Part I - DMA API¶

To get the DMA API, you must #include <linux/dma-mapping.h>. Thisprovides dma_addr_t and the interfaces described below.

A dma_addr_t can hold any valid DMA address for the platform. It can begiven to a device to use as a DMA source or target. A CPU cannot referencea dma_addr_t directly because there may be translation between its physicaladdress space and the DMA address space.

Part Ia - Using large DMA-coherent buffers¶

void *dma_alloc_coherent(struct device *dev, size_t size,                   dma_addr_t *dma_handle, gfp_t flag)

Coherent memory is memory for which a write by either the device orthe processor can immediately be read by the processor or devicewithout having to worry about caching effects. (You may however needto make sure to flush the processor’s write buffers before tellingdevices to read that memory.)

This routine allocates a region of <size> bytes of coherent memory.

It returns a pointer to the allocated region (in the processor’s virtualaddress space) or NULL if the allocation failed.

It also returns a <dma_handle> which may be cast to an unsigned integer thesame width as the bus and given to the device as the DMA address base ofthe region.

Note: coherent memory can be expensive on some platforms, and theminimum allocation length may be as big as a page, so you shouldconsolidate your requests for coherent memory as much as possible.The simplest way to do that is to use the dma_pool calls (see below).

The flag parameter allows the caller to specify theGFP_ flags (seekmalloc()) for the allocation (the implementation may ignore flags that affectthe location of the returned memory, like GFP_DMA).

voiddma_free_coherent(struct device *dev, size_t size, void *cpu_addr,                  dma_addr_t dma_handle)

Free a previously allocated region of coherent memory. dev, size and dma_handlemust all be the same as those passed intodma_alloc_coherent(). cpu_addr mustbe the virtual address returned bydma_alloc_coherent().

Note that unlike the sibling allocation call, this routine may only be calledwith IRQs enabled.

Part Ib - Using small DMA-coherent buffers¶

To get this part of the DMA API, you must #include <linux/dmapool.h>

Many drivers need lots of small DMA-coherent memory regions for DMAdescriptors or I/O buffers. Rather than allocating in units of a pageor more usingdma_alloc_coherent(), you can use DMA pools. These workmuch like astructkmem_cache, except that they use the DMA-coherent allocator,not__get_free_pages(). Also, they understand common hardware constraintsfor alignment, like queue heads needing to be aligned on N-byte boundaries.

structdma_pool*dma_pool_create_node(constchar*name,structdevice*dev,size_tsize,size_talign,size_tboundary,intnode)¶

Creates a pool of coherent DMA memory blocks.

voiddma_pool_destroy(structdma_pool*pool)¶

destroys a pool of dma memory blocks.

void*dma_pool_alloc(structdma_pool*pool,gfp_tmem_flags,dma_addr_t*handle)¶

get a block of coherent memory

voiddma_pool_free(structdma_pool*pool,void*vaddr,dma_addr_tdma)¶

put block back into dma pool

structdma_pool*dmam_pool_create(constchar*name,structdevice*dev,size_tsize,size_talign,size_tallocation)¶

Manageddma_pool_create()

voiddmam_pool_destroy(structdma_pool*pool)¶

Manageddma_pool_destroy()

void*dma_pool_zalloc(structdma_pool*pool,gfp_tmem_flags,dma_addr_t*handle)¶

Get a zero-initialized block of DMA coherent memory.

Part Ic - DMA addressing limitations¶

DMA mask is a bit mask of the addressable region for the device. In other words,if applying the DMA mask (a bitwise AND operation) to the DMA address of amemory region does not clear any bits in the address, then the device canperform DMA to that memory region.

All the below functions which set a DMA mask may fail if the requested maskcannot be used with the device, or if the device is not capable of doing DMA.

intdma_set_mask_and_coherent(struct device *dev, u64 mask)

Updates both streaming and coherent DMA masks.

Returns: 0 if successful and a negative error if not.

intdma_set_mask(struct device *dev, u64 mask)

Updates only the streaming DMA mask.

Returns: 0 if successful and a negative error if not.

intdma_set_coherent_mask(struct device *dev, u64 mask)

Updates only the coherent DMA mask.

Returns: 0 if successful and a negative error if not.

u64dma_get_required_mask(struct device *dev)

This API returns the mask that the platform requires tooperate efficiently. Usually this means the returned maskis the minimum required to cover all of memory. Examining therequired mask gives drivers with variable descriptor sizes theopportunity to use smaller descriptors as necessary.

