Date:	Sun, 12 Jun 2016 01:01:32 +0200
From:	Henrik Austad <henrik@...tad.us>
To:	linux-kernel@...r.kernel.org
Cc:	linux-media@...r.kernel.org, alsa-devel@...r.kernel.org,
	netdev@...r.kernel.org, henrk@...tad.us,
	Henrik Austad <haustad@...co.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [very-RFC 4/8] Add TSN header for the driver

From: Henrik Austad <haustad@...co.com>

This defines the general TSN headers for network packets, the
shim-interface and the central 'tsn_list' structure.

Cc: "David S. Miller" <davem@...emloft.net>
Signed-off-by: Henrik Austad <haustad@...co.com>
---
 include/linux/tsn.h | 806 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 806 insertions(+)
 create mode 100644 include/linux/tsn.h

diff --git a/include/linux/tsn.h b/include/linux/tsn.h
new file mode 100644
index 0000000..0e1f732b
--- /dev/null
+++ b/include/linux/tsn.h
@@ -0,0 +1,806 @@
+/*   TSN - Time Sensitive Networking
+ *
+ *   Copyright (C) 2016- Henrik Austad <haustad@...co.com>
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ */
+#ifndef _TSN_H
+#define _TSN_H
+#include <linux/list.h>
+#include <linux/configfs.h>
+#include <linux/hrtimer.h>
+
+/* The naming here can be a bit confusing as we call it TSN but naming
+ * suggests 'AVB'. Reason: IEE 1722 was written before the working group
+ * was renamed to Time Sensitive Networking.
+ *
+ * To be precise. TSN describes the protocol for shipping data, AVB is a
+ * medialayer which you can build on top of TSN.
+ *
+ * For this reason the frames are given avb-names whereas the functions
+ * use tsn_-naming.
+ */
+
+/* 7 bit value 0x00 - 0x7F */
+enum avtp_subtype {
+	AVTP_61883_IIDC = 0,
+	AVTP_MMA = 0x1,
+	AVTP_MAAP = 0x7e,
+	AVTP_EXPERIMENTAL = 0x7f,
+};
+
+/*		NOTE NOTE NOTE !!
+ * The headers below use bitfields extensively and verifications
+ * are needed when using little-endian vs big-endian systems.
+ */
+
+/* Common part of avtph header
+ *
+ * AVB Transport Protocol Common Header
+ *
+ * Defined in 1722-2011 Sec. 5.2
+ */
+struct avtp_ch {
+#if defined(__LITTLE_ENDIAN_BITFIELD)
+	/* use avtp_subtype enum.
+	 */
+	u8 subtype:7;
+
+	/* Controlframe: 1
+	 * Dataframe   : 0
+	 */
+	u8 cd:1;
+
+	/* Type specific data, part 1 */
+	u8 tsd_1:4;
+
+	/* In current version of AVB, only 0 is valid, all other values
+	 * are reserved for future versions.
+	 */
+	u8 version:3;
+
+	/* Valid StreamID in frame
+	 *
+	 * ControlData not related to a specific stream should clear
+	 * this (and have stream_id = 0), _all_ other values should set
+	 * this to 1.
