CODEWITHSUNDEEP
cmemory-managementlinux-kernelmmu
john s.
john s.

Reputation: 517

__pa returns physical address out of range

Working on the arm core adsp-sc572 (Kernel 4.0), I figured out __pa is not working correct. Allocating a big buffer (e.g.500kB) for DMA using kmalloc gives me a virtual address. When using __pa() -> it sometimes returns an address out of the physical address range. e.g. 0x88AB0000

0x87FFFFFF
[..] -> 128MB RAM
0x80000000

__pa calls __virt_to_phys -> __pv_stub which I do not unterstand.

static inline phys_addr_t __virt_to_phys(unsigned long x)
{
    phys_addr_t t;

    if (sizeof(phys_addr_t) == 4) {
        __pv_stub(x, t, "add", __PV_BITS_31_24);
    } else {
        __pv_stub_mov_hi(t);
        __pv_add_carry_stub(x, t);
    }
    return t;
}

Where is the beginning of the virtual address space defined? How does __pv_stub work? Why does __pa return an invalid address?

Upvotes: 1

Views: 1040

Answers (1)

Dahui
Dahui

Reputation: 146

Pleae refers to https://www.linuxquestions.org/questions/linux-kernel-70/physical-address-on-a-x86-64-bit-machine-914781-print/ , it may be helpful to you.

Part of the kernel virtual address space in Linux is a contiguous mapping of physical memory. If you use the __pa macro on a virtual address in that range you get the corresponding physical address. But are the page tables in that range or what does __pa do if you use it in some other range? I don't know.

Upvotes: 2

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