Reply-To: <imran.badr@cavium.com>
From: "Imran Badr" <imran.badr@cavium.com>
To: "'Manfred Spraul'" <manfred@colorfullife.com>,
       <linux-kernel@vger.kernel.org>
Subject: RE: Calculating kernel logical address ..
Date: Fri, 6 Sep 2002 10:13:36 -0700
Message-ID: <00e901c255c8$bd144e80$9e10a8c0@IMRANPC>
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-----Original Message-----
From: Manfred Spraul [mailto:manfred@colorfullife.com]
Sent: Friday, September 06, 2002 8:44 AM
To: Imran Badr
Cc: linux-kernel@vger.kernel.org
Subject: Re: Calculating kernel logical address ..


> adr = user_address;
> pgd_offset(current->mm, adr);
>
> if (!pgd_none(*pgd)) {
> 	pmd = pmd_offset(pgd, adr);
> 	if (!pmd_none(*pmd)) {
> 		ptep = pte_offset(pmd, adr);
> 		pte = *ptep;
> 		if(pte_present(pte)) {
> 			kaddr  = (unsigned long) page_address(pte_page(pte));
> 			kaddr |= (adr & (PAGE_SIZE - 1));
> 		}
> 	}
> }
>
> Will this code always give me correct kernel logical address?
>
What about

	kmalloc_buffer+(user_address-vma->vm_start)

?
A driver should avoid accessing the page tables.

--
	Manfred

I was wondering if the code which I am using, will always give me addresses
no matter whether HIGHMEM is defined in kernel configuration or not. I
belive that it should not be problem because I am mmap'ing kmalloc'ed memory
which always returns mapped memory. But whats happeing in my lab is
different. If I define HIGHMEM in kernel configuration and install 2GB of
memory in my server then I see a crash in the kernel where I try to access
kaddr calculated bu above code. Any idea?

The problem with your suggestion is that at the point where user gives me an
address for DMA, I do not know what kmalloc_buffer and vma->vm_start values
are. Also, if there are more than one processes accessing the driver, then
how am I going to keep track of all mmap'ed memory.

Thanks,
Imran.


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