Requesting the required mask does not alter the current mask. If youwish to take advantage of it, you should issue adma_set_mask()call to set the mask to the value returned.

size_tdma_max_mapping_size(struct device *dev);

Returns the maximum size of a mapping for the device. The size parameterof the mapping functions likedma_map_single(),dma_map_page() andothers should not be larger than the returned value.

size_tdma_opt_mapping_size(struct device *dev);

Returns the maximum optimal size of a mapping for the device.

Mapping larger buffers may take much longer in certain scenarios. Inaddition, for high-rate short-lived streaming mappings, the upfront timespent on the mapping may account for an appreciable part of the totalrequest lifetime. As such, if splitting larger requests incurs nosignificant performance penalty, then device drivers are advised tolimit total DMA streaming mappings length to the returned value.

booldma_need_sync(struct device *dev, dma_addr_t dma_addr);

Returns %true if dma_sync_single_for_{device,cpu} calls are required totransfer memory ownership. Returns %false if those calls can be skipped.

unsigned longdma_get_merge_boundary(struct device *dev);

Returns the DMA merge boundary. If the device cannot merge any DMA addresssegments, the function returns 0.

Part Id - Streaming DMA mappings¶

Streaming DMA allows to map an existing buffer for DMA transfers and thenunmap it when finished. Map functions are not guaranteed to succeed, so thereturn value must be checked.

dma_addr_tdma_map_single(struct device *dev, void *cpu_addr, size_t size,               enum dma_data_direction direction)

Maps a piece of processor virtual memory so it can be accessed by thedevice and returns the DMA address of the memory.

The DMA API uses a strongly typed enumerator for its direction:

voiddma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,                 enum dma_data_direction direction)

Unmaps the region previously mapped. All the parameters passed inmust be identical to those passed to (and returned by)dma_map_single().

dma_addr_tdma_map_page(struct device *dev, struct page *page,             unsigned long offset, size_t size,             enum dma_data_direction direction)voiddma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,               enum dma_data_direction direction)

API for mapping and unmapping for pages. All the notes and warningsfor the other mapping APIs apply here. Also, although the <offset>and <size> parameters are provided to do partial page mapping, it isrecommended that you never use these unless you really know what thecache width is.

dma_addr_tdma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,                 enum dma_data_direction dir, unsigned long attrs)voiddma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,                   enum dma_data_direction dir, unsigned long attrs)

API for mapping and unmapping for MMIO resources. All the notes andwarnings for the other mapping APIs apply here. The API should only beused to map device MMIO resources, mapping of RAM is not permitted.

intdma_mapping_error(struct device *dev, dma_addr_t dma_addr)

In some circumstancesdma_map_single(),dma_map_page() anddma_map_resource()will fail to create a mapping. A driver can check for these errors by testingthe returned DMA address withdma_mapping_error(). A non-zero return valuemeans the mapping could not be created and the driver should take appropriateaction (e.g. reduce current DMA mapping usage or delay and try again later).

intdma_map_sg(struct device *dev, struct scatterlist *sg,           int nents, enum dma_data_direction direction)

Maps a scatter/gather list for DMA. Returns the number of DMA address segmentsmapped, which may be smaller than <nents> passed in if several consecutivesglist entries are merged (e.g. with an IOMMU, or if some adjacent segmentsjust happen to be physically contiguous).

Please note that the sg cannot be mapped again if it has been mapped once.The mapping process is allowed to destroy information in the sg.

As with the other mapping interfaces,dma_map_sg() can fail. When itdoes, 0 is returned and a driver must take appropriate action. It iscritical that the driver do something, in the case of a block driveraborting the request or even oopsing is better than doing nothing andcorrupting the filesystem.

With scatterlists, you use the resulting mapping like this:

int i, count = dma_map_sg(dev, sglist, nents, direction);struct scatterlist *sg;for_each_sg(sglist, sg, count, i) {        hw_address[i] = sg_dma_address(sg);        hw_len[i] = sg_dma_len(sg);}

where nents is the number of entries in the sglist.

The implementation is free to merge several consecutive sglist entriesinto one. The returned number is the actual number of sg entries itmapped them to. On failure, 0 is returned.

Then you should loop count times (note: this can be less than nents times)and usesg_dma_address() andsg_dma_len() macros where you previouslyaccessed sg->address and sg->length as shown above.

voiddma_unmap_sg(struct device *dev, struct scatterlist *sg,             int nents, enum dma_data_direction direction)

Unmap the previously mapped scatter/gather list. All the parametersmust be the same as those and passed in to the scatter/gather mappingAPI.

