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We pretty much assume everything is DMA safe already, the majority of
transfers go to/from kmalloc()ed scratch buffers so actually are DMA
safe but for bulk transfers we use the caller buffer and there might be
some problem users.

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So, pretty much the situation I2C was in before this patch set: we
pretty much assume DMA safety but there might be problem users.

It's a bit different in that it's much more likely that a SPI controller
will actually do DMA than an I2C one since the speeds are higher and
there's frequent applications that do large transfers so it's more
likely that people will do the right thing as issues would tend to come
up if they don't.

Yes, for SPI this is true. I was thinking more of regmap with its
usage of different transport mechanisms. But I guess they should all be
DMA safe because some of them need to be DMA safe?

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I can't really think of a particularly good way of
handling it off the top of my head, obviously not setting the flag is
easy but doesn't get the benefit while always using a bounce buffer
would involve lots of unneeded memcpy().  Doing _dmasafe() isn't
particularly appealing either but might be what we end up with.

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Okay. That sounds you are fine with the approach taken here, in general?

It's hard to summon enthusiasm but yes, without changes to the DMA stuff
it's probably as good as we can do.

That sums it up pretty nicely. However, despite even my limited
enthusiasm for this extra flag, I think this set of rules has value. For
some platforms, DMA works more or less by coincidence and can lead to
surprises with e.g. virtual stacks. This is not good engineering
practice, I'd say.

I am going to apply this series now. Thanks for your input!