James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
This document describes the DMA API. For a more gentle introduction
-phrased in terms of the pci_ equivalents (and actual examples) see
-DMA-mapping.txt
+of the API (and actual examples) see
+Documentation/DMA-API-HOWTO.txt.
-This API is split into two pieces. Part I describes the API and the
-corresponding pci_ API. Part II describes the extensions to the API
-for supporting non-consistent memory machines. Unless you know that
-your driver absolutely has to support non-consistent platforms (this
-is usually only legacy platforms) you should only use the API
-described in part I.
+This API is split into two pieces. Part I describes the API. Part II
+describes the extensions to the API for supporting non-consistent
+memory machines. Unless you know that your driver absolutely has to
+support non-consistent platforms (this is usually only legacy
+platforms) you should only use the API described in part I.
-Part I - pci_ and dma_ Equivalent API
+Part I - dma_ API
-------------------------------------
-To get the pci_ API, you must #include <linux/pci.h>
To get the dma_ API, you must #include <linux/dma-mapping.h>
void *
dma_alloc_coherent(struct device *dev, size_t size,
- dma_addr_t *dma_handle, int flag)
-void *
-pci_alloc_consistent(struct pci_dev *dev, size_t size,
- dma_addr_t *dma_handle)
+ dma_addr_t *dma_handle, gfp_t flag)
Consistent memory is memory for which a write by either the device or
the processor can immediately be read by the processor or device
devices to read that memory.)
This routine allocates a region of <size> bytes of consistent memory.
-it also returns a <dma_handle> which may be cast to an unsigned
+It also returns a <dma_handle> which may be cast to an unsigned
integer the same width as the bus and used as the physical address
base of the region.
The flag parameter (dma_alloc_coherent only) allows the caller to
specify the GFP_ flags (see kmalloc) for the allocation (the
-implementation may chose to ignore flags that affect the location of
-the returned memory, like GFP_DMA). For pci_alloc_consistent, you
-must assume GFP_ATOMIC behaviour.
+implementation may choose to ignore flags that affect the location of
+the returned memory, like GFP_DMA).
void
-dma_free_coherent(struct device *dev, size_t size, void *cpu_addr
- dma_addr_t dma_handle)
-void
-pci_free_consistent(struct pci_dev *dev, size_t size, void *cpu_addr
+dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
dma_addr_t dma_handle)
Free the region of consistent memory you previously allocated. dev,
size and dma_handle must all be the same as those passed into the
consistent allocate. cpu_addr must be the virtual address returned by
-the consistent allocate
+the consistent allocate.
+
+Note that unlike their sibling allocation calls, these routines
+may only be called with IRQs enabled.
Part Ib - Using small dma-coherent buffers
Many drivers need lots of small dma-coherent memory regions for DMA
descriptors or I/O buffers. Rather than allocating in units of a page
or more using dma_alloc_coherent(), you can use DMA pools. These work
-much like a struct kmem_cache, except that they use the dma-coherent allocator
+much like a struct kmem_cache, except that they use the dma-coherent allocator,
not __get_free_pages(). Also, they understand common hardware constraints
-for alignment, like queue heads needing to be aligned on N byte boundaries.
+for alignment, like queue heads needing to be aligned on N-byte boundaries.
struct dma_pool *
dma_pool_create(const char *name, struct device *dev,
size_t size, size_t align, size_t alloc);
- struct pci_pool *
- pci_pool_create(const char *name, struct pci_device *dev,
- size_t size, size_t align, size_t alloc);
-
The pool create() routines initialize a pool of dma-coherent buffers
for use with a given device. It must be called in a context which
can sleep.
from this pool must not cross 4KByte boundaries.
- void *dma_pool_alloc(struct dma_pool *pool, int gfp_flags,
- dma_addr_t *dma_handle);
-
- void *pci_pool_alloc(struct pci_pool *pool, int gfp_flags,
+ void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
dma_addr_t *dma_handle);
This allocates memory from the pool; the returned memory will meet the size
and alignment requirements specified at creation time. Pass GFP_ATOMIC to
-prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks)
+prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),
pass GFP_KERNEL to allow blocking. Like dma_alloc_coherent(), this returns
two values: an address usable by the cpu, and the dma address usable by the
pool's device.
void dma_pool_free(struct dma_pool *pool, void *vaddr,
dma_addr_t addr);
- void pci_pool_free(struct pci_pool *pool, void *vaddr,
- dma_addr_t addr);
-
This puts memory back into the pool. The pool is what was passed to
-the pool allocation routine; the cpu and dma addresses are what
+the pool allocation routine; the cpu (vaddr) and dma addresses are what
were returned when that routine allocated the memory being freed.