Note: <nents> must be the number you passed in,not the number ofDMA address entries returned.

voiddma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle,                        size_t size,                        enum dma_data_direction direction)voiddma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle,                           size_t size,                           enum dma_data_direction direction)voiddma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,                    int nents,                    enum dma_data_direction direction)voiddma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,                       int nents,                       enum dma_data_direction direction)

Synchronise a single contiguous or scatter/gather mapping for the CPUand device. With the sync_sg API, all the parameters must be the sameas those passed into the sg mapping API. With the sync_single API,you can use dma_handle and size parameters that aren’t identical tothose passed into the single mapping API to do a partial sync.

See alsodma_map_single().

dma_addr_tdma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,                     enum dma_data_direction dir,                     unsigned long attrs)voiddma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,                       size_t size, enum dma_data_direction dir,                       unsigned long attrs)intdma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,                 int nents, enum dma_data_direction dir,                 unsigned long attrs)voiddma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,                   int nents, enum dma_data_direction dir,                   unsigned long attrs)

The four functions above are just like the counterpart functionswithout the _attrs suffixes, except that they pass an optionaldma_attrs.

The interpretation of DMA attributes is architecture-specific, andeach attribute should be documented inDMA attributes.

If dma_attrs are 0, the semantics of each of these functionsis identical to those of the corresponding functionwithout the _attrs suffix. As a resultdma_map_single_attrs()can generally replacedma_map_single(), etc.

As an example of the use of the*_attrs functions, here’s howyou could pass an attribute DMA_ATTR_FOO when mapping memoryfor DMA:

#include <linux/dma-mapping.h>/* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and* documented in Documentation/core-api/dma-attributes.rst */...        unsigned long attr;        attr |= DMA_ATTR_FOO;        ....        n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr);        ....

Architectures that care about DMA_ATTR_FOO would check for itspresence in their implementations of the mapping and unmappingroutines, e.g.::

void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,                             size_t size, enum dma_data_direction dir,                             unsigned long attrs){        ....        if (attrs & DMA_ATTR_FOO)                /* twizzle the frobnozzle */        ....}

Part Ie - IOVA-based DMA mappings¶

These APIs allow a very efficient mapping when using an IOMMU. They are anoptional path that requires extra code and are only recommended for driverswhere DMA mapping performance, or the space usage for storing the DMA addressesmatter. All the considerations from the previous section apply here as well.

bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state,            phys_addr_t phys, size_t size);

Is used to try to allocate IOVA space for mapping operation. If it returnsfalse this API can’t be used for the given device and the normal streamingDMA mapping API should be used. Thestructdma_iova_state is allocatedby the driver and must be kept around until unmap time.

static inline bool dma_use_iova(struct dma_iova_state *state)

Can be used by the driver to check if the IOVA-based API is used after acall to dma_iova_try_alloc. This can be useful in the unmap path.

int dma_iova_link(struct device *dev, struct dma_iova_state *state,            phys_addr_t phys, size_t offset, size_t size,            enum dma_data_direction dir, unsigned long attrs);

Is used to link ranges to the IOVA previously allocated. The start of allbut the first call to dma_iova_link for a given state must be alignedto the DMA merge boundary returned bydma_get_merge_boundary()), andthe size of all but the last range must be aligned to the DMA merge boundaryas well.

int dma_iova_sync(struct device *dev, struct dma_iova_state *state,            size_t offset, size_t size);

Must be called to sync the IOMMU page tables for IOVA-range mapped by one ormore calls todma_iova_link().

For drivers that use a one-shot mapping, all ranges can be unmapped and theIOVA freed by calling:

void dma_iova_destroy(struct device *dev, struct dma_iova_state *state,             size_t mapped_len, enum dma_data_direction dir,             unsigned long attrs);

Alternatively drivers can dynamically manage the IOVA space by unmappingand mapping individual regions. In that case

void dma_iova_unlink(struct device *dev, struct dma_iova_state *state,            size_t offset, size_t size, enum dma_data_direction dir,            unsigned long attrs);

is used to unmap a range previously mapped, and

void dma_iova_free(struct device *dev, struct dma_iova_state *state);

is used to free the IOVA space. All regions must have been unmapped usingdma_iova_unlink() before callingdma_iova_free().

Part II - Non-coherent DMA allocations¶

These APIs allow to allocate pages that are guaranteed to be DMA addressableby the passed in device, but which need explicit management of memory ownershipfor the kernel vs the device.

