I'm trying to reduce the memory footprint of a driver which
makes heavy use of ddi_dma_mem_alloc()
I need to allocate 1520 byte buffers which cannot cross a 4KB
boundary. Note the 4KB requirement is irrespective of the host
page size -- the device cannot DMA across a 4KB boundary.
I'm currently using the following dma_attr:
static ddi_dma_attr_t dma_attr = {
DMA_ATTR_V0, /* version number. */
(uint64_t)0, /* low address */
(uint64_t)0xffffffffffffffffULL, /* high address */
(uint64_t)0x7ffffff, /* address counter max */
(uint64_t)2048, /* alignment */
(uint_t)0x7f, /* burstsizes for 32b and 64b xfers */
(uint32_t)0x1, /* minimum transfer size */
(uint64_t)0x7fffffff, /* maximum transfer size */
UINT64_MAX, /* maximum segment size */
1, /* scatter/gather list length */
1, /* granularity */
0 /* attribute flags */
};
The "real_length" coming back from these allocations is 2048, and
I was expecting to see 1/2 the buffers I get back aligned on a 2KB
boundary, and 1/2 on a 4KB boundary.
However, a quick check shows that all the buffers are coming
back aligned on a 2KB boundary. Eg, all the physical addresses
end in 0x800. My assumption is that since I'm never seeing
physical addresses ending in 0x000, then I must be wasting
an entire 4KB page. Also, kmastat shows these allocations winding
up in kmem_io_512M_4096
How can I satisfy the "don't cross a 4KB boundary" requirement
and still fit 2 buffers into a single 4KB page and save the system
some memory?
BTW, I was testing on an amd64 running snv100
Thanks,
Drew
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