P4080 PCIe Adapter SDK User Guide Production Release

Freescale Semiconductor
write memory directly using DMA. Buffers used for peripherals’ DMA must be allocated from memory
with special properties:
• The memory must be physically contiguous since the peripheral does not access memory via the
core’s MMU (or any page-mode I/O MMU).
• The memory must be addressable by the peripheral.
• The memory must not be swapped out by Linux while the device is accessing it.
• For user space drivers, there must be an efficient mechanism to convert the physical addresses used
by the peripheral hardware to and from the effective addresses used in a user space process or
thread’s address space.
• For BMan and DPAA usage, it is often convenient for software to allocate memory from physically
contiguous regions that are quite large.
• It is desirable to use the Power core’s TLB1 mechanism to map large physically contiguous
memory regions of this sort. This increases performance by reducing the number of MMU-related
interrupts that must be processed. A single TLB0 entry can map only a single 4K page. A single
TLB1 entry, in contrast, can map a large (but variable-sized) page. One TLB1 entry can do the work
of many TLB0 entries in circumstances such as this one.
Memory that meets the criteria above is called DMA memory. Drivers for all DMA-capable devices must
use DMA memory for buffers. This is true for both conventional in-kernel drivers and user space drivers.
It is a hardware implication of peripherals’ relationships to cores and MMUs.
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