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+                     Dynamic DMA mapping Guide
+                     =========================
+
+                 David S. Miller <davem@redhat.com>
+                 Richard Henderson <rth@cygnus.com>
+                  Jakub Jelinek <jakub@redhat.com>
+
+This is a guide to device driver writers on how to use the DMA API
+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())).
+
+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
+mapped only for the time they are actually used and unmapped after the DMA
+transfer.
+
+The following API will work of course even on platforms where no such
+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_*).
+
+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.
+
+                         What memory is DMA'able?
+
+The first piece of information you must know is what kernel memory can
+be used with the DMA mapping facilities.  There has been an unwritten
+set of rules regarding this, and this text is an attempt to finally
+write them down.
+
+If you acquired your memory via the page allocator
+(i.e. __get_free_page*()) or the generic memory allocators
+(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
+that memory using the addresses returned from those routines.
+
+This means specifically that you may _not_ use the memory/addresses
+returned from vmalloc() for DMA.  It is possible to DMA to the
+_underlying_ memory mapped into a vmalloc() area, but this requires
+walking page tables to get the physical addresses, and then
+translating each of those pages back to a kernel address using
+something like __va().  [ EDIT: Update this when we integrate
+Gerd Knorr's generic code which does this. ]
+
+This rule also means that you may use neither kernel image addresses
+(items in data/text/bss segments), nor module image addresses, nor
+stack addresses for DMA.  These could all be mapped somewhere entirely
+different than the rest of physical memory.  Even if those classes of
+memory could physically work with DMA, you'd need to ensure the I/O
+buffers were cacheline-aligned.  Without that, you'd see cacheline
+sharing problems (data corruption) on CPUs with DMA-incoherent caches.
+(The CPU could write to one word, DMA would write to a different one
+in the same cache line, and one of them could be overwritten.)
+
+Also, this means that you cannot take the return of a kmap()
+call and DMA to/from that.  This is similar to vmalloc().
+
+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
+
+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 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.
+
+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():
+
+        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
+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.
+
+This means that in the failure case, you have three 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.
+
+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.
+
+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");
+                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 (!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 {
+                printk(KERN_WARNING
+                       "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(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))) {
+                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");
+                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().
+
+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");
+                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.