+	 */
+	u8 sv:1;
+#elif defined(__BIG_ENDIAN_BITFIELD)
+	u8 cd:1;
+	u8 subtype:7;
+	u8 sv:1;
+	u8 version:3;
+	u8 tsd_1:4;
+#else
+#error "Unknown Endianness, cannot determine bitfield ordering"
+#endif
+	/* Type specific data (adjacent to tsd_1, but split due to bitfield) */
+	u16 tsd_2;
+	u64 stream_id;
+
+	/*
+	 * payload by subtype
+	 */
+	u8 pbs[0];
+} __packed;
+
+/* AVTPDU Common Control header format
+ * IEEE 1722#5.3
+ */
+struct avtpc_header {
+#if defined(__LITTLE_ENDIAN_BITFIELD)
+	u8 subtype:7;
+	u8 cd:1;
+	u8 control_data:4;
+	u8 version:3;
+	u8 sv:1;
+	u16 control_data_length:11;
+	u16 status:5;
+#elif defined(__BIG_ENDIAN_BITFIELD)
+	u8 cd:1;
+	u8 subtype:7;
+	u8 sv:1;
+	u8 version:3;
+	u8 control_data:4;
+	u16 status:5;
+	u16 control_data_length:11;
+#else
+#error "Unknown Endianness, cannot determine bitfield ordering"
+#endif
+	u64 stream_id;
+} __packed;
+
+/* AVTP common stream data AVTPDU header format
+ * IEEE 1722#5.4
+ */
+struct avtpdu_header {
+#if defined(__LITTLE_ENDIAN_BITFIELD)
+	u8 subtype:7;
+	u8 cd:1;
+
+	/* avtp_timestamp valid */
+	u8 tv: 1;
+
+	/* gateway_info valid */
+	u8 gv:1;
+
+	/* reserved */
+	u8 r:1;
+
+	/*
+	 * Media clock Restart toggle
+	 */
+	u8 mr:1;
+
+	u8 version:3;
+
+	/* StreamID valid */
+	u8 sv:1;
+	u8 seqnr;
+
+	/* Timestamp uncertain */
+	u8 tu:1;
+	u8 r2:7;
+#elif defined(__BIG_ENDIAN_BITFIELD)
+	u8 cd:1;
+	u8 subtype:7;
+
+	u8 sv:1;
+	u8 version:3;
+	u8 mr:1;
+	u8 r:1;
+	u8 gv:1;
+	u8 tv: 1;
+
+	u8 seqnr;
+	u8 r2:7;
+	u8 tu:1;
+#else
+#error "Unknown Endianness, cannot determine bitfield ordering"
+#endif
+
+	u64 stream_id;
+
+	u32 avtp_timestamp;
+	u32 gateway_info;
+
+	/* Stream Data Length */
+	u16 sd_len;
+
+	/* Protocol specific header, derived from avtp_subtype */
+	u16 psh;
+
+	/* Stream Payload Data 0 to n octets
+	 * n so that total size < MTU
+	 */
+	u8 data[0];
+} __packed;
+
+
+/**
+ * struct tsn_list - The top level container of TSN
+ *
+ * This is what tsn_configfs refers to as 'tier-0'
+ *
+ * @head List of TSN cards
+ * @lock lock protecting global entries
+ * @tsn_subsys Ref to ConfigFS subsystem
+ *
+ * @running: hrtimer is running driving data out
+ * @tsn_timer: hrtimer container
+ * @num_avail Number of available TSN NICs exposed through ConfigFS
+ */
+struct tsn_list {
+	struct list_head head;
+	struct mutex lock;
+	struct configfs_subsystem tsn_subsys;
+
+	/*
+	 * TSN-timer is running. Not to be confused with the per-link
+	 * disabled flag which indicates if a remote client, like aplay,
+	 * is pushing data to it.
+	 */
+	atomic_t running;
+	struct hrtimer tsn_timer;
+	unsigned int period_ns;
+
+
+	size_t num_avail;
+};
+
+/**
+ * struct tsn_nic
+ *
+ * Individual TSN-capable NICs, or 'tier-1' struct
+ *
+ * @list linked list of all TSN NICs
+ * @group configfs group
+ * @dev corresponding net_device
+ * @dma_size : size of the DMA buffer
+ * @dma_handle: housekeeping DMA-stuff
+ * @dma_mem : pointer to memory region we're using for DMAing to the NIC
+ * @name Name of NIC (same as name in dev), TO BE REMOVED
+ * @txq Size of Tx-queue. TO BE REMOVED
+ * @rx_registered flag indicating if a handler is registered for the nic
+ * @capable: if the NIC is capable for proper TSN traffic or if it must
+ *	     be emulated in software.
+ *
+ */
+struct tsn_nic {
+	struct list_head list;
+	struct config_group group;
+	struct net_device *dev;
+	struct tsn_list *tsn_list;
+
+	size_t dma_size;
+	dma_addr_t dma_handle;
+	void *dma_mem;
+
+	char *name;
+	int txq;
+	u8 rx_registered:1;
+	u8 capable:1;
+	u8 reserved:6;
+};
+
+struct tsn_shim_ops;
+/**
+ * tsn_link - Structure describing a single TSN link
+ *
+ */
+struct tsn_link {
+	/*
+	 * Lock for protecting the buffer
+	 */
+	spinlock_t lock;
+
+	struct config_group group;
+	struct tsn_nic *nic;
+	struct hlist_node node;
+
+	/* The link itself is active, and the tsn_core will treat it as
+	 * an active participant and feed data from it to the
+	 * network. This places some restrictions on which attributes
+	 * can be changed.
