[Bug] VCHIQ functional test broken

[Russell King - ARM Linux]

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.

> 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.

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

-- 
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