void dma_pool_destroy(struct dma_pool *pool);
- void pci_pool_destroy(struct pci_pool *pool);
-
The pool destroy() routines free the resources of the pool. They must be
called in a context which can sleep. Make sure you've freed all allocated
memory back to the pool before you destroy it.
int
dma_supported(struct device *dev, u64 mask)
-int
-pci_dma_supported(struct device *dev, u64 mask)
Checks to see if the device can support DMA to the memory described by
mask.
int
dma_set_mask(struct device *dev, u64 mask)
+
+Checks to see if the mask is possible and updates the device
+parameters if it is.
+
+Returns: 0 if successful and a negative error if not.
+
int
-pci_set_dma_mask(struct pci_device *dev, u64 mask)
+dma_set_coherent_mask(struct device *dev, u64 mask)
Checks to see if the mask is possible and updates the device
parameters if it is.
u64
dma_get_required_mask(struct device *dev)
-After setting the mask with dma_set_mask(), this API returns the
-actual mask (within that already set) that the platform actually
-requires to operate efficiently. Usually this means the returned mask
+This API returns the mask that the platform requires to
+operate efficiently. Usually this means the returned mask
is the minimum required to cover all of memory. Examining the
required mask gives drivers with variable descriptor sizes the
opportunity to use smaller descriptors as necessary.
Requesting the required mask does not alter the current mask. If you
-wish to take advantage of it, you should issue another dma_set_mask()
-call to lower the mask again.
+wish to take advantage of it, you should issue a dma_set_mask()
+call to set the mask to the value returned.
Part Id - Streaming DMA mappings
dma_addr_t
dma_map_single(struct device *dev, void *cpu_addr, size_t size,
enum dma_data_direction direction)
-dma_addr_t
-pci_map_single(struct device *dev, void *cpu_addr, size_t size,
- int direction)
Maps a piece of processor virtual memory so it can be accessed by the
device and returns the physical handle of the memory.
However the dma_ API uses a strongly typed enumerator for its
direction:
-DMA_NONE = PCI_DMA_NONE no direction (used for
- debugging)
-DMA_TO_DEVICE = PCI_DMA_TODEVICE data is going from the
- memory to the device
-DMA_FROM_DEVICE = PCI_DMA_FROMDEVICE data is coming from
- the device to the
- memory
-DMA_BIDIRECTIONAL = PCI_DMA_BIDIRECTIONAL direction isn't known
+DMA_NONE no direction (used for debugging)
+DMA_TO_DEVICE data is going from the memory to the device
+DMA_FROM_DEVICE data is coming from the device to the memory
+DMA_BIDIRECTIONAL direction isn't known
Notes: Not all memory regions in a machine can be mapped by this
API. Further, regions that appear to be physically contiguous in
kernel virtual space may not be contiguous as physical memory. Since
this API does not provide any scatter/gather capability, it will fail
-if the user tries to map a non physically contiguous piece of memory.
+if the user tries to map a non-physically contiguous piece of memory.
For this reason, it is recommended that memory mapped by this API be
-obtained only from sources which guarantee to be physically contiguous
+obtained only from sources which guarantee it to be physically contiguous
(like kmalloc).
Further, the physical address of the memory must be within the
dma_mask of the device (the dma_mask represents a bit mask of the
-addressable region for the device. i.e. if the physical address of
+addressable region for the device. I.e., if the physical address of
the memory anded with the dma_mask is still equal to the physical
address, then the device can perform DMA to the memory). In order to
ensure that the memory allocated by kmalloc is within the dma_mask,
-the driver may specify various platform dependent flags to restrict
+the driver may specify various platform-dependent flags to restrict
the physical memory range of the allocation (e.g. on x86, GFP_DMA
guarantees to be within the first 16Mb of available physical memory,
as required by ISA devices).
DMA_TO_DEVICE synchronisation must be done after the last modification
of the memory region by the software and before it is handed off to
-the driver. Once this primitive is used. Memory covered by this
-primitive should be treated as read only by the device. If the device
+the driver. Once this primitive is used, memory covered by this
+primitive should be treated as read-only by the device. If the device
may write to it at any point, it should be DMA_BIDIRECTIONAL (see
below).
