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    SubjectRe: [PATCH v2 1/5] DMA-API: Clarify physical/bus address distinction
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    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@google.com>

    Looks great!

    Acked-by: Arnd Bergmann <arnd@arndb.de>

    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


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