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Re: Bus error after successful mmap of physical address

To: Alex Gonzalez <langabe@gmail.com>
Subject: Re: Bus error after successful mmap of physical address
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 22 Aug 2007 19:08:27 +0100
Cc: linux-mips <linux-mips@linux-mips.org>
In-reply-to: <c58a7a270708221031g3fba98d5u8507c2aafd4e16b4@mail.gmail.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <c58a7a270708221031g3fba98d5u8507c2aafd4e16b4@mail.gmail.com>
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On Wed, Aug 22, 2007 at 06:31:22PM +0100, Alex Gonzalez wrote:

> I am sure there is a basic reason why this is not working but I just
> can't see it.
> 
> I am booting with mem=512MB and trying to access a memory region at
> 0xC0000000 mapped by a fixed TLB entry.

Is 0xC0000000 a physical or virtual address.  If it's a virtual address
your mapping will conflict with other mappings generated by the kernel and
you will need additional hacks to protect the address space from being
used by the kernel for other purposes.

> My driver does,
> 
> vma->vm_flags = vma->vm_flags | VM_IO | VM_RESERVED ;
> vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot) ;
>
> // open device
>       vadr = mmap( NULL , 1024*1024 ,
> PROT_WRITE|PROT_READ,MAP_NORESERVE|MAP_SHARED,device,0xC0000000);
                                                       ^^^^^^^^^^

This is the reason why I was asking if 0xC0000000 was a physical address.
mmap needs a physical address.

>       if(vadr == MAP_FAILED)
>       {
>               perror("mmap failed.\n");
>               exit(-1);
>       }
> 
> 
> That goes OK, but then if I try to read or write from vadr I get a "Bus 
> error".

Assuming device is a /dev/mem descriptor that is looking ok.  However - you
will be getting an uncached mapping from mmap so the performance will suck
rocks through a straw.

Anyway, you have 64-bit hardware, use it.  On a 64-bit kernel you can just
address all your memory through XKPHYS without the need for any TLB entries.

  Ralf

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