• Dan Williams's avatar
    async_tx: add the async_tx api · 9bc89cd8
    Dan Williams authored
    The async_tx api provides methods for describing a chain of asynchronous
    bulk memory transfers/transforms with support for inter-transactional
    dependencies.  It is implemented as a dmaengine client that smooths over
    the details of different hardware offload engine implementations.  Code
    that is written to the api can optimize for asynchronous operation and the
    api will fit the chain of operations to the available offload resources. 
     
    	I imagine that any piece of ADMA hardware would register with the
    	'async_*' subsystem, and a call to async_X would be routed as
    	appropriate, or be run in-line. - Neil Brown
    
    async_tx exploits the capabilities of struct dma_async_tx_descriptor to
    provide an api of the following general format:
    
    struct dma_async_tx_descriptor *
    async_<operation>(..., struct dma_async_tx_descriptor *depend_tx,
    			dma_async_tx_callback cb_fn, void *cb_param)
    {
    	struct dma_chan *chan = async_tx_find_channel(depend_tx, <operation>);
    	struct dma_device *device = chan ? chan->device : NULL;
    	int int_en = cb_fn ? 1 : 0;
    	struct dma_async_tx_descriptor *tx = device ?
    		device->device_prep_dma_<operation>(chan, len, int_en) : NULL;
    
    	if (tx) { /* run <operation> asynchronously */
    		...
    		tx->tx_set_dest(addr, tx, index);
    		...
    		tx->tx_set_src(addr, tx, index);
    		...
    		async_tx_submit(chan, tx, flags, depend_tx, cb_fn, cb_param);
    	} else { /* run <operation> synchronously */
    		...
    		<operation>
    		...
    		async_tx_sync_epilog(flags, depend_tx, cb_fn, cb_param);
    	}
    
    	return tx;
    }
    
    async_tx_find_channel() returns a capable channel from its pool.  The
    channel pool is organized as a per-cpu array of channel pointers.  The
    async_tx_rebalance() routine is tasked with managing these arrays.  In the
    uniprocessor case async_tx_rebalance() tries to spread responsibility
    evenly over channels of similar capabilities.  For example if there are two
    copy+xor channels, one will handle copy operations and the other will
    handle xor.  In the SMP case async_tx_rebalance() attempts to spread the
    operations evenly over the cpus, e.g. cpu0 gets copy channel0 and xor
    channel0 while cpu1 gets copy channel 1 and xor channel 1.  When a
    dependency is specified async_tx_find_channel defaults to keeping the
    operation on the same channel.  A xor->copy->xor chain will stay on one
    channel if it supports both operation types, otherwise the transaction will
    transition between a copy and a xor resource.
    
    Currently the raid5 implementation in the MD raid456 driver has been
    converted to the async_tx api.  A driver for the offload engines on the
    Intel Xscale series of I/O processors, iop-adma, is provided in a later
    commit.  With the iop-adma driver and async_tx, raid456 is able to offload
    copy, xor, and xor-zero-sum operations to hardware engines.
     
    On iop342 tiobench showed higher throughput for sequential writes (20 - 30%
    improvement) and sequential reads to a degraded array (40 - 55%
    improvement).  For the other cases performance was roughly equal, +/- a few
    percentage points.  On a x86-smp platform the performance of the async_tx
    implementation (in synchronous mode) was also +/- a few percentage points
    of the original implementation.  According to 'top' on iop342 CPU
    utilization drops from ~50% to ~15% during a 'resync' while the speed
    according to /proc/mdstat doubles from ~25 MB/s to ~50 MB/s.
     
    The tiobench command line used for testing was: tiobench --size 2048
    --block 4096 --block 131072 --dir /mnt/raid --numruns 5
    * iop342 had 1GB of memory available
    
    Details:
    * if CONFIG_DMA_ENGINE=n the asynchronous path is compiled away by making
      async_tx_find_channel a static inline routine that always returns NULL
    * when a callback is specified for a given transaction an interrupt will
      fire at operation completion time and the callback will occur in a
      tasklet.  if the the channel does not support interrupts then a live
      polling wait will be performed
    * the api is written as a dmaengine client that requests all available
      channels
    * In support of dependencies the api implicitly schedules channel-switch
      interrupts.  The interrupt triggers the cleanup tasklet which causes
      pending operations to be scheduled on the next channel
    * Xor engines treat an xor destination address differently than a software
      xor routine.  To the software routine the destination address is an implied
      source, whereas engines treat it as a write-only destination.  This patch
      modifies the xor_blocks routine to take a an explicit destination address
      to mirror the hardware.
    
    Changelog:
    * fixed a leftover debug print
    * don't allow callbacks in async_interrupt_cond
    * fixed xor_block changes
    * fixed usage of ASYNC_TX_XOR_DROP_DEST
    * drop dma mapping methods, suggested by Chris Leech
    * printk warning fixups from Andrew Morton
    * don't use inline in C files, Adrian Bunk
    * select the API when MD is enabled
    * BUG_ON xor source counts <= 1
    * implicitly handle hardware concerns like channel switching and
      interrupts, Neil Brown
    * remove the per operation type list, and distribute operation capabilities
      evenly amongst the available channels
    * simplify async_tx_find_channel to optimize the fast path
    * introduce the channel_table_initialized flag to prevent early calls to
      the api
    * reorganize the code to mimic crypto
    * include mm.h as not all archs include it in dma-mapping.h
    * make the Kconfig options non-user visible, Adrian Bunk
    * move async_tx under crypto since it is meant as 'core' functionality, and
      the two may share algorithms in the future
    * move large inline functions into c files
    * checkpatch.pl fixes
    * gpl v2 only correction
    
    Cc: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
    Acked-By: default avatarNeilBrown <neilb@suse.de>
    9bc89cd8
Kconfig 195 Bytes