Documentation/DMA-API-HOWTO: update dma_mask sections

We don't require drivers to guess a DMA mask that might actually
match the system capabilities any more, so fix up the documentation
to clear this up.
Signed-off-by: Christoph Hellwig <hch@lst.de>

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;
 	}