Re: [RFC PATCH V1 1/6] dt-bindings: mt8183: Add binding for DIP shared memory

Rob Herring
2019-05-14T16:19:38Z

On Tue, May 7, 2019 at 9:22 AM Frederic Chen <frederic.chen@mediatek.com> wrote:
>
> Dear Rob,
>
> I appreciate your comments.
>
> On Mon, 2019-04-29 at 20:15 -0500, Rob Herring wrote:
> > On Wed, Apr 17, 2019 at 06:45:06PM +0800, Frederic Chen wrote:
> > > This patch adds the binding for describing the shared memory
> > > used to exchange configuration and tuning data between the
> > > co-processor and Digital Image Processing (DIP) unit of the
> > > camera ISP system on Mediatek SoCs.
> > >
> > > Signed-off-by: Frederic Chen <frederic.chen@mediatek.com>
> > > ---
> > > .../mediatek,reserve-memory-dip_smem.txt | 45 +++++++++++++++++++
> > > 1 file changed, 45 insertions(+)
> > > create mode 100644 Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-dip_smem.txt
> > >
> > > diff --git a/Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-dip_smem.txt b/Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-dip_smem.txt
> > > new file mode 100644
> > > index 000000000000..64c001b476b9
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-dip_smem.txt
> > > @@ -0,0 +1,45 @@
> > > +Mediatek DIP Shared Memory binding
> > > +
> > > +This binding describes the shared memory, which serves the purpose of
> > > +describing the shared memory region used to exchange data between Digital
> > > +Image Processing (DIP) and co-processor in Mediatek SoCs.
> > > +
> > > +The co-processor doesn't have the iommu so we need to use the physical
> > > +address to access the shared buffer in the firmware.
> > > +
> > > +The Digital Image Processing (DIP) can access memory through mt8183 IOMMU so
> > > +it can use dma address to access the memory region.
> > > +(See iommu/mediatek,iommu.txt for the detailed description of Mediatek IOMMU)
> > > +
> > > +
> > > +Required properties:
> > > +
> > > +- compatible: must be "mediatek,reserve-memory-dip_smem"
> >
> > Don't use '_'.
>
> I got it. I will use "mediatek,reserve-memory-dip-smem" instead in next
> version of the patch
>
> >
> > > +
> > > +- reg: required for static allocation (see reserved-memory.txt for
> > > + the detailed usage)
> > > +
> > > +- alloc-range: required for dynamic allocation. The range must
> > > + between 0x00000400 and 0x100000000 due to the co-processer's
> > > + addressing limitation
> >
> > Generally, you should pick either static or dynamic allocation for a
> > given binding. Static if there's some address restriction or sharing,
> > dynamic if not.
> >
> > Sounds like static in this case.
> >
>
> DIP reserved memory has address restriction so it is the static case. I
> would like to remove the dynamic allocation part and modify the
> description as following:
>
> - reg: required for DIP. The range must be between 0x00000400 and
> 0x100000000 due to the co-processor's addressing limitation.
> The size must be 26MB. Please see reserved-memory.txt for the
> detailed usage.

You can use dma-ranges to define addressing translations and
restrictions like this. That will in turn set the device's dma-mask to
ensure allocations are done in a region that is addressable.

But if you have a known, fixed size, then a carve out with
reserved-memory is fine.

Rob