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    SubjectSpeed of RAM reserved with memmap kernel command line option.
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    Hello all, 

    We are writing a PCI-e "frame grabber-like" driver for linux 2.6 kernel.

    For exchanging data with the board, we would like to be able to use a big piece of consecutive memory.
    -> we reserve 512 megs after the first Gig of RAM by setting "memmap=512M$1024M" in grub.conf.

    In order to give access to that memory in user space, I use "remap_pfn_range" :

    int myboard_mmap(struct file *filep, struct vm_area_struct *vma) {
    unsigned int board;

    board = MINOR(filep->f_dentry->d_inode->i_rdev);
    vma->vm_flags |= VM_LOCKED;

    if(remap_pfn_range(vma, vma->vm_start, myboard_var[board].dmablkp>>PAGE_SHIFT, DMA_BUFFER_SIZE, vma->vm_page_prot)) {
    printk(KERN_ERR "myboard: myboard_mmap: failed to mmap\n");
    return -EAGAIN;
    }

    return 0;
    }

    (I have set myboard_var[board].dmablkp to the address I set in grub.conf: 0x40000000)

    However when writing or reading that part of memory, speed is terrible.
    I compared the performances to the ones I can reach using a piece of memory got with malloc:
    For transfering 128 megs:
    With malloc"ed" RAM : 23053us
    With "mmap"ed RAM : 1913389us

    I guess cache is not used. Does anyone know if there is a way to speed this up? How could we set that area to "cached"?

    We have seen this post:
    http://www.linux-mips.org/archives/linux-mips/2006-02/msg00120.html

    And we tried adding this to our driver:

    static inline pgprot_t pgprot_cached(pgprot_t _prot)

    {
    unsigned long prot = pgprot_val(_prot);
    prot = (prot & ~_CACHE_MASK);
    return __pgprot(prot);
    }

    vma->vm_page_prot = pgprot_cached(vma->vm_page_prot);

    But it did not help...

    Thank you in advance!

    Best regards,

    Charles




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