On 15/05/2017 15:54, Stefan Wahren wrote: > 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? I'm sure you mean copy_to_user(), and the answer is "it's complicated". Depending on the relative timing, this code can also be called from a kernel thread, so I doubt that would work. Phil _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
