On Tuesday 06 May 2014 16:48:19 Bjorn Helgaas wrote: > The DMA-API documentation sometimes refers to "physical addresses" when it > really means "bus addresses." Sometimes these are identical, but they may > be different if the bridge leading to the bus performs address translation. > Update the documentation to use "bus address" when appropriate. > > Also, consistently capitalize "DMA", use parens with function names, use > dev_printk() in examples, and reword a few sections for clarity. > > Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx> Looks great! Acked-by: Arnd Bergmann <arnd@xxxxxxxx> Just some minor comments that you may include if you like (my Ack holds if you don't as well). > @@ -30,16 +28,16 @@ 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_*). > +the bus-specific DMA API (e.g. pci_dma_*). It might make sense to change the example to dma_map_* rather than pci_dma_*, which is rarely used these days. I think there was at one point a move to replace remove the include/asm-generic/pci-dma-compat.h APIs. > 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 or bus address for the platform and should be used > +everywhere you hold a DMA address returned from the DMA mapping functions > +or a bus address read from a device register such as a PCI BAR. The PCI BAR example is misleading I think: While the raw value of the BAR would be a dma_addr_t that can be used for pci-pci DMA, we normally only deal with translated BARs from pci_resource_*, which would be a resource_size_t in the same space as phys_addr_t, which has the PCI mem_offset added in. > + * A dma_addr_t can hold any valid DMA or bus address for the platform. > + * It can be given to a device to use as a DMA source or target, or it may > + * appear on the bus when a CPU performs programmed I/O. A CPU cannot > + * reference a dma_addr_t directly because there may be translation between > + * its physical address space and the bus address space. On a similar note, I think the part 'or it may appear on the bus when a CPU performs programmed I/O' is somewhat misleading: While true in theory, we would never use a dma_addr_t to store an address to be used for PIO, because the CPU needs to use either the phys_addr_t value associated with the physical MMIO address or the __iomem pointer for the virtually mapped address. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
