with example pseudo-code. For a concise description of the API, see
DMA-API.txt.
-Most of the 64bit platforms have special hardware that translates bus
-addresses (DMA addresses) into physical addresses. This is similar to
-how page tables and/or a TLB translates virtual addresses to physical
-addresses on a CPU. This is needed so that e.g. PCI devices can
-access with a Single Address Cycle (32bit DMA address) any page in the
-64bit physical address space. Previously in Linux those 64bit
-platforms had to set artificial limits on the maximum RAM size in the
-system, so that the virt_to_bus() static scheme works (the DMA address
-translation tables were simply filled on bootup to map each bus
-address to the physical page __pa(bus_to_virt())).
+ CPU and DMA addresses
+
+There are several kinds of addresses involved in the DMA API, and it's
+important to understand the differences.
+
+The kernel normally uses virtual addresses. Any address returned by
+kmalloc(), vmalloc(), and similar interfaces is a virtual address and can
+be stored in a "void *".
+
+The virtual memory system (TLB, page tables, etc.) translates virtual
+addresses to CPU physical addresses, which are stored as "phys_addr_t" or
+"resource_size_t". The kernel manages device resources like registers as
+physical addresses. These are the addresses in /proc/iomem. The physical
+address is not directly useful to a driver; it must use ioremap() to map
+the space and produce a virtual address.
+
+I/O devices use a third kind of address: a "bus address". If a device has
+registers at an MMIO address, or if it performs DMA to read or write system
+memory, the addresses used by the device are bus addresses. In some
+systems, bus addresses are identical to CPU physical addresses, but in
+general they are not. IOMMUs and host bridges can produce arbitrary
+mappings between physical and bus addresses.
+
+From a device's point of view, DMA uses the bus address space, but it may
+be restricted to a subset of that space. For example, even if a system
+supports 64-bit addresses for main memory and PCI BARs, it may use an IOMMU
+so devices only need to use 32-bit DMA addresses.
+
+Here's a picture and some examples:
+
+ CPU CPU Bus
+ Virtual Physical Address
+ Address Address Space
+ Space Space
+
+ +-------+ +------+ +------+
+ | | |MMIO | Offset | |
+ | | Virtual |Space | applied | |
+ C +-------+ --------> B +------+ ----------> +------+ A
+ | | mapping | | by host | |
+ +-----+ | | | | bridge | | +--------+
+ | | | | +------+ | | | |
+ | CPU | | | | RAM | | | | Device |
+ | | | | | | | | | |
+ +-----+ +-------+ +------+ +------+ +--------+
+ | | Virtual |Buffer| Mapping | |
+ X +-------+ --------> Y +------+ <---------- +------+ Z
+ | | mapping | RAM | by IOMMU
+ | | | |
+ | | | |
+ +-------+ +------+
+
+During the enumeration process, the kernel learns about I/O devices and
+their MMIO space and the host bridges that connect them to the system. For
+example, if a PCI device has a BAR, the kernel reads the bus address (A)
+from the BAR and converts it to a CPU physical address (B). The address B
+is stored in a struct resource and usually exposed via /proc/iomem. When a
+driver claims a device, it typically uses ioremap() to map physical address
+B at a virtual address (C). It can then use, e.g., ioread32(C), to access
+the device registers at bus address A.
+
+If the device supports DMA, the driver sets up a buffer using kmalloc() or
+a similar interface, which returns a virtual address (X). The virtual
+memory system maps X to a physical address (Y) in system RAM. The driver
+can use virtual address X to access the buffer, but the device itself
+cannot because DMA doesn't go through the CPU virtual memory system.
+
+In some simple systems, the device can do DMA directly to physical address
+Y. But in many others, there is IOMMU hardware that translates DMA
+addresses to physical addresses, e.g., it translates Z to Y. This is part
+of the reason for the DMA API: the driver can give a virtual address X to
+an interface like dma_map_single(), which sets up any required IOMMU
+mapping and returns the DMA address Z. The driver then tells the device to
+do DMA to Z, and the IOMMU maps it to the buffer at address Y in system
+RAM.
