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

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

On 2022-03-25 10:25, Maxime Bizon wrote:
> 
> On Thu, 2022-03-24 at 12:26 -0700, Linus Torvalds wrote:
> 
>>
>> It's actually very natural in that situation to flush the caches from
>> the CPU side again. And so dma_sync_single_for_device() is a fairly
>> reasonable thing to do in that situation.
>>
> 
> In the non-cache-coherent scenario, and assuming dma_map() did an
> initial cache invalidation, you can write this:
> 
> rx_buffer_complete_1(buf)
> {
> 	invalidate_cache(buf, size)
> 	if (!is_ready(buf))
> 		return;
> 	<proceed with receive>
> }
> 
> or
> 
> rx_buffer_complete_2(buf)
> {
> 	if (!is_ready(buf)) {
> 		invalidate_cache(buf, size)
> 		return;
> 	}
> 	<proceed with receive>
> }
> 
> The latter is preferred for performance because dma_map() did the
> initial invalidate.
> 
> Of course you could write:
> 
> rx_buffer_complete_3(buf)
> {
> 	invalidate_cache(buf, size)
> 	if
> (!is_ready(buf)) {
> 		invalidate_cache(buf, size)
> 		return;
> 	}
> 	
> <proceed with receive>
> }
> 
> 
> but it's a waste of CPU cycles
> 
> So I'd be very cautious assuming sync_for_cpu() and sync_for_device()
> are both doing invalidation in existing implementation of arch DMA ops,
> implementers may have taken some liberty around DMA-API to avoid
> unnecessary cache operation (not to blame them).

Right, if you have speculatively-prefetching caches, you have to 
invalidate DMA_FROM_DEVICE in unmap/sync_for_cpu, since a cache may have 
pulled in a snapshot of partly-written data at any point beforehand. But 
if you don't, then you can simply invalidate up-front in 
map/sync_for_device to tie in with the other directions, and trust that 
it stays that way for the duration.

What muddies the waters a bit is that the opposite combination 
sync_for_cpu(DMA_TO_DEVICE) really *should* always be a no-op, and I for 
one have already made the case for eliding that in code elsewhere, but 
it doesn't necessarily hold for the inverse here, hence why I'm not sure 
there even is a robust common solution for peeking at a live 
DMA_FROM_DEVICE buffer.

Robin.

> For example looking at arch/arm/mm/dma-mapping.c, for DMA_FROM_DEVICE
> 
> sync_single_for_device()
>    => __dma_page_cpu_to_dev()
>      => dma_cache_maint_page(op=dmac_map_area)
>        => cpu_cache.dma_map_area()
> 
> sync_single_for_cpu()
>    => __dma_page_dev_to_cpu()
>      =>
> __dma_page_cpu_to_dev(op=dmac_unmap_area)
>        =>
> cpu_cache.dma_unmap_area()
> 
> dma_map_area() always does cache invalidate.
> 
> But for a couple of CPU variant, dma_unmap_area() is a noop, so
> sync_for_cpu() does nothing.
> 
> Toke's patch will break ath9k on those platforms (mostly silent
> breakage, rx corruption leading to bad performance)
> 
> 
>> There's a fair number of those dma_sync_single_for_device() things
>> all over. Could we find mis-uses and warn about them some way? It
>> seems to be a very natural thing to do in this context, but bounce
>> buffering does make them very fragile.
> 
> At least in network drivers, there are at least two patterns:
> 
> 1) The issue at hand, hardware mixing rx_status and data inside the
> same area. Usually very old hardware, very quick grep in network
> drivers only revealed slicoss.c. Probably would have gone unnoticed if
> ath9k hardware wasn't so common.
> 
> 
> 2) The very common "copy break" pattern. If a received packet is
> smaller than a certain threshold, the driver rx path is changed to do:
> 
>   sync_for_cpu()
>   alloc_small_skb()
>   memcpy(small_skb, rx_buffer_data)
>   sync_for_device()
> 
> Original skb is left in the hardware, this reduces memory wasted.
> 
> This pattern is completely valid wrt DMA-API, the buffer is always
> either owned by CPU or device.
> 
>