Re: [never mind] kiobufs and highmem

From: Ingo Oeser (ingo.oeser@informatik.tu-chemnitz.de)
Date: Fri Jul 19 2002 - 17:39:18 EST

On Fri, Jul 19, 2002 at 08:00:00AM -0700, William D Waddington wrote:
> I haven't yet figured out how to allocate a (possibly) non-contiguous
> buffer, since vmalloc is frowned on, or how to populate the kiobuf
> with its pages.

Don't use the kiobuf, since it is bloated.

Try this instead:

/* Pin down user pages and put them into a scatter gather list */
int sg_map_user_pages(struct scatterlist *sgl, const unsigned int max_pages,
                unsigned long uaddr, size_t count, int rw) {
        int res, i;
        unsigned int nr_pages=((uaddr & ~PAGE_MASK) + count - 1 + ~PAGE_MASK) >> PAGE_SHIFT;
        struct page *pages[max_pages];

        /* User attempted Overflow! */
        if ((uaddr + count) < uaddr)
                return -EINVAL;

        /* To big */
        if (nr_pages > max_pages)
                return -ENOMEM;

        /* Hmm? */
        if (count == 0)
                return 0;

        /* Not enough SGL entries provided! */
        BUG_ON((((uaddr & ~PAGE_MASK) + count + ~PAGE_MASK) >> PAGE_SHIFT) > max_pages );

        down_read(&current->mm->mmap_sem);
        res = get_user_pages(
                        current,
                        current->mm,
                        uaddr,
                        nr_pages,
                        rw == READ, /* logic is perversed^Wreversed here :-( */
                        0, /* don't force */
                        &pages[0],
                        NULL);
        up_read(&current->mm->mmap_sem);

        /* Errors and no page mapped should return here */
        if (res <= 0) return res;

        memset(sgl, 0, sizeof(*sgl) * nr_pages);

        sgl[0].page = pages[0];
        sgl[0].offset = uaddr & PAGE_MASK;
        
        /* Page crossing transfers need these adjustments */
        if (res > 1) {
                for (i=1; i < res ; i++) {
                        sgl[i].offset = 0;
                        sgl[i].page = pages[i];
                        sgl[i].length = PAGE_SIZE;
                }
                sgl[0].length = PAGE_SIZE - sgl[0].offset;
                count -= sgl[0].length;
                count -= (res - 2) * PAGE_SIZE;
        }

        sgl[res - 1].length = count;

        return res;
}

/* And unmap them... */
int sg_unmap_user_pages(struct scatterlist *sgl, const unsigned int nr_pages) {
        int i;

        for (i=0; i < nr_pages; i++)
                page_cache_release(sgl[i].page);

        return 0;
}

That will give you nearly the same. You just have to lock_page()
the pages and UnlockPage() yourself.

Don't give to high values for max_pages, because this might
overflow your stack.

Do the pci_map_sg() after sg_map_user_pages() and pci_unmap_sg()
before sg_unmap_user_pages().

This should work. If you have highmem pages and your device can't
handle >32-bit physical addresses, then kmap() them
before pci_map_sg()ing them and kunmap() them after
pci_unmap_sg()ing.

kiobufs are (next to) useless for character io devices.

Hope that helps

Regards

Ingo Oeser 

-- 
Science is what we can tell a computer. Art is everything else. --- D.E.Knuth
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This archive was generated by hypermail 2b29 : Tue Jul 23 2002 - 22:00:31 EST