Re: [PATCH v6 2/2] media: V3s: Add support for Allwinner CSI.

[Maxime Ripard]

On Wed, Jan 31, 2018 at 10:37:37AM +0100, Arnd Bergmann wrote:
> On Wed, Jan 31, 2018 at 8:29 AM, Maxime Ripard
> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
> > Hi Thierry,
> >
> > On Tue, Jan 30, 2018 at 11:01:50AM +0100, Thierry Reding wrote:
> >> On Tue, Jan 30, 2018 at 10:59:16AM +0100, Thierry Reding wrote:
> >> > On Tue, Jan 30, 2018 at 10:24:48AM +0100, Arnd Bergmann wrote:
> >> > > On Tue, Jan 30, 2018 at 8:54 AM, Maxime Ripard
> >> > > <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
> >> > > > On Mon, Jan 29, 2018 at 03:34:02PM +0100, Arnd Bergmann wrote:
> >> > > >> On Mon, Jan 29, 2018 at 10:25 AM, Linus Walleij
> >> > > >> <linus.walleij@xxxxxxxxxx> wrote:
> >> > > >> > On Mon, Jan 29, 2018 at 9:25 AM, Maxime Ripard
> >> > > >> > <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
> >> > > >> >> On Sat, Jan 27, 2018 at 05:14:26PM +0100, Linus Walleij wrote:
> >> > >
> >> > > >>
> >> > > >> At one point we had discussed adding a 'dma-masters' property that
> >> > > >> lists all the buses on which a device can be a dma master, and
> >> > > >> the respective properties of those masters (iommu, coherency,
> >> > > >> offset, ...).
> >> > > >>
> >> > > >> IIRC at the time we decided that we could live without that complexity,
> >> > > >> but perhaps we cannot.
> >> > > >
> >> > > > Are you talking about this ?
> >> > > > https://elixir.free-electrons.com/linux/latest/source/Documentation/devicetree/bindings/dma/dma.txt#L41
> >> > > >
> >> > > > It doesn't seem to be related to that issue to me. And in our
> >> > > > particular cases, all the devices are DMA masters, the RAM is just
> >> > > > mapped to another address.
> >> > >
> >> > > No, that's not the one I was thinking of. The idea at the time was much
> >> > > more generic, and not limited to dma engines. I don't recall the details,
> >> > > but I think that Thierry was either involved or made the proposal at the
> >> > > time.
> >> >
> >> > Yeah, I vaguely remember discussing something like this before. A quick
> >> > search through my inbox yielded these two threads, mostly related to
> >> > IOMMU but I think there were some mentions about dma-ranges and so on as
> >> > well. I'll have to dig deeper into those threads to refresh my memories,
> >> > but I won't get around to it until later today.
> >> >
> >> > If someone wants to read up on this in the meantime, here are the links:
> >> >
> >> >     https://lkml.org/lkml/2014/4/27/346
> >> >     http://lists.infradead.org/pipermail/linux-arm-kernel/2014-May/257200.html
> >> >
> >> > From a quick glance the issue of dma-ranges was something that we hand-
> >> > waved at the time.
> >>
> >> Also found this, which seems to be relevant as well:
> >>
> >>       http://lists.infradead.org/pipermail/linux-arm-kernel/2014-May/252715.html
> >>
> >> Adding Dave.
> >
> > Thanks for the pointers, I started to read through it.
> >
> > I guess we have to come up with two solutions here: a short term one
> > to address the users we already have in the kernel properly, and a
> > long term one where we could use that discussion as a starting point.
> >
> > For the short term one, could we just set the device dma_pfn_offset to
> > PHYS_OFFSET at probe time, and use our dma_addr_t directly later on,
> > or would this also cause some issues?
> 
> That would certainly be an improvement over the current version,
> it keeps the hack more localized. That's fine with me.

Ok, we'll do that in that driver and convert the existing drivers
then.

> Note that both PHYS_OFFSET and dma_pfn_offset have architecture
> specific meanings and they could in theory change, so ideally we'd
> do that fixup somewhere in arch/arm/mach-sunxi/ at boot time before
> the driver gets probed, but this wouldn't work on arm64 if we need
> it there too.

Unfortunately, we do :/

> > For the long term plan, from what I read from the discussion, it's
> > mostly centered around IOMMU indeed, and we don't have that. What we
> > would actually need is to break the assumption that the DMA "parent"
> > bus is the DT node's parent bus.
> >
> > And I guess this could be done quite easily by adding a dma-parent
> > property with a phandle to the bus controller, that would have a
> > dma-ranges property of its own with the proper mapping described
> > there. It should be simple enough to support, but is there anything
> > that could prevent something like that to work properly (such as
> > preventing further IOMMU-related developments that were described in
> > those mail threads).
> 
> I've thought about it a little bit now. A dma-parent property would nicely
> solve two problems:
> 
> - a device on a memory mapped control bus that is a bus master on
>   a different bus. This is the case we are talking about here AFAICT
> 
> - a device that is on a different kind of bus (i2c, spi, usb, ...) but also
>   happens to be a dma master on another bus. I suspect we have
>   some of these today and they work by accident because we set the
>   dma_mask and dma_map_ops quite liberally in the DT probe code,
>   but it really shouldn't work according to our bindings. We may also
>   have drivers that work around the issue by forcing the correct dma
>   mask and map_ops today, which makes them work but is rather
>   fragile.

Ok, I'll give it a shot then.

> I can think of a couple of other problems that may or may not be
> relevant in the future that would require a more complex solution:
> 
> - a device that is a bus master on more than one bus, e.g. a
>   DMA engine that can copy between the CPU address space and
>   another memory controller that is not visible to the CPU
> 
> - a device that is connected to main memory both through an IOMMU
>   and directly through its parent bus, and the device itself is in
>   control over which of the two it uses (usually the IOMMU would
>   contol whether a device is bypassing translation)
> 
> - a device that has a single DMA address space with some form
>   of non-linear mapping to one or more parent buses. Some of these
>   can be expressed using the parent's dma-ranges properties, but
>   our code currently only looks at the first entry in dma-ranges.

As far as I know, we're in neither of these cases.

> Another problem is the interaction with the dma-ranges and iommu
> properties. I have not found any real problems here, but we certainly
> need to be careful to define what happens in all combinations and
> make sure that we document it well in the bindings and have those
> reviewed by the affected parties, at least the ARM and PowerPC
> architecture folks as well as the Nvidia and Renesas platform
> maintainers, which in my experience have the most complex DMA
> hardware.

I guess we can discuss it during the review cycles.

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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