Centos 7.x: Physical buffer allocation

zvivered · 09-19-2019, 02:13 AM

Hello,

Under Centos 7.x (64) I tried to allocate a 32MB contiguous physical memory with:

void *p = dma_alloc_coherent (NULL, 0x2000000, &DmaHandle, GFP_KERNEL);

Also tried:

void *p = __get_free_pages (GFP_KERNEL, __get_order (0x2000000));

In both cases I got p=0 and few error messages in dmesg.

It works fine if the size is 0x400000 (4MB).

Is there a solution without changing the kernel boot command line parameters ?

Thank you,
Zvika

zvivered · 09-20-2019, 03:58 PM

Hello,

I tried to add cma=32M to the kernel parameters.
After boot I got:

more /proc/cmdline
BOOT_IMAGE=/vmlinuz-3.10.0-957.10.1.el7.x86_64 root=/dev/mapper/centos-root ro c
rashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet cma=32M

But;
dma_alloc_coherent (NULL, 0x2000000, &DmaHandle, GFP_KERNEL);
still returns NULL.

According to kernel-parameters.txt;
cma=nn[MG] [ARM,KNL]
Sets the size of kernel global memory area for contiguous
memory allocations. For more information, see
include/linux/dma-contiguous.h

Does it mean that cma is not relevant in x86 ?

Thank you,
Zvika