Hi, how come reading memory from /dev/mem using pread(2) or mmap(2)
will give diffent results?
I run the little prog below 10 times and it always give the following
results
# for i in `seq 1 10`; do ./bar 0x4000000 ; done
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
thanx
----------------------------------- bar.c ---------------------------------
#include <unistd.h>
#include <stdio.h>
#include <inttypes.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <errno.h>
int
main(int argc, char** argv)
{
uint32_t *addr, item;
uint32_t offset, val;
int fd, update = 0;
if (argc < 2)
return 1;
offset = strtoul(argv[1], NULL, 0);
if (argc > 2) {
val = strtoul(argv[2], NULL, 0);
update = 1;
}
fd = open("/dev/mem", O_RDWR, 0666);
addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, 0x8080|MAP_SHARED, fd, offset);
if (addr)
{
printf("%8.8x: %8.8x", offset, *addr);
if (update) {
*addr = val;
msync(addr, 4096, MS_SYNC);
printf("-> %8.8x\n", *addr);
} else
printf("\n");
pread(fd, &item, sizeof(item), offset);
printf("pread(%8.8x): %8.8x\n", offset, item);
munmap(addr, 4096);
}
close(fd);
}
On Sat, 2005-09-17 at 12:10 -0400, Linh Dang wrote:
> Hi, how come reading memory from /dev/mem using pread(2) or mmap(2)
> will give diffent results?
because you're being evil ;)
mmap of /dev/mem for *ram* is special. To avoid cache aliases and other
evils, you can only mmap non-ram realistically on /dev/mem.
Why are you using /dev/mem in the first place, it's a sure sign that
you're doing something really wrong in your design...
Arjan van de Ven <[email protected]> wrote:
> On Sat, 2005-09-17 at 12:10 -0400, Linh Dang wrote:
>> Hi, how come reading memory from /dev/mem using pread(2) or mmap(2)
>> will give diffent results?
>
> because you're being evil ;)
>
> mmap of /dev/mem for *ram* is special. To avoid cache aliases and
> other evils, you can only mmap non-ram realistically on /dev/mem.
>
> Why are you using /dev/mem in the first place, it's a sure sign that
> you're doing something really wrong in your design...
Thanx for the reply, what I'm doing is writing a driver (based on
mem.c) to export a block of ram to (other masters on) the PCI bus. The
driver does:
1. get a contiguous block of ram using alloc_pages()
2. export (via host-bridge hw setting) the block to the pci
bus
3. provide the .mmap() method in the driver to let userspace
to mmap the device
In doing so, I encountered the inconsistencies of mmap(2) vs
read(2)/write(2). The work around I found is to SetPageReserved() on
all the pages got from alloc_pages(). But unfortunately I have no
clues why it's so. The vm code is not the easiest one to read
(compared to let's say the network code.)
--
Linh Dang