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mmap'd physical memory not updating user virtual

Jun 21, 2009  By debrupam
 in

I am writing a user space MAC driver in Linux 2.6.23 hosted on a Arm11 mpcore. A contiguous block of physical memory is allocated and mmap'd to userspace.

/dev/mem with O_SYNC flag is used to mmap so that the user virtual memory is uncached (with no much benefit).

Kernel
------
kptr = kmalloc(sz, GFP_DMA|GFP_KERNEL);
pptr = __pa(kptr);

User
----
fd = open("/dev/mem", O_RDWR|O_SYNC);
uptr = mmap(0, sz, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_LOCKED,fd, pptr);

The map'd user virtual memory is carved into network buffers and used for DMA.The MAC is successfully able to DMA ethernet frames to the physical memory. From the Rx interrupt handler bh (kernel logical memory) the frames received could be viewed and the cache functions also work fine. However, the mmap'd memory in user space does not reflect the changes.

Why is the physical memory mapped to user virtual does not reflect the chages ?

Or any suggestions on what could be going wrong here ?

thanks,
Deb Rupam Banerjee

P.S: Please note that all buffer alignment's are taken care of.

‹ Query about "Function call interrupts" entry in /proc/interrupts file non cached memory for user space DMA ›

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Hi Deb, How did you solve

dono8's picture
Submitted by dono8 (not verified) on Tue, 09/08/2009 - 21:20.

Hi Deb,

How did you solve this issue? I am also facing similar problem with ARM9. I have reserved 4M Bytes of memory at boot time, for using with the DMA and using the following code:

#define DMAMEM_START 0x73C00000

dma_fd = open("/dev/mem", O_RDWR | O_SYNC);
dma_mem = mmap(0, MSP2_DEV_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, dma_fd, DMAMEM_START);

DMA_write(DMAMEM_START, INST_Add, 16);
err = msync(dma_mem, (size_t) 64, MS_SYNC | MS_INVALIDATE);
DMA_read(INST_Add, DMAMEM_START+32, 16);

Using DMA_READ, I can see the data at DMAMEM_START, but sometime it is correct sometime incorrect.

I appreciate your step by step help!

Thanks,
Aa

DEVMEM

debrupam's picture
Submitted by debrupam on Thu, 08/27/2009 - 07:48.

Mitch,

Thanks for your comments. I was able to solve the problem as follows:

I was using the cache (flush/inv) routines used on the kernel logical address and was able to see the DMA'd data. The same idea I used for the user virtual mapping and the DMA'd data was visible in the user space mapping.

thanks,
Deb

DEVMEM

Mitch Frazier's picture
Submitted by Mitch Frazier on Sun, 06/21/2009 - 10:09.

Presumably mmap() is returning a valid pointer, correct? And you're not seeing any other errors occur, correct? You just never see the data change in the mapped area.

Does your kernel have CONFIG_STRICT_DEVMEM enabled?

Mitch Frazier is an Associate Editor for Linux Journal.