From 28405d8d9ce05f5bd869ef8b48da5086f9527d73 Mon Sep 17 00:00:00 2001 From: Dan Williams Date: Mon, 5 Jan 2009 17:14:31 -0700 Subject: [PATCH] async_tx, dmaengine: document channel allocation and api rework "Wouldn't it be better if the dmaengine layer made sure it didn't pass the same channel several times to a client? I mean, you seem concerned that the memcpy() API should be transparent and easy to use, but the whole registration interface is just ridiculously complicated..." - Haavard The dmaengine and async_tx registration/allocation interface is indeed needlessly complicated. This redesign has the following goals: 1/ Simplify reference counting: dma channels are not something one would expect to be hotplugged, it should be an exceptional event handled by drivers not something clients should be mandated to handle in a callback. The common case channel removal event is 'rmmod ', which for simplicity should be disallowed if the channel is in use. 2/ Add an interface for requesting exclusive access to a channel suitable to device-to-memory users. 3/ Convert all memory-to-memory users over to a common allocator, the goal here is to not have competing channel allocation schemes. The only competition should be between device-to-memory exclusive allocations and the memory-to-memory usage case where channels are shared between multiple "clients". Cc: Haavard Skinnemoen Cc: Neil Brown Cc: Jeff Garzik Reviewed-by: Andrew Morton Signed-off-by: Dan Williams --- Documentation/crypto/async-tx-api.txt | 96 ++++++++++++++++------------------- Documentation/dmaengine.txt | 1 + 2 files changed, 45 insertions(+), 52 deletions(-) create mode 100644 Documentation/dmaengine.txt diff --git a/Documentation/crypto/async-tx-api.txt b/Documentation/crypto/async-tx-api.txt index c1e9545..9f59fcb 100644 --- a/Documentation/crypto/async-tx-api.txt +++ b/Documentation/crypto/async-tx-api.txt @@ -13,9 +13,9 @@ 3.6 Constraints 3.7 Example -4 DRIVER DEVELOPER NOTES +4 DMAENGINE DRIVER DEVELOPER NOTES 4.1 Conformance points -4.2 "My application needs finer control of hardware channels" +4.2 "My application needs exclusive control of hardware channels" 5 SOURCE @@ -150,6 +150,7 @@ ops_run_* and ops_complete_* routines in drivers/md/raid5.c for more implementation examples. 4 DRIVER DEVELOPMENT NOTES + 4.1 Conformance points: There are a few conformance points required in dmaengine drivers to accommodate assumptions made by applications using the async_tx API: @@ -158,58 +159,49 @@ accommodate assumptions made by applications using the async_tx API: 3/ Use async_tx_run_dependencies() in the descriptor clean up path to handle submission of dependent operations -4.2 "My application needs finer control of hardware channels" -This requirement seems to arise from cases where a DMA engine driver is -trying to support device-to-memory DMA. The dmaengine and async_tx -implementations were designed for offloading memory-to-memory -operations; however, there are some capabilities of the dmaengine layer -that can be used for platform-specific channel management. -Platform-specific constraints can be handled by registering the -application as a 'dma_client' and implementing a 'dma_event_callback' to -apply a filter to the available channels in the system. Before showing -how to implement a custom dma_event callback some background of -dmaengine's client support is required. - -The following routines in dmaengine support multiple clients requesting -use of a channel: -- dma_async_client_register(struct dma_client *client) -- dma_async_client_chan_request(struct dma_client *client) - -dma_async_client_register takes a pointer to an initialized dma_client -structure. It expects that the 'event_callback' and 'cap_mask' fields -are already initialized. - -dma_async_client_chan_request triggers dmaengine to notify the client of -all channels that satisfy the capability mask. It is up to the client's -event_callback routine to track how many channels the client needs and -how many it is currently using. The dma_event_callback routine returns a -dma_state_client code to let dmaengine know the status of the -allocation. - -Below is the example of how to extend this functionality for -platform-specific filtering of the available channels beyond the -standard capability mask: - -static enum dma_state_client -my_dma_client_callback(struct dma_client *client, - struct dma_chan *chan, enum dma_state state) -{ - struct dma_device *dma_dev; - struct my_platform_specific_dma *plat_dma_dev; - - dma_dev = chan->device; - plat_dma_dev = container_of(dma_dev, - struct my_platform_specific_dma, - dma_dev); - - if (!plat_dma_dev->platform_specific_capability) - return DMA_DUP; - - . . . -} +4.2 "My application needs exclusive control of hardware channels" +Primarily this requirement arises from cases where a DMA engine driver +is being used to support device-to-memory operations. A channel that is +performing these operations cannot, for many platform specific reasons, +be shared. For these cases the dma_request_channel() interface is +provided. + +The interface is: +struct dma_chan *dma_request_channel(dma_cap_mask_t mask, + dma_filter_fn filter_fn, + void *filter_param); + +Where dma_filter_fn is defined as: +typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param); + +When the optional 'filter_fn' parameter is set to NULL +dma_request_channel simply returns the first channel that satisfies the +capability mask. Otherwise, when the mask parameter is insufficient for +specifying the necessary channel, the filter_fn routine can be used to +disposition the available channels in the system. The filter_fn routine +is called once for each free channel in the system. Upon seeing a +suitable channel filter_fn returns DMA_ACK which flags that channel to +be the return value from dma_request_channel. A channel allocated via +this interface is exclusive to the caller, until dma_release_channel() +is called. + +The DMA_PRIVATE capability flag is used to tag dma devices that should +not be used by the general-purpose allocator. It can be set at +initialization time if it is known that a channel will always be +private. Alternatively, it is set when dma_request_channel() finds an +unused "public" channel. + +A couple caveats to note when implementing a driver and consumer: +1/ Once a channel has been privately allocated it will no longer be + considered by the general-purpose allocator even after a call to + dma_release_channel(). +2/ Since capabilities are specified at the device level a dma_device + with multiple channels will either have all channels public, or all + channels private. 5 SOURCE -include/linux/dmaengine.h: core header file for DMA drivers and clients + +include/linux/dmaengine.h: core header file for DMA drivers and api users drivers/dma/dmaengine.c: offload engine channel management routines drivers/dma/: location for offload engine drivers include/linux/async_tx.h: core header file for the async_tx api diff --git a/Documentation/dmaengine.txt b/Documentation/dmaengine.txt new file mode 100644 index 0000000..0c1c2f6 --- /dev/null +++ b/Documentation/dmaengine.txt @@ -0,0 +1 @@ +See Documentation/crypto/async-tx-api.txt -- 2.7.4