+	 *
+	 * 1: active
+	 * 0: inactive
+	 */
+	atomic_t active;
+
+	u64 timer_period_ns;
+
+	/* Pointer to media-specific data.
+	 * e.g. struct avb_chip
+	 */
+	void *media_chip;
+
+	u64 stream_id;
+
+	/*
+	 * The max required size for a _single_ TSN frame.
+	 *
+	 * To be used instead of channels and sample_freq.
+	 */
+	u16 max_payload_size;
+	u16 shim_header_size;
+
+	/*
+	 * Size of buffer (in bytes) to use when handling data to/from
+	 * NIC.
+	 *
+	 * Smaller size will result in client being called more often
+	 * but also provides lower latencies.
+	 */
+	size_t buffer_size;
+	size_t used_buffer_size;
+
+	/*
+	 * Used when frames are constructed and shipped to the network
+	 * layer. If this is true, 0-frames will be sent insted of data
+	 * from the buffer.
+	 */
+	atomic_t buffer_active;
+
+	/*
+	 * ringbuffer for incoming or outging traffic
+	 * +-----------------------------------+
+	 * |                  ##########       |
+	 * +-----------------------------------+
+	 * ^                  ^         ^      ^
+	 * buffer           tail      head    end
+	 *
+	 * Buffer: start of memory area
+	 * tail: first byte of data in buffer
+	 * head: first unused slot in which to store new data
+	 *
+	 * head,tail is used to represent the position of 'live data' in
+	 * the buffer.
+	 */
+	void *buffer;
+	void *head;
+	void *tail;
+	void *end;
+
+	/* Number of bytes to run refill/drain callbacks */
+	size_t low_water_mark;
+	size_t high_water_mark;
+
+
+	/*
+	 * callback ops.
+	 */
+	struct tsn_shim_ops *ops;
+
+	/*
+	 * EndStation Type
+	 *
+	 * Either Talker or Listener
+	 *
+	 * 1: We are *Talker*, i.e. producing data to send
+	 * 0: We are *Listener*, i.e. we receive data from another ES.
+	 *
+	 * This is for a single link, so even though an end-station can
+	 * be both Talker *and* Listener, a link can only be one.
+	 */
+	u8 estype_talker;
+
+	/*
+	 * Link will use buffer managed by the shim. For this to work,
+	 * the shim must:
+	 *
+	 * - call tsn_use_external_buffer(link, size);
+	 * - provide tsn_shim_buffer_swap(link) in tsn_shim_ops
+	 */
+	u8 external_buffer;
+
+	u8 last_seqnr;
+
+	/*
+	 * Class can be either A or B
+	 *
+	 * ClassA: every 125us
+	 * ClassB: every 250us
+	 *
+	 * This will also affect how large each frame will be.
+	 */
+	u8 class_a:1;
+
+	/*
+	 * Any AVTP data stream must set the 802.1Q vlan id and priority
+	 * Code point. This should be obtained from MSRP, default values
+	 * are:
+	 *
+	 * pvid: SR_PVID 2
+	 * pcp: Class A: 3
+	 *	Class B: 2
+	 *
+	 * See IEEE 802.1Q-2011, Sec 35.2.2.9.3 and table 6-6 in 6.6.2
+	 * for details
+	 */
+	u8 pcp_a:3;
+	u8 pcp_b:3;
+	u16 vlan_id:12;
+
+	u8 remote_mac[6];
+};
+
+/**
+ * tsn_link_on - make link active
+ *
+ * This cause most of the attributes to be treated read-only since we
+ * will have to re-negotiate with the network if most of these
+ * parameters change.
+ *
+ * Note: this means that the link will be handled by the rx-handler or
+ * the timer callback, but until the link_buffer is set active (via
+ * tsn_lb_on()), actual data is not moved.
+ *
+ * @link: link being set to active
+ */
+static inline void tsn_link_on(struct tsn_link *link)
+{
+	if (link)
+		atomic_set(&link->active, 1);
+}
+
+/**
+ * tsn_link_off - make link inactive
+ *
+ * The link will now be ignored by timer callback or the
+ * rx-handler. Attributes can be mostly freely changed (we assume that
+ * userspace sets values that are negotiated properly).
