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* DMA-able FIFO interface
* Copyright (C) 2012 Peter Hurley <>
* 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
* GNU General Public License for more details.
#ifndef _DMA_FIFO_H_
#define _DMA_FIFO_H_
* The design basis for the DMA FIFO is to provide an output side that
* complies with the streaming DMA API design that can be DMA'd from directly
* (without additional copying), coupled with an input side that maintains a
* logically consistent 'apparent' size (ie, bytes in + bytes avail is static
* for the lifetime of the FIFO).
* DMA output transactions originate on a cache line boundary and can be
* variably-sized. DMA output transactions can be retired out-of-order but
* the FIFO will only advance the output in the original input sequence.
* This means the FIFO will eventually stall if a transaction is never retired.
* Chunking the output side into cache line multiples means that some FIFO
* memory is unused. For example, if all the avail input has been pended out,
* then the in and out markers are re-aligned to the next cache line.
* The maximum possible waste is
* (cache line alignment - 1) * (max outstanding dma transactions)
* This potential waste requires additional hidden capacity within the FIFO
* to be able to accept input while the 'apparent' size has not been reached.
* Additional cache lines (ie, guard area) are used to minimize DMA
* fragmentation when wrapping at the end of the FIFO. Input is allowed into the
* guard area, but the in and out FIFO markers are wrapped when DMA is pended.
#define DMA_FIFO_GUARD 3 /* # of cache lines to reserve for the guard area */
struct dma_fifo {
unsigned in;
unsigned out; /* updated when dma is pended */
unsigned done; /* updated upon dma completion */
struct {
unsigned corrupt:1;
int size; /* 'apparent' size of fifo */
int guard; /* ofs of guard area */
int capacity; /* size + reserved */
int avail; /* # of unused bytes in fifo */
unsigned align; /* must be power of 2 */
int tx_limit; /* max # of bytes per dma transaction */
int open_limit; /* max # of outstanding allowed */
int open; /* # of outstanding dma transactions */
struct list_head pending; /* fifo markers for outstanding dma */
void *data;
struct dma_pending {
struct list_head link;
void *data;
unsigned len;
unsigned next;
unsigned out;
static inline void dp_mark_completed(struct dma_pending *dp)
dp->data += 1;
static inline bool dp_is_completed(struct dma_pending *dp)
return (unsigned long)dp->data & 1UL;
void dma_fifo_init(struct dma_fifo *fifo);
int dma_fifo_alloc(struct dma_fifo *fifo, int size, unsigned align,
int tx_limit, int open_limit, gfp_t gfp_mask);
void dma_fifo_free(struct dma_fifo *fifo);
void dma_fifo_reset(struct dma_fifo *fifo);
int dma_fifo_in(struct dma_fifo *fifo, const void *src, int n);
int dma_fifo_out_pend(struct dma_fifo *fifo, struct dma_pending *pended);
int dma_fifo_out_complete(struct dma_fifo *fifo,
struct dma_pending *complete);
/* returns the # of used bytes in the fifo */
static inline int dma_fifo_level(struct dma_fifo *fifo)
return fifo->size - fifo->avail;
/* returns the # of bytes ready for output in the fifo */
static inline int dma_fifo_out_level(struct dma_fifo *fifo)
return fifo->in - fifo->out;
/* returns the # of unused bytes in the fifo */
static inline int dma_fifo_avail(struct dma_fifo *fifo)
return fifo->avail;
/* returns true if fifo has max # of outstanding dmas */
static inline bool dma_fifo_busy(struct dma_fifo *fifo)
return fifo->open == fifo->open_limit;
/* changes the max size of dma returned from dma_fifo_out_pend() */
static inline int dma_fifo_change_tx_limit(struct dma_fifo *fifo, int tx_limit)
tx_limit = round_down(tx_limit, fifo->align);
fifo->tx_limit = max_t(int, tx_limit, fifo->align);
return 0;
#endif /* _DMA_FIFO_H_ */