RE: [GIT PULL] DMA-mapping framework updates for 3.4

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Linus,

On Friday, March 23, 2012 10:36 PM Linus Torvalds wrote:

> On Tue, Mar 20, 2012 at 12:24 AM, Marek Szyprowski
> <m.szyprowski@samsung.com> wrote:
> >
> > =A0git://git.infradead.org/users/kmpark/linux-samsung =
dma-mapping-next
> >
> > Those patches introduce a new alloc method (with support for memory
> > attributes) in dma_map_ops structure, which will later replace
> > dma_alloc_coherent and dma_alloc_writecombine functions.
>=20
> So I'm quite unhappy with these patches.
>=20
> Here's just the few problems I saw from some *very* quick look-through
> of the git tree:
>=20
>  - I'm not seeing ack's from the architecture maintainers for the
> patches that change some architecture.

Ok, I've asked personally each respective maintainer for an ack or=20
comments. Before I've sent my pull request there were only a few =
comments
on the mailing lists, but this topic have been discussed at ELC-E in =
Prague
and Linaro Memory-management summit in Budapest (May 2011).
=20
>  - Even more importantly, what I really want is acks and comments from
> the people who are expected to *use* this.

The plan is to use it as a base for further cleanup in the dma-mapping=20
implementations, especially on ARM architecture. The changes are =
designed
in such a way to keep compatibility with the existing users of the API.=20
ARM will be the first architecture which will use the new attributes.=20
The main clients for this new API will be mainly multimedia drivers =
(v4l2,
drm) and dma_buf buffer sharing. The advantage of this approach is the=20
fact that the same drivers can be used on other architectures without =
any
changes in the dma calls. The attributes which are not supported by the
architecture will be simply ignored.

>  - it looks like patches break compilation half-way through the
> series. Just one example I noticed: the "x86 adaptation" patch changes
> the functions in lib/swiotlb.c, but afaik ia64 *also* uses those. So
> now ia64 is broken until a couple of patches later. I suspect there
> are other examples like that.

Ok, I missed this and I will fix this issue asap.

>  - the sign-off chains are odd. What happened there? Several patches
> are signed off by Kyungmin Park, but he doesn't seem to be "in the
> chain" at all. Whazzup? (*)

That was caused by our internal flow of the patches, but I see that it=20
made only a lot of confusion. I got my own git repository at=20
git.linaro.org and I will resolve these sign-off issues correctly there.

>  - Finally, how/why are "dma attributes" different from the per-device
> dma limits ("device_dma_parameters")

Device dma parameters are global for all dma mapping operations for the=20
specified device, while dma attributes can be set for each allocation or
mapping call. Dma attributes are mainly used to provide some hints to =
the
dma mapping core, which might improve speed/performance/throughput for
some particular sw&hw architectures. Unsupported attributes are ignored,
so the in the worst case a driver gets coherent mapping.

Best regards
--=20
Marek Szyprowski
Samsung Poland R&D Center