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. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
