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

[Liviu Dudau <liviu@dudau.co.uk>]

On Wed, Jan 31, 2018 at 08:42:12AM +0100, Maxime Ripard wrote:
> Hi Liviu,

Hi Maxime,

> 
> On Wed, Jan 31, 2018 at 03:08:08AM +0000, Liviu Dudau wrote:
> > On Fri, Jan 26, 2018 at 11:00:41AM +0800, Yong wrote:
> > > Hi Maxime,
> > > 
> > > On Fri, 26 Jan 2018 09:46:58 +0800
> > > Yong <yong.deng@magewell.com> wrote:
> > > 
> > > > Hi Maxime,
> > > > 
> > > > Do you have any experience in solving this problem?
> > > > It seems the PHYS_OFFSET maybe undeclared when the ARCH is not arm.
> > > 
> > > Got it.
> > > Should I add 'depends on ARM' in Kconfig?
> > 
> > No, I don't think you should do that, you should fix the code.
> > 
> > The dma_addr_t addr that you've got is ideally coming from dma_alloc_coherent(),
> > in which case the addr is already "suitable" for use by the device (because the
> > bus where the device is attached to does all the address translations).
> 
> Like we're discussing in that other part of the thread with Thierry
> and Arnd, things are slightly more complicated than that :)

Yeah, sorry, my threading of the discussion was broken and I've seen the rest of the 
thread after I have replied. My bad!

> 
> In our case, the bus where the device is attached will not do the
> address translations, and shouldn't.

In my view, the bus is already doing address translation at physical level, AFAIU it
remaps the memory to zero. What you (we?) need is a simple bus driver that registers
the correct virt_to_bus()/bus_to_virt() hooks for the device that do this translation
at the DMA API level as well.

> 
> > If you apply PHYS_OFFSET forcefully to it you might get unexpected
> > results.
> 
> Out of curiosity, what would be these unexpected results?

If in the future (or a parallel world setup) the device is sitting behind an IOMMU, the
addr value might well be smaller than PHYS_OFFSET and you will under-wrap, possibly
starting to hit kernel physical addresses (or anything sitting at the top of the physical
memory).

>From my time playing with IOMMUs and PCI domains, I've learned to treat the dma_addr_t as
a cookie value and never try to do arithmetics with it.

Best regards,
Liviu

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