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

[Petr Tesařík <petr@xxxxxxxxxxx>]

On Mon, 15 Apr 2024 12:23:22 +0000
Michael Kelley <mhklinux@xxxxxxxxxxx> wrote:

> 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?

Oh, sure, there's nothing broken ATM. It's merely a cleanup. The API is
asymmetric and thus confusing. You get a final sync by default if you
call swiotlb_tbl_unmap_single(), but you don't get an initial sync by
default if you call swiotlb_tbl_map_single(). This is difficult to
remember, so potential new users of the API may incorrectly assume that
an initial sync is done, or that a final sync is not done.

And yes, when moving the code, all current users of
swiotlb_tbl_map_single() should specify DMA_ATTR_SKIP_CPU_SYNC.

Petr T