+ *
+ * @link: link to deactivate
+ */
+static inline void tsn_link_off(struct tsn_link *link)
+{
+	if (link)
+		atomic_set(&link->active, 0);
+}
+
+/**
+ * tsn_link_is_on - query link to see if it is active
+ *
+ * Mostly used by tsn_configfs to respect the "read-only" once link is
+ * configured and made active.
+ *
+ * @link active link
+ * @returns 1 if active/on, 0 otherwise
+ */
+static inline int tsn_link_is_on(struct tsn_link *link)
+{
+	if (link)
+		return atomic_read(&link->active);
+	return 0;
+}
+
+/**
+ * tsn_set_buffer_size - adjust buffersize to match a shim
+ *
+ * This will not allocate (or deallcoate) memory, just adjust how much
+ * of the buffer allocated in tsn_prepare_link is being used. tsn_
+ * expects tsn_clear_buffer_size() to be invoked when stream is closed.
+ */
+int tsn_set_buffer_size(struct tsn_link *link, size_t bsize);
+int tsn_clear_buffer_size(struct tsn_link *link);
+
+/**
+ * tsn_buffer_write write data into the buffer from shim
+ *
+ * This is called from the shim-driver when more data is available and
+ * data needs to be pushed out to the network.
+ *
+ * NOTE: This is used when TSN handles the databuffer. This will not be
+ *	 needed for "shim-hosted" buffers.
+ *
+ * _If_ this function is called when the link is inactive, it will
+ * _enable_ the link (i.e. link will mark the buffer as 'active'). Do
+ * not copy data into the buffer unless you are ready to start sending
+ * frames!
+ *
+ * @link active link
+ * @src	the buffer to copy data from
+ * @bytes bytes to copy
+ * @return bytes copied from link->buffer or negative error
+ */
+int tsn_buffer_write(struct tsn_link *link, void *src, size_t bytes);
+
+
+/**
+ * tsn_buffer_read - read data from link->buffer and give to shim
+ *
+ * When we act as a listener, this is what the shim (should|will) call
+ * to grab data. It typically grabs much more data than the _net
+ * equivalent. It also do not trigger a refill-event the same way
+ * buffer_read_net does.
+ *
+ * @param link current link that holds the buffer
+ * @param buffer the buffer to copy into, must be at least of size bytes
+ * @param bytes number of bytes.
+ *
+ * Note that this routine does NOT CARE about channels, samplesize etc,
+ * it is a _pure_ copy that handles ringbuffer wraps etc.
+ *
+ * This function have side-effects as it will update internal tsn_link
+ * values.
+ *
+ * @return Bytes copied into link->buffer, negative value upon error.
+ */
+int tsn_buffer_read(struct tsn_link *link, void *buffer, size_t bytes);
+
+/**
+ * tsn_lb_enable - TSN Link Buffer Enable
+ *
+ * Mark the link as "buffer-enabled" which will let the core start
+ * shifting data in/out of the buffer instead of ignoring incoming
+ * frames or sending "nullframes".
+ *
+ * This is for the network-end of the tsn-buffer, i.e.
+ * - when enabled frames *from* the network will be inserted into the buffer,
+ * - or frames going *out* will include data from the buffer instead of sending
+ *   null-frames.
+ *
+ * When disabled, data will be zero'd, e.g Tx will send NULL-frames and
+ * Rx will silently drop the frames.
+ *
+ * @link: active link
+ */
+static inline void tsn_lb_enable(struct tsn_link *link)
+{
+	if (link)
+		atomic_set(&link->buffer_active, 1);
+}
+
+/**
+ * tsn_lb_disable - stop using the buffer for the net-side of TSN
+ *
+ * When we close a stream, we do not necessarily tear down the link, and
+ * we need to handle the data in some way.
+ */
+static inline void tsn_lb_disable(struct tsn_link *link)
+{
+	if (link)
+		atomic_set(&link->buffer_active, 0);
+}
+
+/**
+ * tsn_lb() - query if we have disabled pushing of data to/from link-buffer
+ *
+ * @param struct tsn_link *link - active link
+ * @returns 1 if link is enabled
+ */
+static inline int tsn_lb(struct tsn_link *link)
+{
+	if (link)
+		return atomic_read(&link->buffer_active);
+
+	/* if link is NULL; buffer not active */
+	return 0;
+}
+
+
+/**
+ * Shim ops - what tsn_core use when calling back into the shim. All ops
+ * must be reentrant.
