[Xen-devel] Re: mthca use of dma_sync_single is bogus

["Michael S. Tsirkin" <mst@xxxxxxxxxxxxxxxxxx>]

> Quoting Roland Dreier <rdreier@xxxxxxxxx>:
> Subject: mthca use of dma_sync_single is bogus
> 
> It seems the problems running mthca in a Xen domU have uncovered a bug
> in mthca: mthca uses dma_sync_single in mthca_arbel_write_mtt_seg()
> and mthca_arbel_map_phys_fmr() to sync the MTTs that get written.
> However, Documentation/DMA-API.txt says:
> 
>     void
>     dma_sync_single(struct device *dev, dma_addr_t dma_handle, size_t size,
>               enum dma_data_direction direction)
> 
>     synchronise a single contiguous or scatter/gather mapping.  All the
>     parameters must be the same as those passed into the single mapping
>     API.
> 
> and mthca is *not* following this clear rule: it is trying to sync
> only a subrange of the mapping.

Yes, this looks like a bug.

> Later on in the document, there is:
> 
>     void
>     dma_sync_single_range(struct device *dev, dma_addr_t dma_handle,
>                     unsigned long offset, size_t size,
>                     enum dma_data_direction direction)
>     
>     does a partial sync.  starting at offset and continuing for size.  You
>     must be careful to observe the cache alignment and width when doing
>     anything like this.  You must also be extra careful about accessing
>     memory you intend to sync partially.
> 
> but that is in a section dealing with non-consistent memory so it's
> not entirely clear to me whether it's kosher to use this as mthca
> wants.

This is under Part II - Advanced dma_ usage - I don't think it's dealing with
non-consistent memory only (e.g. dma_declare_coherent_memory is there), and this
looks like a good fit.  Most functions here work for both consistent and
non-consistent memory...  What makes you suspicious?

> The other alternative is to put the MTT table in coherent memory just
> like the MPT table.  That might be the best solution I suppose...
> 
> Michael, anyone else, thoughts on this?

Certainly easy ...

I'm concerned that MTTs need a fair amount of memory,
while the amount of coherent memory might be limited.
Not that non-coherent memory systems are widespread ...

-- 
MST

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