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Question on Au1550 ...

Subject: Question on Au1550 ...
From: "Alexey Shinkin" <>
Date: Mon, 18 Oct 2004 19:01:30 +0700
Original-recipient: rfc822;
Hi , All !

I am porting a linux driver for PCI device and some applications form x86 platform to AMD Au1550 MIPS , running MontaVista Linux 2.4.20-mvl31.1550.

I got the  following  problem :

The driver allocates some memory (SharedMemory) for communication with PCI card , using function pci_alloc_consistent(). PCI card can write to this memory using DMA . On kernel level driver correctly reads values, written by the board , and the board correctly reads values, written by driver.

The goal is to make this memoty visible from user level application. The following approach is used on õ86 and works : 1. Application calls mmap( 0, Size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0 ) ;

2. driver implements mmap() function that just prepares some vma structure fields for nopage operetions :
        vma->vm_flags |= VM_RESERVED;        // no swapping
        vma->vm_ops  = &device_vmops; // contains nopage func
        vma->vm_file   = filp;

3. Application gets pointer and accesses all mapped pages (reads a byte from each page) - this causes call of nopage function in driver . The nopage function calculates VirtualAddress from SharedMemory address (received after pci_alloc_consistent()),
then  does   KernelPage= virt_to_page(VirtualAddress)   ,
  and returns KernelPage.

4. Now Application reads from SharedMemory , using a pointer , given by mmap() . This works fine on x86 platform , but on MIPS application cannot see expected contents of the memory - it reads garbage .

What I have  tried :
- to insert vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot) in driver?s implementation of mmap() ;
  -  to use  remap_page_range() instead of vm_ops .

Result is the same ? application reads garbage . If application write some values into the memory - it reads garbage . The values written by application are not seen from kernel level .

I am new to MIPS so maybe I missed something obvious ? Is this right approach on this platform at all ?

Thank you in advance!

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