DMA_FROM_DEVICE synchronisation must be done before the driver
accesses data that may be changed by the device. This memory should
-be treated as read only by the driver. If the driver needs to write
+be treated as read-only by the driver. If the driver needs to write
to it at any point, it should be DMA_BIDIRECTIONAL (see below).
DMA_BIDIRECTIONAL requires special handling: it means that the driver
memory is handed off to the device (to make sure all memory changes
are flushed from the processor) and once before the data may be
accessed after being used by the device (to make sure any processor
-cache lines are updated with data that the device may have changed.
+cache lines are updated with data that the device may have changed).
void
dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
enum dma_data_direction direction)
-void
-pci_unmap_single(struct pci_dev *hwdev, dma_addr_t dma_addr,
- size_t size, int direction)
Unmaps the region previously mapped. All the parameters passed in
must be identical to those passed in (and returned) by the mapping
dma_map_page(struct device *dev, struct page *page,
unsigned long offset, size_t size,
enum dma_data_direction direction)
-dma_addr_t
-pci_map_page(struct pci_dev *hwdev, struct page *page,
- unsigned long offset, size_t size, int direction)
void
dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
enum dma_data_direction direction)
-void
-pci_unmap_page(struct pci_dev *hwdev, dma_addr_t dma_address,
- size_t size, int direction)
API for mapping and unmapping for pages. All the notes and warnings
for the other mapping APIs apply here. Also, although the <offset>
cache width is.
int
-dma_mapping_error(dma_addr_t dma_addr)
-
-int
-pci_dma_mapping_error(dma_addr_t dma_addr)
+dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
In some circumstances dma_map_single and dma_map_page will fail to create
a mapping. A driver can check for these errors by testing the returned
-dma address with dma_mapping_error(). A non zero return value means the mapping
-could not be created and the driver should take appropriate action (eg
+dma address with dma_mapping_error(). A non-zero return value means the mapping
+could not be created and the driver should take appropriate action (e.g.
reduce current DMA mapping usage or delay and try again later).
int
dma_map_sg(struct device *dev, struct scatterlist *sg,
int nents, enum dma_data_direction direction)
- int
- pci_map_sg(struct pci_dev *hwdev, struct scatterlist *sg,
- int nents, int direction)
-Maps a scatter gather list from the block layer.
-
-Returns: the number of physical segments mapped (this may be shorted
-than <nents> passed in if the block layer determines that some
-elements of the scatter/gather list are physically adjacent and thus
-may be mapped with a single entry).
+Returns: the number of physical segments mapped (this may be shorter
+than <nents> passed in if some elements of the scatter/gather list are
+physically or virtually adjacent and an IOMMU maps them with a single
+entry).
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.
int i, count = dma_map_sg(dev, sglist, nents, direction);
struct scatterlist *sg;
- for (i = 0, sg = sglist; i < count; i++, sg++) {
+ for_each_sg(sglist, sg, count, i) {
hw_address[i] = sg_dma_address(sg);
hw_len[i] = sg_dma_len(sg);
}
void
dma_unmap_sg(struct device *dev, struct scatterlist *sg,
int nhwentries, enum dma_data_direction direction)
- void
- pci_unmap_sg(struct pci_dev *hwdev, struct scatterlist *sg,
- int nents, int direction)
-unmap the previously mapped scatter/gather list. All the parameters
+Unmap the previously mapped scatter/gather list. All the parameters
must be the same as those and passed in to the scatter/gather mapping
API.
physical entries returned.
void
-dma_sync_single(struct device *dev, dma_addr_t dma_handle, size_t size,
- enum dma_data_direction direction)
+dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
+ enum dma_data_direction direction)
void
-pci_dma_sync_single(struct pci_dev *hwdev, dma_addr_t dma_handle,
- size_t size, int direction)
+dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
+ enum dma_data_direction direction)
void
-dma_sync_sg(struct device *dev, struct scatterlist *sg, int nelems,
- enum dma_data_direction direction)
+dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
+ enum dma_data_direction direction)
void
-pci_dma_sync_sg(struct pci_dev *hwdev, struct scatterlist *sg,
- int nelems, int direction)
+dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
+ enum dma_data_direction direction)
-synchronise a single contiguous or scatter/gather mapping. All the
-parameters must be the same as those passed into the single mapping
-API.
+Synchronise a single contiguous or scatter/gather mapping for the cpu
+and device. With the sync_sg API, all the parameters must be the same
+as those passed into the single mapping API. With the sync_single API,
+you can use dma_handle and size parameters that aren't identical to
+those passed into the single mapping API to do a partial sync.
Notes: You must do this:
See also dma_map_single().