So that Linux can use the dynamic DMA mapping, it needs some help from the
drivers, namely it has to take into account that DMA addresses should be
hardware exists.
Note that the DMA API works with any bus independent of the underlying
-microprocessor architecture. You should use the DMA API rather than
-the bus specific DMA API (e.g. pci_dma_*).
+microprocessor architecture. You should use the DMA API rather than the
+bus-specific DMA API, i.e., use the dma_map_*() interfaces rather than the
+pci_map_*() interfaces.
First of all, you should make sure
#include <linux/dma-mapping.h>
-is in your driver. This file will obtain for you the definition of the
-dma_addr_t (which can hold any valid DMA address for the platform)
-type which should be used everywhere you hold a DMA (bus) address
-returned from the DMA mapping functions.
+is in your driver, which provides the definition of dma_addr_t. This type
+can hold any valid DMA address for the platform and should be used
+everywhere you hold a DMA address returned from the DMA mapping functions.
What memory is DMA'able?
because this shows that you did think about these issues wrt. your
device.
-The query is performed via a call to dma_set_mask():
+The query is performed via a call to dma_set_mask_and_coherent():
- int dma_set_mask(struct device *dev, u64 mask);
+ int dma_set_mask_and_coherent(struct device *dev, u64 mask);
-The query for consistent allocations is performed via a call to
-dma_set_coherent_mask():
+which will query the mask for both streaming and coherent APIs together.
+If you have some special requirements, then the following two separate
+queries can be used instead:
- int dma_set_coherent_mask(struct device *dev, u64 mask);
+ The query for streaming mappings is performed via a call to
+ dma_set_mask():
+
+ int dma_set_mask(struct device *dev, u64 mask);
+
+ The query for consistent allocations is performed via a call
+ to dma_set_coherent_mask():
+
+ int dma_set_coherent_mask(struct device *dev, u64 mask);
Here, dev is a pointer to the device struct of your device, and mask
is a bit mask describing which bits of an address your device
supports. It returns zero if your card can perform DMA properly on
the machine given the address mask you provided. In general, the
-device struct of your device is embedded in the bus specific device
-struct of your device. For example, a pointer to the device struct of
-your PCI device is pdev->dev (pdev is a pointer to the PCI device
+device struct of your device is embedded in the bus-specific device
+struct of your device. For example, &pdev->dev is a pointer to the
+device struct of a PCI device (pdev is a pointer to the PCI device
struct of your device).
If it returns non-zero, your device cannot perform DMA properly on
The standard 32-bit addressing device would do something like this:
- if (dma_set_mask(dev, DMA_BIT_MASK(32))) {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
+ if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
+ dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
} else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
using_dac = 0;
} else {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
+ dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
int using_dac, consistent_using_dac;
- if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
+ if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) {
using_dac = 1;
- consistent_using_dac = 1;
- dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
- } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
+ consistent_using_dac = 1;
+ } else if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
using_dac = 0;
consistent_using_dac = 0;
- dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
} else {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
+ dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
-dma_set_coherent_mask() will always be able to set the same or a
-smaller mask as dma_set_mask(). However for the rare case that a
-device driver only uses consistent allocations, one would have to
-check the return value from dma_set_coherent_mask().
+The coherent mask will always be able to set the same or a smaller mask as
+the streaming mask. However for the rare case that a device driver only
+uses consistent allocations, one would have to check the return value from
+dma_set_coherent_mask().
Finally, if your device can only drive the low 24-bits of
address you might do something like:
if (dma_set_mask(dev, DMA_BIT_MASK(24))) {
- printk(KERN_WARNING
- "mydev: 24-bit DMA addressing not available.\n");
+ dev_warn(dev, "mydev: 24-bit DMA addressing not available\n");
goto ignore_this_device;
}
-When dma_set_mask() is successful, and returns zero, the kernel saves
-away this mask you have provided. The kernel will use this
-information later when you make DMA mappings.