If you don’t understand how cache line coherency works between a processor andan I/O device, you should not be using this part of the API.

struct page *dma_alloc_pages(struct device *dev, size_t size, dma_addr_t *dma_handle,                enum dma_data_direction dir, gfp_t gfp)

This routine allocates a region of <size> bytes of non-coherent memory. Itreturns a pointer to firststructpage for the region, or NULL if theallocation failed. The resultingstructpage can be used for everything astructpage is suitable for.

It also returns a <dma_handle> which may be cast to an unsigned integer thesame width as the bus and given to the device as the DMA address base ofthe region.

The dir parameter specified if data is read and/or written by the device,seedma_map_single() for details.

The gfp parameter allows the caller to specify theGFP_ flags (seekmalloc()) for the allocation, but rejects flags used to specify a memoryzone such as GFP_DMA or GFP_HIGHMEM.

Before giving the memory to the device,dma_sync_single_for_device() needsto be called, and before reading memory written by the device,dma_sync_single_for_cpu(), just like for streaming DMA mappings that arereused.

voiddma_free_pages(struct device *dev, size_t size, struct page *page,                dma_addr_t dma_handle, enum dma_data_direction dir)

Free a region of memory previously allocated usingdma_alloc_pages().dev, size, dma_handle and dir must all be the same as those passed intodma_alloc_pages(). page must be the pointer returned bydma_alloc_pages().

intdma_mmap_pages(struct device *dev, struct vm_area_struct *vma,               size_t size, struct page *page)

Map an allocation returned fromdma_alloc_pages() into a user address space.dev and size must be the same as those passed intodma_alloc_pages().page must be the pointer returned bydma_alloc_pages().

void *dma_alloc_noncoherent(struct device *dev, size_t size,                dma_addr_t *dma_handle, enum dma_data_direction dir,                gfp_t gfp)

This routine is a convenient wrapper around dma_alloc_pages that returns thekernel virtual address for the allocated memory instead of the page structure.

voiddma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,                dma_addr_t dma_handle, enum dma_data_direction dir)

Free a region of memory previously allocated usingdma_alloc_noncoherent().dev, size, dma_handle and dir must all be the same as those passed intodma_alloc_noncoherent(). cpu_addr must be the virtual address returned bydma_alloc_noncoherent().

struct sg_table *dma_alloc_noncontiguous(struct device *dev, size_t size,                        enum dma_data_direction dir, gfp_t gfp,                        unsigned long attrs);

This routine allocates <size> bytes of non-coherent and possibly non-contiguousmemory. It returns a pointer tostructsg_table that describes the allocatedand DMA mapped memory, or NULL if the allocation failed. The resulting memorycan be used forstructpage mapped into a scatterlist are suitable for.

The return sg_table is guaranteed to have 1 single DMA mapped segment asindicated by sgt->nents, but it might have multiple CPU side segments asindicated by sgt->orig_nents.

The dir parameter specified if data is read and/or written by the device,seedma_map_single() for details.

The gfp parameter allows the caller to specify theGFP_ flags (seekmalloc()) for the allocation, but rejects flags used to specify a memoryzone such as GFP_DMA or GFP_HIGHMEM.

The attrs argument must be either 0 or DMA_ATTR_ALLOC_SINGLE_PAGES.

Before giving the memory to the device,dma_sync_sgtable_for_device() needsto be called, and before reading memory written by the device,dma_sync_sgtable_for_cpu(), just like for streaming DMA mappings that arereused.

voiddma_free_noncontiguous(struct device *dev, size_t size,                       struct sg_table *sgt,                       enum dma_data_direction dir)

Free memory previously allocated usingdma_alloc_noncontiguous(). dev, size,and dir must all be the same as those passed intodma_alloc_noncontiguous().sgt must be the pointer returned bydma_alloc_noncontiguous().

void *dma_vmap_noncontiguous(struct device *dev, size_t size,        struct sg_table *sgt)

Return a contiguous kernel mapping for an allocation returned fromdma_alloc_noncontiguous(). dev and size must be the same as those passed intodma_alloc_noncontiguous(). sgt must be the pointer returned bydma_alloc_noncontiguous().