+dma_addr_t
+dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
+ enum dma_data_direction dir,
+ struct dma_attrs *attrs)
+
+void
+dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
+ size_t size, enum dma_data_direction dir,
+ struct dma_attrs *attrs)
+
+int
+dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
+ int nents, enum dma_data_direction dir,
+ struct dma_attrs *attrs)
+
+void
+dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
+ int nents, enum dma_data_direction dir,
+ struct dma_attrs *attrs)
+
+The four functions above are just like the counterpart functions
+without the _attrs suffixes, except that they pass an optional
+struct dma_attrs*.
+
+struct dma_attrs encapsulates a set of "dma attributes". For the
+definition of struct dma_attrs see linux/dma-attrs.h.
+
+The interpretation of dma attributes is architecture-specific, and
+each attribute should be documented in Documentation/DMA-attributes.txt.
+
+If struct dma_attrs* is NULL, the semantics of each of these
+functions is identical to those of the corresponding function
+without the _attrs suffix. As a result dma_map_single_attrs()
+can generally replace dma_map_single(), etc.
+
+As an example of the use of the *_attrs functions, here's how
+you could pass an attribute DMA_ATTR_FOO when mapping memory
+for DMA:
+
+#include <linux/dma-attrs.h>
+/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
+ * documented in Documentation/DMA-attributes.txt */
+...
+
+ DEFINE_DMA_ATTRS(attrs);
+ dma_set_attr(DMA_ATTR_FOO, &attrs);
+ ....
+ n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
+ ....
+
+Architectures that care about DMA_ATTR_FOO would check for its
+presence in their implementations of the mapping and unmapping
+routines, e.g.:
+
+void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
+ size_t size, enum dma_data_direction dir,
+ struct dma_attrs *attrs)
+{
+ ....
+ int foo = dma_get_attr(DMA_ATTR_FOO, attrs);
+ ....
+ if (foo)
+ /* twizzle the frobnozzle */
+ ....
+
Part II - Advanced dma_ usage
-----------------------------
-Warning: These pieces of the DMA API have no PCI equivalent. They
-should also not be used in the majority of cases, since they cater for
-unlikely corner cases that don't belong in usual drivers.
+Warning: These pieces of the DMA API should not be used in the
+majority of cases, since they cater for unlikely corner cases that
+don't belong in usual drivers.
If you don't understand how cache line coherency works between a
processor and an I/O device, you should not be using this part of the
void *
dma_alloc_noncoherent(struct device *dev, size_t size,
- dma_addr_t *dma_handle, int flag)
+ dma_addr_t *dma_handle, gfp_t flag)
Identical to dma_alloc_coherent() except that the platform will
choose to return either consistent or non-consistent memory as it sees
dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
dma_addr_t dma_handle)
-free memory allocated by the nonconsistent API. All parameters must
+Free memory allocated by the nonconsistent API. All parameters must
be identical to those passed in (and returned by
dma_alloc_noncoherent()).
int
dma_is_consistent(struct device *dev, dma_addr_t dma_handle)
-returns true if the device dev is performing consistent DMA on the memory
+Returns true if the device dev is performing consistent DMA on the memory
area pointed to by the dma_handle.
int
dma_get_cache_alignment(void)
-returns the processor cache alignment. This is the absolute minimum
+Returns the processor cache alignment. This is the absolute minimum
alignment *and* width that you must observe when either mapping
memory or doing partial flushes.
Notes: This API may return a number *larger* than the actual cache
line, but it will guarantee that one or more cache lines fit exactly
into the width returned by this call. It will also always be a power
-of two for easy alignment
-
-void
-dma_sync_single_range(struct device *dev, dma_addr_t dma_handle,
- unsigned long offset, size_t size,
- enum dma_data_direction direction)
-
-does a partial sync. starting at offset and continuing for size. You
-must be careful to observe the cache alignment and width when doing
-anything like this. You must also be extra careful about accessing
-memory you intend to sync partially.
+of two for easy alignment.
void
-dma_cache_sync(void *vaddr, size_t size,
+dma_cache_sync(struct device *dev, void *vaddr, size_t size,
enum dma_data_direction direction)
Do a partial sync of memory that was allocated by
dma_addr_t device_addr, size_t size, int
flags)
-
Declare region of memory to be handed out by dma_alloc_coherent when
it's asked for coherent memory for this device.
bus_addr is the physical address to which the memory is currently
assigned in the bus responding region (this will be used by the
-platform to perform the mapping)
+platform to perform the mapping).
device_addr is the physical address the device needs to be programmed
with actually to address this memory (this will be handed out as the
-dma_addr_t in dma_alloc_coherent())
+dma_addr_t in dma_alloc_coherent()).
size is the size of the area (must be multiples of PAGE_SIZE).