+When dma_set_mask() or dma_set_mask_and_coherent() is successful, and
+returns zero, the kernel saves away this mask you have provided. The
+kernel will use this information later when you make DMA mappings.
There is a case which we are aware of at this time, which is worth
mentioning in this documentation. If your device supports multiple
card->playback_enabled = 1;
} else {
card->playback_enabled = 0;
- printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.\n",
+ dev_warn(dev, "%s: Playback disabled due to DMA limitations\n",
card->name);
}
if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
card->record_enabled = 1;
} else {
card->record_enabled = 0;
- printk(KERN_WARNING "%s: Record disabled due to DMA limitations.\n",
+ dev_warn(dev, "%s: Record disabled due to DMA limitations\n",
card->name);
}
Think of "consistent" as "synchronous" or "coherent".
The current default is to return consistent memory in the low 32
- bits of the bus space. However, for future compatibility you should
+ bits of the DMA space. However, for future compatibility you should
set the consistent mask even if this default is fine for your
driver.
transfer, unmapped right after it (unless you use dma_sync_* below)
and for which hardware can optimize for sequential accesses.
- This of "streaming" as "asynchronous" or "outside the coherency
+ Think of "streaming" as "asynchronous" or "outside the coherency
domain".
Good examples of what to use streaming mappings for are:
Size is the length of the region you want to allocate, in bytes.
This routine will allocate RAM for that region, so it acts similarly to
-__get_free_pages (but takes size instead of a page order). If your
+__get_free_pages() (but takes size instead of a page order). If your
driver needs regions sized smaller than a page, you may prefer using
the dma_pool interface, described below.
dma_set_coherent_mask(). This is true of the dma_pool interface as
well.
-dma_alloc_coherent returns two values: the virtual address which you
+dma_alloc_coherent() returns two values: the virtual address which you
can use to access it from the CPU and dma_handle which you pass to the
card.
-The cpu return address and the DMA bus master address are both
+The CPU virtual address and the DMA address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size. This invariant
exists (for example) to guarantee that if you allocate a chunk
dma_free_coherent(dev, size, cpu_addr, dma_handle);
where dev, size are the same as in the above call and cpu_addr and
-dma_handle are the values dma_alloc_coherent returned to you.
+dma_handle are the values dma_alloc_coherent() returned to you.
This function may not be called in interrupt context.
If your driver needs lots of smaller memory regions, you can write
-custom code to subdivide pages returned by dma_alloc_coherent,
+custom code to subdivide pages returned by dma_alloc_coherent(),
or you can use the dma_pool API to do that. A dma_pool is like
-a kmem_cache, but it uses dma_alloc_coherent not __get_free_pages.
+a kmem_cache, but it uses dma_alloc_coherent(), not __get_free_pages().
Also, it understands common hardware constraints for alignment,
like queue heads needing to be aligned on N byte boundaries.
struct dma_pool *pool;
- pool = dma_pool_create(name, dev, size, align, alloc);
+ pool = dma_pool_create(name, dev, size, align, boundary);
The "name" is for diagnostics (like a kmem_cache name); dev and size
are as above. The device's hardware alignment requirement for this
type of data is "align" (which is expressed in bytes, and must be a
power of two). If your device has no boundary crossing restrictions,
-pass 0 for alloc; passing 4096 says memory allocated from this pool
+pass 0 for boundary; passing 4096 says memory allocated from this pool
must not cross 4KByte boundaries (but at that time it may be better to
-go for dma_alloc_coherent directly instead).
+use dma_alloc_coherent() directly instead).
-Allocate memory from a dma pool like this:
+Allocate memory from a DMA pool like this:
cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
-flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
-holding SMP locks), SLAB_ATOMIC otherwise. Like dma_alloc_coherent,
+flags are GFP_KERNEL if blocking is permitted (not in_interrupt nor
+holding SMP locks), GFP_ATOMIC otherwise. Like dma_alloc_coherent(),
this returns two values, cpu_addr and dma_handle.
