[Bug] VCHIQ functional test broken

[Phil Elwell]

On 13/05/2017 10:30, Russell King - ARM Linux wrote:
> On Sat, May 13, 2017 at 11:07:28AM +0200, Stefan Wahren wrote:
>> In the meantime this issue has been fixed by Phil [1].
> 
> Right - definitely a driver bug.  Mapping more memory for DMA than is
> actually going to be DMA'd to and expecting data to be preserved is
> really horrid.

That feature was added during the upstreaming process, and as Stefan says
there is an outstanding patch for it.

>> Unfortunately i found another issue. If i enable CONFIG_HIGHMEM in
>> the kernel config, the data during functional test gets corrupted.
>> Phil said it's caused by the usage of get_user_pages() [2].
> 
> Without knowing who "Phil" is in that thread, but...
> 
>    HIGHMEM is a problem because you can't use get_user_pages on pages in
>    HIGHMEM.
> 
> is an interesting statement, and without any reasoning or evidence.
> 
> I also believe it to be incorrect.  get_user_pages() returns an array
> of struct page pointers for the user memory, calling flush_dcache_page()
> and flush_anon_page() on them to ensure that any kernel mapping is
> coherent with what is in userspace.
> 
> As far as returning the array of page pointers, get_user_pages() doesn't
> care whether they're lowmem or highmem.
> 
> flush_dcache_page() doesn't care either - if it wants to flush the page
> and the page is a highmem page, it will temporarily map it before
> flushing it.
> 
> flush_anon_page() is a no-op for all non-aliasing caches.
> 
> get_user_pages() works fine for whatever memory on other platforms and
> drivers such as etnaviv, so I think this comes down to the vchip driver
> doing things in ways that the kernel interfaces its using don't expect -
> exactly like the "lets pass full pages to the DMA API" broken-ness.

See previous comment.

> I would like to hear the justification for that statement, but without
> any justification, I assert that the statement is false.

I am the Phil in question, and the off-the-cuff comment was the result of
a hazy memory of issues encountered with VCHIQ bulk transfers as a Broadcom
employee (which would have been on a 2.6 kernel). I suspect there may have
been some use of kernel virtual addresses as an intermediate representation,
but I no longer have access to that code.

If get_user_pages is HIGHMEM-safe (and I can see why it would be), then the
cause of the corruption Stefan saw is probably the special handling of
unaligned reads, specifically:

			memcpy((char *)page_address(pages[0]) +
				pagelist->offset,
				fragments,
				head_bytes);

Phil