Hi Iain,

I can reproduce the problem - if the shared memory object already exists and has physical memory assigned to it. 

When I run your code for the first time, it works perfectly. From then on, it will fail on shm_ctl. This kind of makes 
sense, since you're trying to associate physical memory with a shared memory object which already has physical memory 
associated with it. It's mostly the error code that is misleading.

Cheers,
Thomas

-----Original Message-----
From: Iain Martin [mailto:community-noreply@qnx.com] 
Sent: Dienstag, 13. März 2012 13:54
To: ostech-core_os
Subject: shm_ctl() fails when setting up a shared memory physical DMA buffer for a PCI card

I am trying to set up a shared memory buffer for DMA, so I can DMA directly in/out from one process and memcpy in/out 
from other processes but shm_ctl() fails. I have been "doing the dance" with shm_open(), shm_ctl(), mmap() and 
mem_offset64() but not managed to combine these in a way that enables the call to shm_ctl to succeed with the physical 
flag set. There are other threads  on this topic and I have followed their suggestions but they don't work for me. (for 
example: http://www.openqnx.com/newsgroups/viewtopic.php?f=13&t=12398) The "tested example" that thread works for me but
 it specifies the phyiscal address directly and does not obtain it from an mmap allocation followed by mem_offset64. As 
soon as I pass the mmap allocation physical address to shm_ctl(), it fails with "error: Function not implemented".
 I'm testing on a 4 core x86 PC.


An extract from my code is:

/* Allocate contiguous memory for DMA buffer */
/* byte size is page aligned */
uint32_t *  ptr = mmap(NULL, bytesize,   PROT_READ | PROT_WRITE | PROT_NOCACHE,   MAP_PHYS | MAP_ANON,  NOFD,  0);

/* Get the physical addresss, note that mem_offset fails so I use mem_offset64 */ mem_offset64(ptr, NOFD, bytesize, &
physical_addr_64, &contig_len

/* Create a shared memory object */
int fd = shm_open("/turnip", O_RDWR | O_CREAT, 0644);

/* Overlay the shared memory object to the physical memory address */ if (shm_ctl(fd, SHMCTL_GLOBAL|SHMCTL_PHYS, 
physical_addr_64, bytesize) == -1) {
   perror("shm_open");
   exit(EXIT_FAILURE);
}

If I replace physical_addr_64 with 0xa0000, shm_ctl() works but this seems too risky because I am allocating a large 
amount for memory for driver DMA buffers so I'm not happy just picking any physical address for a driver buffer.

If I just use mmap and memoffset_64 to have a DMA buffer in process memory space this works fine and if I create a 
shared memory objects, this also works fine. I have read conflicting accounts of using ltruncate to allocate the 
physical buffer but I couldn't get that to work either and Ithink I'm going round in circles now!

Any help or advice would be greatly appreciated. Is it possible to allocate a DMA buffer that can be shared between 
processes?





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OSTech
http://community.qnx.com/sf/go/post92072