summaryrefslogtreecommitdiffstats
path: root/Documentation
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/DMA-API-HOWTO.txt121
-rw-r--r--Documentation/DMA-API.txt29
2 files changed, 45 insertions, 105 deletions
diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index f0cc3f772265..1a721d0f35c8 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -146,114 +146,75 @@ What about block I/O and networking buffers? The block I/O and
networking subsystems make sure that the buffers they use are valid
for you to DMA from/to.
-DMA addressing limitations
+DMA addressing capabilities
==========================
-Does your device have any DMA addressing limitations? For example, is
-your device only capable of driving the low order 24-bits of address?
-If so, you need to inform the kernel of this fact.
+By default, the kernel assumes that your device can address 32-bits of DMA
+addressing. For a 64-bit capable device, this needs to be increased, and for
+a device with limitations, it needs to be decreased.
-By default, the kernel assumes that your device can address the full
-32-bits. For a 64-bit capable device, this needs to be increased.
-And for a device with limitations, as discussed in the previous
-paragraph, it needs to be decreased.
+Special note about PCI: PCI-X specification requires PCI-X devices to support
+64-bit addressing (DAC) for all transactions. And at least one platform (SGI
+SN2) requires 64-bit consistent allocations to operate correctly when the IO
+bus is in PCI-X mode.
-Special note about PCI: PCI-X specification requires PCI-X devices to
-support 64-bit addressing (DAC) for all transactions. And at least
-one platform (SGI SN2) requires 64-bit consistent allocations to
-operate correctly when the IO bus is in PCI-X mode.
+For correct operation, you must set the DMA mask to inform the kernel about
+your devices DMA addressing capabilities.
-For correct operation, you must interrogate the kernel in your device
-probe routine to see if the DMA controller on the machine can properly
-support the DMA addressing limitation your device has. It is good
-style to do this even if your device holds the default setting,
-because this shows that you did think about these issues wrt. your
-device.
-
-The query is performed via a call to dma_set_mask_and_coherent()::
+This is performed via a call to dma_set_mask_and_coherent()::
int dma_set_mask_and_coherent(struct device *dev, u64 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:
+which will set the mask for both streaming and coherent APIs together. If you
+have some special requirements, then the following two separate calls can be
+used instead:
- The query for streaming mappings is performed via a call to
+ The setup 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
+ The setup 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, &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).
+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. Often the
+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
-this platform, and attempting to do so will result in undefined
-behavior. You must either use a different mask, or not use DMA.
+These calls usually return zero to indicated your device can perform DMA
+properly on the machine given the address mask you provided, but they might
+return an error if the mask is too small to be supportable on the given
+system. If it returns non-zero, your device cannot perform DMA properly on
+this platform, and attempting to do so will result in undefined behavior.
+You must not use DMA on this device unless the dma_set_mask family of
+functions has returned success.
-This means that in the failure case, you have three options:
+This means that in the failure case, you have two options:
-1) Use another DMA mask, if possible (see below).
-2) Use some non-DMA mode for data transfer, if possible.
-3) Ignore this device and do not initialize it.
+1) Use some non-DMA mode for data transfer, if possible.
+2) Ignore this device and do not initialize it.
-It is recommended that your driver print a kernel KERN_WARNING message
-when you end up performing either #2 or #3. In this manner, if a user
-of your driver reports that performance is bad or that the device is not
-even detected, you can ask them for the kernel messages to find out
-exactly why.
+It is recommended that your driver print a kernel KERN_WARNING message when
+setting the DMA mask fails. In this manner, if a user of your driver reports
+that performance is bad or that the device is not even detected, you can ask
+them for the kernel messages to find out exactly why.
-The standard 32-bit addressing device would do something like this::
+The standard 64-bit addressing device would do something like this::
- if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
+ if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) {
dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
-Another common scenario is a 64-bit capable device. The approach here
-is to try for 64-bit addressing, but back down to a 32-bit mask that
-should not fail. The kernel may fail the 64-bit mask not because the
-platform is not capable of 64-bit addressing. Rather, it may fail in
-this case simply because 32-bit addressing is done more efficiently
-than 64-bit addressing. For example, Sparc64 PCI SAC addressing is
-more efficient than DAC addressing.
-
-Here is how you would handle a 64-bit capable device which can drive
-all 64-bits when accessing streaming DMA::
-
- int using_dac;
+If the device only supports 32-bit addressing for descriptors in the
+coherent allocations, but supports full 64-bits for streaming mappings
+it would look like this:
- if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
- using_dac = 1;
- } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
- using_dac = 0;
- } else {
- dev_warn(dev, "mydev: No suitable DMA available\n");
- goto ignore_this_device;
- }
-
-If a card is capable of using 64-bit consistent allocations as well,
-the case would look like this::
-
- int using_dac, consistent_using_dac;
-
- if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) {
- using_dac = 1;
- consistent_using_dac = 1;
- } else if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
- using_dac = 0;
- consistent_using_dac = 0;
- } else {
+ if (dma_set_mask(dev, DMA_BIT_MASK(64))) {
dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 607c1e75e5aa..586643642910 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -566,8 +566,7 @@ boundaries when doing this.
int
dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
- dma_addr_t device_addr, size_t size, int
- flags)
+ dma_addr_t device_addr, size_t size);
Declare region of memory to be handed out by dma_alloc_coherent() when
it's asked for coherent memory for this device.
@@ -581,12 +580,6 @@ dma_addr_t in dma_alloc_coherent()).
size is the size of the area (must be multiples of PAGE_SIZE).
-flags can be ORed together and are:
-
-- DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
- Do not allow dma_alloc_coherent() to fall back to system memory when
- it's out of memory in the declared region.
-
As a simplification for the platforms, only *one* such region of
memory may be declared per device.
@@ -605,23 +598,6 @@ unconditionally having removed all the required structures. It is the
driver's job to ensure that no parts of this memory region are
currently in use.
-::
-
- void *
- dma_mark_declared_memory_occupied(struct device *dev,
- dma_addr_t device_addr, size_t size)
-
-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.
-
-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
-------------------------------------------
@@ -696,6 +672,9 @@ dma-api/disabled This read-only file contains the character 'Y'
happen when it runs out of memory or if it was
disabled at boot time
+dma-api/dump This read-only file contains current DMA
+ mappings.
+
dma-api/error_count This file is read-only and shows the total
numbers of errors found.