Sorry for the late reply,

On Thu, 21 Jan 2010 22:38:41 -0800 (PST)
David Miller [off-list ref] wrote:

From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Date: Fri, 22 Jan 2010 14:11:29 +0900

quoted

Even if 'offset' is zero, 'size' still matters, I think. If 'size' is
not a multiple of the cache line size, it's possible that driver
writers who aren't familiar with cache would be surprised (it depends
on the way their drivers use buffers though).

The easiest way for 'completely safe sync for any driver writers' is
asking for all the sync parameters must be the same as those passed
into the single mapping API. If writes knows what they do, they can do
a partial sync with sync_range API. That's the author intention, I
guess.

This is not reasonable.

You have to think about how people actually use these
interfaces.

They have a large buffer, and if they receive a small request they
want to allocate a smaller buffer, copy into that smaller buffer, and
give the larger buffer back to the hardware.

It's an optimization, it performs better this way.

If you make it so that the DMA sync has to cover the entire large
buffer, the whole point of the optimization is taken away.

I talked with James. He is ok with changing (or fixing) this API to
enable users to do a partial sync (I'm ok with that too. I just
guessed that he designed the API in such way intentionally not by
mistake).

Can we safely assume that the arch implementations already round
up/down to the safe boundary internally in this API (they should
already)?

As you know, the patch to remove the description of
dma_sync_single/pci_dma_sync_single/dma_sync_sg/pci_dma_sync_pci that
always require a full sync in DMA-API.txt is already -mm so what we
need to do are:

- adding 'a partial sync' description to PCI-DMA-mapping.txt.
- duplicating the similar description to DMA-API.txt.

I don't like two DMA docs. I like to make pci_dma_* API obsolete. We
have the generic DMA API with generic devices so we are always able to
use the API (as you did with sbus_map_*). The majority arch
implementations safely call the bus specific DMA functions via the
generic DMA API. So there are not many things to do. We can just
convert pci_dma_* to dma_* API slowly.

Opinions?