Am 15.05.2017 um 16:29 schrieb 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); Btw shouldn't we use copy_from_user() at this place? _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