Once a non-contiguous allocation is mapped using this function, theflush_kernel_vmap_range() andinvalidate_kernel_vmap_range() APIs must be usedto manage the coherency between the kernel mapping, the device and user spacemappings (if any).

voiddma_vunmap_noncontiguous(struct device *dev, void *vaddr)

Unmap a kernel mapping returned bydma_vmap_noncontiguous(). dev must be thesame the one passed intodma_alloc_noncontiguous(). vaddr must be the pointerreturned bydma_vmap_noncontiguous().

intdma_mmap_noncontiguous(struct device *dev, struct vm_area_struct *vma,                       size_t size, struct sg_table *sgt)

Map an allocation returned fromdma_alloc_noncontiguous() into a user addressspace. dev and size must be the same as those passed intodma_alloc_noncontiguous(). sgt must be the pointer returned bydma_alloc_noncontiguous().

intdma_get_cache_alignment(void)

Returns the processor cache alignment. This is the absolute minimumalignmentand width that you must observe when either mappingmemory or doing partial flushes.

Part III - Debug drivers use of the DMA API¶

The DMA API as described above has some constraints. DMA addresses must bereleased with the corresponding function with the same size for example. Withthe advent of hardware IOMMUs it becomes more and more important that driversdo not violate those constraints. In the worst case such a violation canresult in data corruption up to destroyed filesystems.

To debug drivers and find bugs in the usage of the DMA API checking code canbe compiled into the kernel which will tell the developer about thoseviolations. If your architecture supports it you can select the “Enabledebugging of DMA API usage” option in your kernel configuration. Enabling thisoption has a performance impact. Do not enable it in production kernels.

If you boot the resulting kernel will contain code which does some bookkeepingabout what DMA memory was allocated for which device. If this code detects anerror it prints a warning message with some details into your kernel log. Anexample warning message may look like this:

WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448        check_unmap+0x203/0x490()Hardware name:forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong        function [device address=0x00000000640444be] [size=66 bytes] [mapped assingle] [unmapped as page]Modules linked in: nfsd exportfs bridge stp llc r8169Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1Call Trace:<IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130[<ffffffff80647b70>] _spin_unlock+0x10/0x30[<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0[<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40[<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0[<ffffffff80252f96>] queue_work+0x56/0x60[<ffffffff80237e10>] enqueue_task_fair+0x20/0x50[<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0[<ffffffff803b78c3>] cpumask_next_and+0x23/0x40[<ffffffff80235177>] find_busiest_group+0x207/0x8a0[<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50[<ffffffff803c7ea3>] check_unmap+0x203/0x490[<ffffffff803c8259>] debug_dma_unmap_phys+0x49/0x50[<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0[<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0[<ffffffff8026df84>] handle_IRQ_event+0x34/0x70[<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150[<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0[<ffffffff8020c093>] ret_from_intr+0x0/0xa<EOI> <4>---[ end trace f6435a98e2a38c0e ]---

The driver developer can find the driver and the device including a stacktraceof the DMA API call which caused this warning.

Per default only the first error will result in a warning message. All othererrors will only silently counted. This limitation exist to prevent the codefrom flooding your kernel log. To support debugging a device driver this canbe disabled via debugfs. See the debugfs interface documentation below fordetails.

The debugfs directory for the DMA API debugging code is called dma-api/. Inthis directory the following files can currently be found:

If you have this code compiled into your kernel it will be enabled by default.If you want to boot without the bookkeeping anyway you can provide‘dma_debug=off’ as a boot parameter. This will disable DMA API debugging.Notice that you can not enable it again at runtime. You have to reboot to doso.

If you want to see debug messages only for a special device driver you canspecify the dma_debug_driver=<drivername> parameter. This will enable thedriver filter at boot time. The debug code will only print errors for thatdriver afterwards. This filter can be disabled or changed later using debugfs.

When the code disables itself at runtime this is most likely because it ranout of dma_debug_entries and was unable to allocate more on-demand. 65536entries are preallocated at boot - if this is too low for you boot with‘dma_debug_entries=<your_desired_number>’ to overwrite the default. Notethat the code allocates entries in batches, so the exact number ofpreallocated entries may be greater than the actual number requested. Thecode will print to the kernel log each time it has dynamically allocatedas many entries as were initially preallocated. This is to indicate that alarger preallocation size may be appropriate, or if it happens continuallythat a driver may be leaking mappings.

voiddebug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);

dma-debug interfacedebug_dma_mapping_error() to debug drivers that failto check DMA mapping errors on addresses returned bydma_map_single() anddma_map_page() interfaces. This interface clears a flag set bydebug_dma_map_phys() to indicate thatdma_mapping_error() has been called bythe driver. When driver does unmap,debug_dma_unmap() checks the flag and ifthis flag is still set, prints warning message that includes call trace thatleads up to the unmap. This interface can be called fromdma_mapping_error()routines to enable DMA mapping error check debugging.