Re: [RFC v2 4/5] dt-bindings: of: Add plumbing for restricted DMA pool

[Claire Chang <tientzu@chromium.org>]

On Sat, Aug 1, 2020 at 4:58 AM Rob Herring <robh@kernel.org> wrote:
>
> On Tue, Jul 28, 2020 at 01:01:39PM +0800, Claire Chang wrote:
> > Introduce the new compatible string, device-swiotlb-pool, for restricted
> > DMA. One can specify the address and length of the device swiotlb memory
> > region by device-swiotlb-pool in the device tree.
> >
> > Signed-off-by: Claire Chang <tientzu@chromium.org>
> > ---
> >  .../reserved-memory/reserved-memory.txt       | 35 +++++++++++++++++++
> >  1 file changed, 35 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > index 4dd20de6977f..78850896e1d0 100644
> > --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > @@ -51,6 +51,24 @@ compatible (optional) - standard definition
> >            used as a shared pool of DMA buffers for a set of devices. It can
> >            be used by an operating system to instantiate the necessary pool
> >            management subsystem if necessary.
> > +        - device-swiotlb-pool: This indicates a region of memory meant to be
>
> swiotlb is a Linux thing. The binding should be independent.
Got it. Thanks for pointing this out.

>
> > +          used as a pool of device swiotlb buffers for a given device. When
> > +          using this, the no-map and reusable properties must not be set, so the
> > +          operating system can create a virtual mapping that will be used for
> > +          synchronization. Also, there must be a restricted-dma property in the
> > +          device node to specify the indexes of reserved-memory nodes. One can
> > +          specify two reserved-memory nodes in the device tree. One with
> > +          shared-dma-pool to handle the coherent DMA buffer allocation, and
> > +          another one with device-swiotlb-pool for regular DMA to/from system
> > +          memory, which would be subject to bouncing. The main purpose for
> > +          restricted DMA is to mitigate the lack of DMA access control on
> > +          systems without an IOMMU, which could result in the DMA accessing the
> > +          system memory at unexpected times and/or unexpected addresses,
> > +          possibly leading to data leakage or corruption. The feature on its own
> > +          provides a basic level of protection against the DMA overwriting buffer
> > +          contents at unexpected times. However, to protect against general data
> > +          leakage and system memory corruption, the system needs to provide a
> > +          way to restrict the DMA to a predefined memory region.
>
> I'm pretty sure we already support per device carveouts and I don't
> understand how this is different.
We use this to bounce streaming DMA in and out of a specially allocated region.
I'll try to merge this with the existing one (i.e., shared-dma-pool)
to see if that
makes things clearer.

>
> What is the last sentence supposed to imply? You need an IOMMU?
The main purpose is to mitigate the lack of DMA access control on
systems without an IOMMU.
For example, we plan to use this plus a MPU for our PCIe WiFi which is
not behind an IOMMU.

>
> >          - vendor specific string in the form <vendor>,[<device>-]<usage>
> >  no-map (optional) - empty property
> >      - Indicates the operating system must not create a virtual mapping
> > @@ -117,6 +135,16 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
> >                       compatible = "acme,multimedia-memory";
> >                       reg = <0x77000000 0x4000000>;
> >               };
> > +
> > +             wifi_coherent_mem_region: wifi_coherent_mem_region {
> > +                     compatible = "shared-dma-pool";
> > +                     reg = <0x50000000 0x400000>;
> > +             };
> > +
> > +             wifi_device_swiotlb_region: wifi_device_swiotlb_region {
> > +                     compatible = "device-swiotlb-pool";
> > +                     reg = <0x50400000 0x4000000>;
> > +             };
> >       };
> >
> >       /* ... */
> > @@ -135,4 +163,11 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
> >               memory-region = <&multimedia_reserved>;
> >               /* ... */
> >       };
> > +
> > +     pcie_wifi: pcie_wifi@0,0 {
> > +             memory-region = <&wifi_coherent_mem_region>,
> > +                      <&wifi_device_swiotlb_region>;
> > +             restricted-dma = <0>, <1>;
> > +             /* ... */
> > +     };
> >  };
> > --
> > 2.28.0.rc0.142.g3c755180ce-goog
> >