Free memory that was allocated from a dma_pool like this:
dma_pool_free(pool, cpu_addr, dma_handle);
-where pool is what you passed to dma_pool_alloc, and cpu_addr and
-dma_handle are the values dma_pool_alloc returned. This function
+where pool is what you passed to dma_pool_alloc(), and cpu_addr and
+dma_handle are the values dma_pool_alloc() returned. This function
may be called in interrupt context.
Destroy a dma_pool by calling:
dma_pool_destroy(pool);
-Make sure you've called dma_pool_free for all memory allocated
+Make sure you've called dma_pool_free() for all memory allocated
from a pool before you destroy the pool. This function may not
be called in interrupt context.
DMA_FROM_DEVICE
DMA_NONE
-One should provide the exact DMA direction if you know it.
+You should provide the exact DMA direction if you know it.
DMA_TO_DEVICE means "from main memory to the device"
DMA_FROM_DEVICE means "from the device to main memory"
size_t size = buffer->len;
dma_handle = dma_map_single(dev, addr, size, direction);
- if (dma_mapping_error(dma_handle)) {
+ if (dma_mapping_error(dev, dma_handle)) {
/*
* reduce current DMA mapping usage,
* delay and try again later or
dma_unmap_single(dev, dma_handle, size, direction);
You should call dma_mapping_error() as dma_map_single() could fail and return
-error. Not all dma implementations support dma_mapping_error() interface.
+error. Not all DMA implementations support the dma_mapping_error() interface.
However, it is a good practice to call dma_mapping_error() interface, which
will invoke the generic mapping error check interface. Doing so will ensure
-that the mapping code will work correctly on all dma implementations without
+that the mapping code will work correctly on all DMA implementations without
any dependency on the specifics of the underlying implementation. Using the
returned address without checking for errors could result in failures ranging
from panics to silent data corruption. A couple of examples of incorrect ways
-to check for errors that make assumptions about the underlying dma
+to check for errors that make assumptions about the underlying DMA
implementation are as follows and these are applicable to dma_map_page() as
well.
goto map_error;
}
-You should call dma_unmap_single when the DMA activity is finished, e.g.
+You should call dma_unmap_single() when the DMA activity is finished, e.g.,
from the interrupt which told you that the DMA transfer is done.
-Using cpu pointers like this for single mappings has a disadvantage,
+Using CPU pointers like this for single mappings has a disadvantage:
you cannot reference HIGHMEM memory in this way. Thus, there is a
-map/unmap interface pair akin to dma_{map,unmap}_single. These
-interfaces deal with page/offset pairs instead of cpu pointers.
+map/unmap interface pair akin to dma_{map,unmap}_single(). These
+interfaces deal with page/offset pairs instead of CPU pointers.
Specifically:
struct device *dev = &my_dev->dev;
size_t size = buffer->len;
dma_handle = dma_map_page(dev, page, offset, size, direction);
- if (dma_mapping_error(dma_handle)) {
+ if (dma_mapping_error(dev, dma_handle)) {
/*
* reduce current DMA mapping usage,
* delay and try again later or
You should call dma_mapping_error() as dma_map_page() could fail and return
error as outlined under the dma_map_single() discussion.
-You should call dma_unmap_page when the DMA activity is finished, e.g.
+You should call dma_unmap_page() when the DMA activity is finished, e.g.,
from the interrupt which told you that the DMA transfer is done.
With scatterlists, you map a region gathered from several regions by:
it should _NOT_ be the 'count' value _returned_ from the
dma_map_sg call.
-Every dma_map_{single,sg} call should have its dma_unmap_{single,sg}
-counterpart, because the bus address space is a shared resource (although
-in some ports the mapping is per each BUS so less devices contend for the
-same bus address space) and you could render the machine unusable by eating
-all bus addresses.
+Every dma_map_{single,sg}() call should have its dma_unmap_{single,sg}()
+counterpart, because the DMA address space is a shared resource and
+you could render the machine unusable by consuming all DMA addresses.
