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List:       linux-rt
Subject:    RE: [rtl] Shared Memory - Segmentation Fault or General Protectio
From:       "Estabridis, Janet P" <EstabridisJP () navair ! navy ! mil>
Date:       1999-08-31 17:59:28
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Jose,
Thanks.  Unfortunately I am using the vremap().  So, if anyone else can help
-- Please.

Janet

> -----Original Message-----
> From:	Marshall Jose [SMTP:Marshall.Jose@jhuapl.edu]
> Sent:	Tuesday, August 31, 1999 10:00 AM
> To:	'RT Linux Group'
> Cc:	Estabridis, Janet P
> Subject:	Re: [rtl] Shared Memory - Segmentation Fault or General
> Protection
> 
> At 07:08 AM 8/31/99 -0700, Estabridis, Janet P wrote:
> 
>  >I have gotten shared memory to work on this machine but my shared memory
>  >block size was small, about 700 bytes to test out my code.  Now I was
> ready
>  >to increase it and I get a Segmenation Fault when I /sbin/insmod my rt
>  >process....
> 
> My experiences may help:
> 
> I am doing DMA transfers from a PCI card directly to shared RAM while 
> within an RT module. Also while in the module, I write a couple of 
> locations in shared-RAM directly.
> 
> At first, I was doing this with a 1MB shared-RAM area and all this seemed 
> to work. But I got core dumps when I changed the shared-RAM size to 4MB!
> 
> Cause: I was following the shared-memory FAQ which didn't mention the need
> 
> to use vremap(). Once I did the vremap() call before touching shared-RAM, 
> my problems went away. I guess that when I was using the 1 MB area, I was 
> simply lucky.
> 
> So: I can apparently DMA to shared-RAM without performing a vremap() 
> because the transfer occurs outside the CPU. But programmed reads/writes 
> must be done using a mapped pointer.
> 
> Hoping this helps,
> 
> Marshall Jose, WA3VPZ
> Johns Hopkins Univ. Applied Physics Lab., 240-228-4367
> | "It could have been an organically based disturbance of the brain -- |
> | perhaps a tumor or a metabolic deficiency -- but after a thorough    |
> | neurological exam it was determined that Byron was simply a jerk."   |
> | -- Jeff Jahnke, runner-up, 7th Annual Bulwer-Lytton Contest.         |
> 
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