RE: [PATCH 1/2] swiotlb: Remove alloc_size argument to swiotlb_tbl_map_single()

[Michael Kelley <mhklinux@xxxxxxxxxxx>]

From: Petr Tesařík <petr@xxxxxxxxxxx> Sent: Monday, April 15, 2024 4:46 AM
> 
> Hi Michael,
> 
> sorry for taking so long to answer. Yes, there was no agreement on the
> removal of the "dir" parameter, but I'm not sure it's because of
> symmetry with swiotlb_sync_*(), because the topic was not really
> discussed.
> 
> The discussion was about the KUnit test suite and whether direction is
> a property of the bounce buffer or of each sync operation. Since DMA API
> defines associates each DMA buffer with a direction, the direction
> parameter passed to swiotlb_sync_*() should match what was passed to
> swiotlb_tbl_map_single(), because that's how it is used by the generic
> DMA code. In other words, if the parameter is kept, it should be kept
> to match dma_map_*().
> 
> However, there is also symmetry with swiotlb_tbl_unmap_single(). This
> function does use the parameter for the final sync. I believe there
> should be a matching initial sync in swiotlb_tbl_map_single(). In
> short, the buffer sync for DMA non-coherent devices should be moved from
> swiotlb_map() to swiotlb_tbl_map_single(). If this sync is not needed,
> then the caller can (and should) include DMA_ATTR_SKIP_CPU_SYNC in
> the flags parameter.
> 
> To sum it up:
> 
> * Do *NOT* remove the "dir" parameter.
> * Let me send a patch which moves the initial buffer sync.
> 

I'm not seeing the need to move the initial buffer sync.  All
callers of swiotlb_tbl_map_single() already have a subsequent
check for a non-coherent device, and a call to 
arch_sync_dma_for_device().  And the Xen code has some 
special handling that probably shouldn't go in
swiotlb_tbl_map_single().  Or am I missing something?

Michael