userspace address to bus address

[Peter Teoh]

while reading this:

http://linux.die.net/man/2/mlock

notice that u have to allocate + write to the memory first, before
mlocking it - did u do that?

(this is to prevent copy-on-write page fault, which means the memory
is allocated.....but is not really resident until written to it....so
allocated memory does not mean the physical memory exist.)

also, check this function out:   drivers/dma/iovlock.c (this function
prepare the memory for IO scatter-gather operation, ie, physical
address must be used, and no segfault is allowed):

struct dma_pinned_list *dma_pin_iovec_pages(struct iovec *iov, size_t len)
{

                if (!access_ok(VERIFY_WRITE, iov[i].iov_base, iov[i].iov_len))
                        goto unpin;

                page_list->nr_pages = num_pages_spanned(&iov[i]);
                page_list->base_address = iov[i].iov_base;

                page_list->pages = pages;
                pages += page_list->nr_pages;

                /* pin pages down */
                down_read(&current->mm->mmap_sem);
                ret = get_user_pages(
                        current,
                        current->mm,
                        (unsigned long) iov[i].iov_base,
                        page_list->nr_pages,
                        1,      /* write */
                        0,      /* force */
                        page_list->pages,
                        NULL);
                up_read(&current->mm->mmap_sem);
=========================
which thus showed the same pattern here, write to it to ensure the TLB
(translation lookaside) exists for the physical address, so there is
no pagefault occurring why hardware read/write the memory bus, which
always goes by physical address (vs everything internal to CPU is
logical/virtual address).

On Tue, Sep 13, 2011 at 1:45 AM, Florian Bauer
<florian.bauer at informatik.stud.uni-erlangen.de> wrote:
> Hi,
> I'm currently writing my first kernel module, but i got stuck with address
> translations. I want my pcie device to access a buffer which was allocated
> in userspace (page aligned, currently smaller than one page to ignore
> scattering). I tried to translate the userspace address to a bus address
> in my ioctrl handler like this:
>
> void __user *data = arg;
> down_read(&current->mm->mmap_sem);
> get_user_pages(current, current->mm, (unsigned long)data, len/*1*/, 1, 0,
> &pages, NULL);
> up_read(&current->mm->mmap_sem);
> void* kvaddr = kmap(pages[0]);
> unsigned long baddr = virt_to_bus(kaddr);
> kunmap(pages[0]);
> page_cache_release(pages[0]);
> //...
>
> data is page-aligned and was locked in userspace with mlock.
> There is some address in baddr, but the device does not write to the
> correct location and sometimes causes kernel freezes. Can someone tell me,
> what is wrong with this code?TheI device works with a address, that was
> received in different way (closed source driver).
> I use 32 bit Ubuntu with an 2.6.32 kernel.
>
>
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>



-- 
Regards,
Peter Teoh