I've seen a couple posts out there about issuing a kernel boot parameter of
mem=XXXM, say for example mem=504M on a 512 MiB system followed by issuing
an ioremap function call from a module/driver such as: ioremap( 0x1F800000,
0x800000 ) where an attempt is made to remap the unclaimed (by the kernel)
physical memory from 504MiB up to 512MiB.
I've tried this route which looks to make great logical sense and I've tried
to actually use the remapped range for DMA with no luck. I've seen posts
from all the way back in 2003 where folks are doing the same thing and have
the same bad results... and no replies to the posts.
Can this ram really be used for DMA?
I'm afraid that since I'm using a PCI device (PLX) with device specific
hardware address decoding on the LOCAL bus side of the PLX, that by taking
this approach, the kernel comes back with virtual addresses that, after
being mapped back to bus addresses still won't allow for DMA to work
properly. Can anyone give some insight into this?
I'm basically trying the following:
boot kernel (2.4.xx) with mem=504M
in my driver, I'm calling
my_virt_address = ioremap( 0x1F800000, 0x800000 );
my_bus_address = virt_to_bus( my_virt_address );
Then, I'm telling my FPGA bus master device (across a PLX chip) that it's
starting DMA address is my_bus_address.
I get interrupts, but no data in my_virt_address. I've seen several posts
exactly like this and have yet to see any answers to these posts...
I've tried an alternative approach of calling one of the alloc_bootmem_xxx
functions early in start_kernel and have had positivie results ONLY on the
montavista 3.1 pro kernel with preemption enabled. On the Denx 2.4.25
Kernel from ELDK 3.1 I just get a locked up/frozen kernel when I trie to use
the alloc_bootmem_xx approach and when I use the mem=504M / ioremap
approach, I don't get any DMA data from either kernel...
Please help!!
Thanks,
Eric
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