-flags can be or'd together and are
+flags can be or'd together and are:
DMA_MEMORY_MAP - request that the memory returned from
dma_alloc_coherent() be directly writable.
DMA_MEMORY_IO - request that the memory returned from
dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.
-One or both of these flags must be present
+One or both of these flags must be present.
DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
dma_alloc_coherent of any child devices of this one (for memory residing
Remove the memory region previously declared from the system. This
API performs *no* in-use checking for this region and will return
unconditionally having removed all the required structures. It is the
-drivers job to ensure that no parts of this memory region are
+driver's job to ensure that no parts of this memory region are
currently in use.
void *
This is used to occupy specific regions of the declared space
(dma_alloc_coherent() will hand out the first free region it finds).
-device_addr is the *device* address of the region requested
+device_addr is the *device* address of the region requested.
-size is the size (and should be a page sized multiple).
+size is the size (and should be a page-sized multiple).
The return value will be either a pointer to the processor virtual
address of the memory, or an error (via PTR_ERR()) if any part of the
region is occupied.
-
+Part III - Debug drivers use of the DMA-API
+-------------------------------------------
+
+The DMA-API as described above as some constraints. DMA addresses must be
+released with the corresponding function with the same size for example. With
+the advent of hardware IOMMUs it becomes more and more important that drivers
+do not violate those constraints. In the worst case such a violation can
+result in data corruption up to destroyed filesystems.
+
+To debug drivers and find bugs in the usage of the DMA-API checking code can
+be compiled into the kernel which will tell the developer about those
+violations. If your architecture supports it you can select the "Enable
+debugging of DMA-API usage" option in your kernel configuration. Enabling this
+option has a performance impact. Do not enable it in production kernels.
+
+If you boot the resulting kernel will contain code which does some bookkeeping
+about what DMA memory was allocated for which device. If this code detects an
+error it prints a warning message with some details into your kernel log. An
+example warning message may look like this:
+
+------------[ cut here ]------------
+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 as
+single] [unmapped as page]
+Modules linked in: nfsd exportfs bridge stp llc r8169
+Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
+Call 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_page+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 stacktrace
+of the DMA-API call which caused this warning.
+
+Per default only the first error will result in a warning message. All other
+errors will only silently counted. This limitation exist to prevent the code
+from flooding your kernel log. To support debugging a device driver this can
+be disabled via debugfs. See the debugfs interface documentation below for
+details.
+
+The debugfs directory for the DMA-API debugging code is called dma-api/. In
+this directory the following files can currently be found:
+
+ dma-api/all_errors This file contains a numeric value. If this
+ value is not equal to zero the debugging code
+ will print a warning for every error it finds
+ into the kernel log. Be careful with this
+ option, as it can easily flood your logs.
+
+ dma-api/disabled This read-only file contains the character 'Y'
+ if the debugging code is disabled. This can
+ happen when it runs out of memory or if it was
+ disabled at boot time
+
+ dma-api/error_count This file is read-only and shows the total
+ numbers of errors found.
+
+ dma-api/num_errors The number in this file shows how many
+ warnings will be printed to the kernel log
+ before it stops. This number is initialized to
+ one at system boot and be set by writing into
+ this file
+
+ dma-api/min_free_entries
+ This read-only file can be read to get the
+ minimum number of free dma_debug_entries the
+ allocator has ever seen. If this value goes
+ down to zero the code will disable itself
+ because it is not longer reliable.
+
+ dma-api/num_free_entries
+ The current number of free dma_debug_entries
+ in the allocator.
+
+ dma-api/driver-filter
+ You can write a name of a driver into this file
+ to limit the debug output to requests from that
+ particular driver. Write an empty string to
+ that file to disable the filter and see
+ all errors again.
+
+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 do
+so.
+
+If you want to see debug messages only for a special device driver you can
+specify the dma_debug_driver=<drivername> parameter. This will enable the
+driver filter at boot time. The debug code will only print errors for that
+driver afterwards. This filter can be disabled or changed later using debugfs.
+
+When the code disables itself at runtime this is most likely because it ran
+out of dma_debug_entries. These entries are preallocated at boot. The number
+of preallocated entries is defined per architecture. If it is too low for you
+boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
+architectural default.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff