[RFC] Kernel panic down to swiotlb when doing insmod a simple driver

[Robin Murphy]

On 13/01/17 11:49, Ard Biesheuvel wrote:
> On 13 January 2017 at 11:47, Robin Murphy <robin.murphy at arm.com> wrote:
>> On 13/01/17 11:25, Ard Biesheuvel wrote:
>>> On 13 January 2017 at 11:03, Robin Murphy <robin.murphy at 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.

Robin.