i = 0; i < packet->packets; i++)
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ctx->packet_size[i] = packet->packet_size[i];
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/* call the data handler to fill in playback data */
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prepare_outbound_urb(ep, ctx);
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err = usb_submit_urb(ctx->urb, GFP_ATOMIC);
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@ -336,6 +390,22 @@ exit_clear:
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clear_bit(ctx->index, &ep->active_mask);
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}
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/**
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* snd_usb_add_endpoint: Add an endpoint to an audio chip
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*
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* @chip: The chip
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* @alts: The USB host interface
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* @ep_num: The number of the endpoint to use
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* @direction: SNDRV_PCM_STREAM_PLAYBACK or SNDRV_PCM_STREAM_CAPTURE
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* @type: SND_USB_ENDPOINT_TYPE_DATA or SND_USB_ENDPOINT_TYPE_SYNC
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*
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* If the requested endpoint has not been added to the given chip before,
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* a new instance is created. Otherwise, a pointer to the previoulsy
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* created instance is returned. In case of any error, NULL is returned.
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*
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* New endpoints will be added to chip->ep_list and must be freed by
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* calling snd_usb_endpoint_free().
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*/
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struct snd_usb_endpoint *snd_usb_add_endpoint(struct snd_usb_audio *chip,
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struct usb_host_interface *alts,
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int ep_num, int direction, int type)
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@ -506,6 +576,9 @@ static void release_urbs(struct snd_usb_endpoint *ep, int force)
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ep->nurbs = 0;
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}
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/*
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* configure a data endpoint
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*/
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static int data_ep_set_params(struct snd_usb_endpoint *ep,
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struct snd_pcm_hw_params *hw_params,
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struct audioformat *fmt,
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@ -629,6 +702,9 @@ out_of_memory:
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return -ENOMEM;
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}
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/*
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* configure a sync endpoint
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*/
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static int sync_ep_set_params(struct snd_usb_endpoint *ep,
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struct snd_pcm_hw_params *hw_params,
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struct audioformat *fmt)
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@ -669,6 +745,15 @@ out_of_memory:
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return -ENOMEM;
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}
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/**
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* snd_usb_endpoint_set_params: configure an snd_endpoint
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*
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* @ep: the endpoint to configure
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*
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* Determine the number of of URBs to be used on this endpoint.
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* An endpoint must be configured before it can be started.
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* An endpoint that is already running can not be reconfigured.
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*/
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int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
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struct snd_pcm_hw_params *hw_params,
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struct audioformat *fmt,
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@ -717,6 +802,19 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
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return err;
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}
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/**
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* snd_usb_endpoint_start: start an snd_usb_endpoint
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*
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* @ep: the endpoint to start
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*
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* A call to this function will increment the use count of the endpoint.
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* In case this not already running, the URBs for this endpoint will be
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* submitted. Otherwise, this function does nothing.
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*
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* Must be balanced to calls of snd_usb_endpoint_stop().
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*
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* Returns an error if the URB submission failed, 0 in all other cases.
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*/
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int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
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{
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int err;
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@ -788,6 +886,17 @@ __error:
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return -EPIPE;
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}
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/**
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* snd_usb_endpoint_stop: stop an snd_usb_endpoint
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*
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* @ep: the endpoint to stop (may be NULL)
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*
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* A call to this function will decrement the use count of the endpoint.
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* In case the last user has requested the endpoint stop, the URBs will
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* actually deactivated.
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*
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* Must be balanced to calls of snd_usb_endpoint_start().
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*/
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void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep,
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int force, int can_sleep, int wait)
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{
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@ -812,6 +921,19 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep,
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}
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}
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/**
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* snd_usb_endpoint_activate: activate an snd_usb_endpoint
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*
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* @ep: the endpoint to activate
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*
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* If the endpoint is not currently in use, this functions will select the
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* correct alternate interface setting for the interface of this endpoint.
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*
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* In case of any active users, this functions does nothing.
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*
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* Returns an error if usb_set_interface() failed, 0 in all other
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* cases.
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*/
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int snd_usb_endpoint_activate(struct snd_usb_endpoint *ep)
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{
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if (ep->use_count != 0)
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@ -835,6 +957,19 @@ int snd_usb_endpoint_activate(struct snd_usb_endpoint *ep)
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return -EBUSY;
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}
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/**
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* snd_usb_endpoint_deactivate: deactivate an snd_usb_endpoint
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*
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* @ep: the endpoint to deactivate
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*
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* If the endpoint is not currently in use, this functions will select the
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* alternate interface setting 0 for the interface of this endpoint.
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*
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* In case of any active users, this functions does nothing.
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*
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* Returns an error if usb_set_interface() failed, 0 in all other
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* cases.
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*/
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int snd_usb_endpoint_deactivate(struct snd_usb_endpoint *ep)
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{
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if (!ep)
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@ -860,6 +995,13 @@ int snd_usb_endpoint_deactivate(struct snd_usb_endpoint *ep)
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return -EBUSY;
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}
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/** snd_usb_endpoint_free: Free the resources of an snd_usb_endpoint
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*
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* @ep: the list header of the endpoint to free
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*
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* This function does not care for the endpoint's use count but will tear
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* down all the streaming URBs immediately and free all resources.
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*/
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void snd_usb_endpoint_free(struct list_head *head)
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{
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struct snd_usb_endpoint *ep;
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@ -869,15 +1011,15 @@ void snd_usb_endpoint_free(struct list_head *head)
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kfree(ep);
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}
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/*
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* process after playback sync complete
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/**
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* snd_usb_handle_sync_urb: parse an USB sync packet
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*
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* Full speed devices report feedback values in 10.14 format as samples per
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* frame, high speed devices in 16.16 format as samples per microframe.
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* Because the Audio Class 1 spec was written before USB 2.0, many high speed
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* devices use a wrong interpretation, some others use an entirely different
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* format. Therefore, we cannot predict what format any particular device uses
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* and must detect it automatically.
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* @ep: the endpoint to handle the packet
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* @sender: the sending endpoint
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* @urb: the received packet
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*
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* This function is called from the context of an endpoint that received
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* the packet and is used to let another endpoint object handle the payload.
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*/
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void snd_usb_handle_sync_urb(struct snd_usb_endpoint *ep,
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struct snd_usb_endpoint *sender,
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@ -889,6 +1031,11 @@ void snd_usb_handle_sync_urb(struct snd_usb_endpoint *ep,
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snd_BUG_ON(ep == sender);
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/*
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* In case the endpoint is operating in implicit feedback mode, prepare
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* and a new outbound URB that has the same layout as the received
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* packet and add it to the list of pending urbs.
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*/
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if (snd_usb_endpoint_implict_feedback_sink(ep) &&
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ep->use_count != 0) {
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@ -938,7 +1085,20 @@ void snd_usb_handle_sync_urb(struct snd_usb_endpoint *ep,
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return;
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}
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/* parse sync endpoint packet */
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/*
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* process after playback sync complete
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*
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* Full speed devices report feedback values in 10.14 format as samples
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* per frame, high speed devices in 16.16 format as samples per
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* microframe.
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*
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* Because the Audio Class 1 spec was written before USB 2.0, many high
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* speed devices use a wrong interpretation, some others use an
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* entirely different format.
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*
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* Therefore, we cannot predict what format any particular device uses
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* and must detect it automatically.
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*/
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if (urb->iso_frame_desc[0].status != 0 ||
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urb->iso_frame_desc[0].actual_length < 3)
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