Re: [PATCH 1/3 v3] dma: document dma_{un}map_{single|sg}_attrs() interface

Michael Ellerman
2008-02-29T02:45:59Z

On Thu, Feb 28, 2008 at 2:24 PM, <akepner@sgi.com> wrote:
>
> Document the new dma_{un}map_{single|sg}_attrs() functions.
>
> diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
> index e69de29..36baea5 100644
> --- a/Documentation/DMA-attributes.txt
> +++ b/Documentation/DMA-attributes.txt
> @@ -0,0 +1,29 @@
> + DMA attributes
> + ==============
> +
> +This document describes the semantics of the DMA attributes that are
> +defined in linux/dma-attrs.h.
> +
> +
> +DMA_ATTR_SYNC_ON_WRITE
> +----------------------
> +
> +DMA_ATTR_SYNC_ON_WRITE is used on the IA64_SGI_SN2 architecture.
> +It provides a mechanism for devices to explicitly order their DMA
> +writes.
> +
> +On IA64_SGI_SN2 machines, DMA may be reordered within the NUMA
> +interconnect. Allowing reordering improves performance, but in some
> +situations it may be necessary to ensure that one DMA write is
> +complete before another is visible. For example, if the device does
> +a DMA write to indicate that data is available in memory, DMA of the
> +"completion indication" can race with DMA of data.
> +
> +When a memory region is mapped with the DMA_ATTR_SYNC_ON_WRITE attribute,
> +a write to that region causes all in-flight DMA to be flushed to memory.
> +Any pending DMA will complete and be visible in memory before the write
> +to the region with the DMA_ATTR_SYNC_ON_WRITE attribute becomes visible.

I'm not clear how this is all meant to work. Your intial patch says
this is an interface to pass "architecture-specific
attributes" from drivers through to the DMA mapping code, which is
fair enough - we want to do something similar.

But it's not clear that DMA_ATTR_SYNC_ON_WRITE is architecture
specific. If I was a driver writer might assume it works on all
platforms. And in fact in patch 3/3 you define it in
linux/dma-attrs.h, so it's not architecture specific.

What is architecture specific is the semantic. DMA_ATTR_SYNC_ON_WRITE
is defined entirely in terms of what it does on ia64 hardware, and a
particular flavour thereof.

It just seems to me we're going to end up with a gazillion flags,
because DMA_ATTR_SYNC_ON_WRITE isn't quite the same semantic as what
Power can do. So we'll have to add
DMA_ATTR_SYNC_ON_WRITE_WRT_OTHER_SYNC_ON_WRITE_MAPPINGS_ONLY and so
on. Or, we end with subtle driver bugs because the semantics aren't
clear across platforms.

I guess I think the attributes should either be truly arch-specific,
ie. DMA_ATTR_IA64_SYNC_ON_WRITE. Or we need to come up with some well
defined, and architecture neutral semantics for what the flags mean.

cheers