Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP

[Robin Murphy <robin.murphy@arm.com>]

On 2022-03-24 10:25, Oleksandr Natalenko wrote:
> Hello.
> 
> On čtvrtek 24. března 2022 6:57:32 CET Christoph Hellwig wrote:
>> On Wed, Mar 23, 2022 at 08:54:08PM +0000, Robin Murphy wrote:
>>> I'll admit I still never quite grasped the reason for also adding the
>>> override to swiotlb_sync_single_for_device() in aa6f8dcbab47, but I think
>>> by that point we were increasingly tired and confused and starting to
>>> second-guess ourselves (well, I was, at least). I don't think it's wrong
>>> per se, but as I said I do think it can bite anyone who's been doing
>>> dma_sync_*() wrong but getting away with it until now. If ddbd89deb7d3
>>> alone turns out to work OK then I'd be inclined to try a partial revert of
>>> just that one hunk.
>>
>> Agreed.  Let's try that first.
>>
>> Oleksandr, can you try the patch below:
>>
>>
>> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
>> index 6db1c475ec827..6c350555e5a1c 100644
>> --- a/kernel/dma/swiotlb.c
>> +++ b/kernel/dma/swiotlb.c
>> @@ -701,13 +701,10 @@ void swiotlb_tbl_unmap_single(struct device *dev, phys_addr_t tlb_addr,
>>   void swiotlb_sync_single_for_device(struct device *dev, phys_addr_t tlb_addr,
>>   		size_t size, enum dma_data_direction dir)
>>   {
>> -	/*
>> -	 * Unconditional bounce is necessary to avoid corruption on
>> -	 * sync_*_for_cpu or dma_ummap_* when the device didn't overwrite
>> -	 * the whole lengt of the bounce buffer.
>> -	 */
>> -	swiotlb_bounce(dev, tlb_addr, size, DMA_TO_DEVICE);
>> -	BUG_ON(!valid_dma_direction(dir));
>> +	if (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL)
>> +		swiotlb_bounce(dev, tlb_addr, size, DMA_TO_DEVICE);
>> +	else
>> +		BUG_ON(dir != DMA_FROM_DEVICE);
>>   }
>>   
>>   void swiotlb_sync_single_for_cpu(struct device *dev, phys_addr_t tlb_addr,
>>
> 
> With this patch the AP works for me.

Cool, thanks for confirming. So I think ath9k probably is doing 
something dodgy with dma_sync_*(), but if Linus prefers to make the 
above change rather than wait for that to get figured out, I believe 
that should be fine.

The crucial part of the "rework" patch is that we'll unconditionally 
initialise the SWIOTLB bounce slot as it's allocated in 
swiotlb_tbl_map_single(), regardless of DMA_ATTR_SKIP_CPU_SYNC. As long 
as that happens, we're safe in terms of leaking data from previous 
mappings, and any possibility for incorrect sync usage to lose 
newly-written DMA data is at least no worse than it always has been. The 
most confusion was around how the proposed DMA_ATTR_OVERWRITE attribute 
would need to interact with DMA_ATTR_SKIP_CPU_SYNC to remain safe but 
still have any useful advantage, so unless and until anyone wants to 
revisit that, this should remain comparatively simple to reason about.

Cheers,
Robin.