+ */
+#define SHIM_NAME_SIZE 32
+struct tsn_shim_ops {
+
+	/* internal linked list used by tsn_core to keep track of all
+	 * shims.
+	 */
+	struct list_head head;
+
+	/**
+	 * name - a unique name identifying this shim
+	 *
+	 * This is what userspace use to indicate to core what SHIM a
+	 * particular link will use. If the name is already present,
+	 * core will reject this name.
+	 */
+	char shim_name[SHIM_NAME_SIZE];
+
+	/**
+	 * probe - callback when a new link of this type is instantiated.
+	 *
+	 * When a new link is brought online, this is called once the
+	 * essential parts of tsn_core has finiesh. Once probe_cb has
+	 * finisehd, the shim _must_ be ready to accept data to/from
+	 * tsn_core. On the other hand, due to the final steps of setup,
+	 * it cannot expect to be called into action immediately after
+	 * probe has finished.
+	 *
+	 * In other words, shim must be ready, but core doesn't have to
+	 *
+	 * @param : a particular link to pass along to the probe-function.
+	 */
+	int (*probe)(struct tsn_link *link);
+
+	/**
+	 * buffer_swap - set a new buffer for the link. [OPTIONAL]
+	 *
+	 * Used when external buffering is enabled.
+	 *
+	 * When called, a new buffer must be returned WITHOUT blocking
+	 * as this will be called from interrupt context.
+	 *
+	 * The buffer returned from the shim must be at least the size
+	 * of used_buffer_size.
+	 *
+	 * @param current link
+	 * @param old_buffer the buffer that are no longer needed
+	 * @param used number of bytes in buffer that has been filled with data.
+	 * @return new buffer to use
+	 */
+	void * (*buffer_swap)(struct tsn_link *link, void *old_buffer,
+			      size_t used);
+
+	/**
+	 * buffer_refill - signal shim that more data is required
+	 * @link Active link
+	 *
+	 * This function should not do anything that can preempt the
+	 * task (kmalloc, sleeping lock) or invoke actions that can take
+	 * a long time to complete.
+	 *
+	 * This will be called from tsn_buffer_read_net() when available
+	 * data in the buffer drops below low_water_mark. It will be
+	 * called with the link-lock *held*
+	 */
+	size_t (*buffer_refill)(struct tsn_link *link);
+
+	/**
+	 * buffer_drain - shim need to copy data from buffer
+	 *
+	 * This will be called from tsn_buffer_write_net() when data in
+	 * the buffer exceeds high_water_mark.
+	 *
+	 * The expected behavior is for the shim to then fill data into
+	 * the buffer via tsn_buffer_write()
+	 */
+	size_t (*buffer_drain)(struct tsn_link *link);
+
+	/**
+	 * media_close - shut down media controller properly
+	 *
+	 * when the link is closed/removed for some reason
+	 * external to the media controller (ALSA soundcard, v4l2 driver
+	 * etc), we call this to clean up.
+	 *
+	 * Normal operation is stopped before media_close is called, but
+	 * all references should be valid. TSN core expects media_close
+	 * to handle any local cleanup, once returned, any references in
+	 * stale tsn_links cannot be trusted.
+	 *
+	 * @link: current link where data is stored
+	 * @returns: 0 upon success, negative on error.
+	 */
+	int (*media_close)(struct tsn_link *link);
+
+	/**
+	 * hdr_size - ask shim how large the header is
+	 *
+	 * Needed when reserving space in skb for transmitting data.
+	 *
+	 * @link: current link where data is stored
+	 * @return: size of header for this shim
+	 */
+	size_t (*hdr_size)(struct tsn_link *link);
+
+	/**
+	 * copy_size - ask client how much from the buffer to include in
+	 *	       the next frame.
+	 *
+	 *	       This is for *outgoing* frames, incoming frames
+	 *	       have 'sd_len' set in the header.
+	 *
+	 *	       Note: copy_size should not return a size larger
+	 *		     than link->max_payload_size
+	 */
+	size_t (*copy_size)(struct tsn_link *link);
+
+	/**
+	 * validate_header - let the shim validate subtype-header
+	 *
+	 * Both psh and data may (or may not) contain headers that need
+	 * validating. This is the responsibility of the shim to
+	 * validate, and ops->valdiate_header() will be called before
+	 * any data is copied from the incoming frame and into the
+	 * buffer.