If you need to use the same streaming DMA region multiple times and touch
the data in between the DMA transfers, the buffer needs to be synced
-properly in order for the cpu and device to see the most uptodate and
+properly in order for the CPU and device to see the most up-to-date and
correct copy of the DMA buffer.
-So, firstly, just map it with dma_map_{single,sg}, and after each DMA
+So, firstly, just map it with dma_map_{single,sg}(), and after each DMA
transfer call either:
dma_sync_single_for_cpu(dev, dma_handle, size, direction);
as appropriate.
Then, if you wish to let the device get at the DMA area again,
-finish accessing the data with the cpu, and then before actually
+finish accessing the data with the CPU, and then before actually
giving the buffer to the hardware call either:
dma_sync_single_for_device(dev, dma_handle, size, direction);
as appropriate.
+PLEASE NOTE: The 'nents' argument to dma_sync_sg_for_cpu() and
+ dma_sync_sg_for_device() must be the same passed to
+ dma_map_sg(). It is _NOT_ the count returned by
+ dma_map_sg().
+
After the last DMA transfer call one of the DMA unmap routines
-dma_unmap_{single,sg}. If you don't touch the data from the first dma_map_*
-call till dma_unmap_*, then you don't have to call the dma_sync_*
-routines at all.
+dma_unmap_{single,sg}(). If you don't touch the data from the first
+dma_map_*() call till dma_unmap_*(), then you don't have to call the
+dma_sync_*() routines at all.
Here is pseudo code which shows a situation in which you would need
to use the dma_sync_*() interfaces.
dma_addr_t mapping;
mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
- if (dma_mapping_error(dma_handle)) {
+ if (dma_mapping_error(cp->dev, dma_handle)) {
/*
* reduce current DMA mapping usage,
* delay and try again later or
}
}
-Drivers converted fully to this interface should not use virt_to_bus any
-longer, nor should they use bus_to_virt. Some drivers have to be changed a
-little bit, because there is no longer an equivalent to bus_to_virt in the
+Drivers converted fully to this interface should not use virt_to_bus() any
+longer, nor should they use bus_to_virt(). Some drivers have to be changed a
+little bit, because there is no longer an equivalent to bus_to_virt() in the
dynamic DMA mapping scheme - you have to always store the DMA addresses
-returned by the dma_alloc_coherent, dma_pool_alloc, and dma_map_single
-calls (dma_map_sg stores them in the scatterlist itself if the platform
+returned by the dma_alloc_coherent(), dma_pool_alloc(), and dma_map_single()
+calls (dma_map_sg() stores them in the scatterlist itself if the platform
supports dynamic DMA mapping in hardware) in your driver structures and/or
in the card registers.
DMA address space is limited on some architectures and an allocation
failure can be determined by:
-- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
+- checking if dma_alloc_coherent() returns NULL or dma_map_sg returns 0
-- checking the returned dma_addr_t of dma_map_single and dma_map_page
+- checking the dma_addr_t returned from dma_map_single() and dma_map_page()
by using dma_mapping_error():
dma_addr_t dma_handle;
dma_unmap_single(array[i].dma_addr);
}
-Networking drivers must call dev_kfree_skb to free the socket buffer
+Networking drivers must call dev_kfree_skb() to free the socket buffer
and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
(ndo_start_xmit). This means that the socket buffer is just dropped in
the failure case.
DEFINE_DMA_UNMAP_LEN(len);
};
-2) Use dma_unmap_{addr,len}_set to set these values.
+2) Use dma_unmap_{addr,len}_set() to set these values.
Example, before:
ringp->mapping = FOO;
dma_unmap_addr_set(ringp, mapping, FOO);
dma_unmap_len_set(ringp, len, BAR);
-3) Use dma_unmap_{addr,len} to access these values.
+3) Use dma_unmap_{addr,len}() to access these values.
Example, before:
dma_unmap_single(dev, ringp->mapping, ringp->len,
projects / firefly-linux-kernel-4.4.55.git / blobdiff