userspace address to bus address

[Peter Teoh]

comments about "consistent memory" from here:

http://kerneltrap.org/node/7194

might be useful for you too.....

On Mon, Oct 10, 2011 at 1:13 PM, Peter Teoh <htmldeveloper at gmail.com> wrote:
> 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.
>>
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>
>
>
> --
> Regards,
> Peter Teoh
>



-- 
Regards,
Peter Teoh