+	 *
+	 * Important: tsn_core expects validate_header to _not_ alter
+	 * the contents of the frame, and ideally, validate_header could
+	 * be called multiple times and give the same result.
+	 *
+	 * @param: active link owning the new data
+	 * @param: start of data-unit header
+	 *
+	 * This function will be called from interrupt-context and MUST
+	 * NOT take any locks.
+	 */
+	int (*validate_header)(struct tsn_link *link,
+			       struct avtpdu_header *header);
+
+	/**
+	 * assemble_header - add shim-specific headers
+	 *
+	 * This adds the headers required by the current shim after the
+	 * generic 1722-header.
+	 *
+	 * @param: active link
+	 * @param: start of data-unit header
+	 * @param: size of data to send in this frame
+	 * @return void
+	 */
+	void (*assemble_header)(struct tsn_link *link,
+				struct avtpdu_header *header, size_t bytes);
+
+	/**
+	 * get_payload_data - get a pointer to where the data is stored
+	 *
+	 * core will use the pointer (or drop it if NULL is returned)
+	 * and copy header->sd_len bytes of *consecutive* data from the
+	 * target memory and into the buffer memory.
+	 *
+	 * This is called with relevant locks held, from interrupt context.
+	 *
+	 * @param link active link
+	 * @param header header of frame, which contains data
+	 * @returns pointer to memory to copy from
+	 */
+	void * (*get_payload_data)(struct tsn_link *link,
+				   struct avtpdu_header *header);
+};
+/**
+ * tsn_shim_register_ops - register shim-callbacks for a given shim
+ *
+ * @param shim_ops - callbacks. The ops-struct should be kept intact for
+ *		     as long as the driver is running.
+ *
+ *
+ */
+int tsn_shim_register_ops(struct tsn_shim_ops *shim_ops);
+
+/**
+ * tsn_shim_deregister_ops - remove callback for module
+ *
+ * Completely remove shim_ops. This will close any links currently using
+ * this shim. Note: the links will be closed, but _not_ removed.
+ *
+ * @param shim_ops ops associated with this shim
+ */
+void tsn_shim_deregister_ops(struct tsn_shim_ops *shim_ops);
+
+/**
+ * tsn_shim_get_active : return the name of the currently loaded shim
+ *
+ * @param current link
+ * @return name of shim (matches an entry from exported triggers)
+ */
+char *tsn_shim_get_active(struct tsn_link *link);
+
+/**
+ * tsn_shim_find_by_name find shim_ops by name
+ *
+ * @param name of shim
+ * @return shim or NULL if not found/error.
+ */
+struct tsn_shim_ops *tsn_shim_find_by_name(const char *name);
+
+/**
+ * tsn_shim_export_probe_triggers - export a list of registered shims
+ *
+ * @param page to write content into
+ * @returns length of data written to page
+ */
+ssize_t tsn_shim_export_probe_triggers(char *page);
+
+/**
+ * tsn_get_framesize - get the size of the next TSN frame to send
+ *
+ * This will call into the shim to get the next chunk of data to
+ * read. Some sanitychecking is performed, i.e.
+ *
+ * 0 <= size <= max_payload_size
+ *
+ * @param struct tsn_link *link active link
+ * @returns size of frame in bytes or negative on error.
+ */
+static inline size_t tsn_shim_get_framesize(struct tsn_link *link)
+{
+	size_t ret;
+
+	ret = link->ops->copy_size(link);
+	if (ret <= link->max_payload_size)
+		return ret;
+	return link->max_payload_size;
+}
+
+/**
+ * tsn_get_hdr_size - get the size of the shim-specific header size
+ *
+ * The shim will add it's own header to the frame.
+ */
+static inline size_t tsn_shim_get_hdr_size(struct tsn_link *link)
+{
+	size_t ret;
+
+	if (!link || !link->ops->hdr_size)
+		return -EINVAL;
+	ret = link->ops->hdr_size(link);
+	if (ret > link->max_payload_size)
+		return -EINVAL;
+	return ret;
+}
+
+#endif	/* _TSN_H */
-- 
2.7.4

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