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 _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
