Messages in this thread | | | From | Ard Biesheuvel <> | Date | Fri, 13 Jan 2017 11:54:40 +0000 | Subject | Re: [RFC] Kernel panic down to swiotlb when doing insmod a simple driver |
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On 13 January 2017 at 11:52, Robin Murphy <robin.murphy@arm.com> wrote: > On 13/01/17 11:49, Ard Biesheuvel wrote: >> On 13 January 2017 at 11:47, Robin Murphy <robin.murphy@arm.com> wrote: >>> On 13/01/17 11:25, Ard Biesheuvel wrote: >>>> On 13 January 2017 at 11:03, Robin Murphy <robin.murphy@arm.com> wrote: >>>>> On 13/01/17 10:00, Shawn Lin wrote: >>>>>> Hi, >>>>>> >>>>>> Sorry for sending this RFC for help as I couldn't find some useful hint >>>>>> to slove my issue by git-log the swiotlb commit from kernel v4.4 to >>>>>> v4.9 and I'm also not familar with these stuff. So could you kindly >>>>>> point me to the right direction to debug it? Thanks. :) >>>>>> >>>>>> -------------------------------------- >>>>>> We just have a very simple wifi driver *built as ko module* which only >>>>>> have a probe function to do the basic init work and call SDIO API to >>>>>> transfer some bytes. >>>>>> >>>>>> Env: kernel 4.4 stable tree, ARM64(rk3399) >>>>>> >>>>>> Two cases are included: >>>>> >>>>> And they are both wrong :) >>>>> >>>>>> The crash case: >>>>>> >>>>>> u8 __aligned(32) buf[PAGE_SIZE]; //global here in ko driver file >>>>> >>>>> It is only valid to do DMA from linear map addresses - I'm not sure if >>>>> the modules area was in the linear map before, but either way it >>>>> probably isn't now (Ard, Mark?). Either way, I don't believe static data >>>>> honours ARCH_DMA_MINALIGN in general, so it's still highly inadvisable. >>>>> >>>> >>>> The __aligned() modifier should work fine: the alignment is propagated >>>> to the ELF section alignment, which in turn is honoured by the module >>>> loader. The problem is that '32' is too low for non-coherent DMA to be >>>> safe. In general, alignments up to 4 KB should work everywhere. >>> >>> Does that alignment also implicitly apply to the size, though? In other >>> words, given: >>> >>> static int X >>> static int __aligned(32) Y; >>> static int Z; >>> >>> is it guaranteed that if, say, X gets placed at .data + 0, so Y goes to >>> .data + 32, then Z *cannot* be placed at .data + 36? >>> >> >> I'm not sure if I understand the question: why would it be incorrect >> for Z to be placed at .data + 36? > > Because they'd then wind up in the same cache line, so non-coherent DMA > to Y will result in concurrent CPU updates to Z being lost/corrupted. > ARCH_DMA_MINALIGN isn't about alignemnt per se, it's about keeping > things in distinct cache writeback granules. >
I understand that. But the original code did
u8 __aligned(32) buf[PAGE_SIZE]; //global here in ko driver file
so there the size is guaranteed to be a multiple of the CWG
So to answer your question: no, the compiler will not round up the size of the allocation to the alignment, it will only align the start.
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