Re: [PATCH v5 0/5] iommu: Support mappings/reservations in reserved-memory regions

Re: [PATCH v5 0/5] iommu: Support mappings/reservations in reserved-memory regions

[Janne Grunau]

Hej,

I'm working on the display controller for Apple silicon SoCs and will 
add some comments with support for it in mind.

added asahi@lists.linux.dev to CC for the Apple silicon related aspects

On 2022-05-12 21:00:47 +0200, Thierry Reding wrote:
> 
> this is another attempt at solving the problem of passing IOMMU
> configuration via device tree. It has significantly evolved since the
> last attempt, based on the discussion that followed. The discussion can
> be found here:
> 
>   https://lore.kernel.org/all/20210423163234.3651547-1-thierry.reding@gmail.com/
> 
> Rather than using a memory-region specifier, this new version introduces
> a new "iommu-addresses" property for the reserved-memory regions
> themselves.

If experimented with both proposed bindings for dcp and I think this 
binding is easer to understand and to work with.

> These are used to describe either a static mapping or
> reservation that should be created for a given device. If both "reg" and
> "iommu-addresses" properties are given, a mapping will be created
> (typically this would be an identity mapping)

dcp on Apple silicon will not use identity mappings. The IOMMU supports 
identity mapping but the pre-configured mappings setup by Apple's system 
firmware will not work with identity mapping. It maps multiple regions 
which are incompatible with a linear identity mapping. In addition the 
embbeded aarch64 micro controllers used in the display subsystem appears 
to use a heap mapped at low IOVA space starting at 0.

> whereas if only an "iommu-addresses" property is specified, a 
> reservation for the specified range will be installed.
> 
> An example is included in the DT bindings, but here is an extract of
> what I've used to test this:
> 
> 	reserved-memory {
> 		#address-cells = <2>;
> 		#size-cells = <2>;
> 		ranges;
> 
> 		/*
> 		 * Creates an identity mapping for the framebuffer that
> 		 * the firmware has setup to scan out a bootsplash from.
> 		 */
> 		fb: framebuffer@92cb2000 {
> 			reg = <0x0 0x92cb2000 0x0 0x00800000>;
> 			iommu-addresses = <&dc0 0x0 0x92cb2000 0x0 0x00800000>;
> 		};

The binding supports mapping the same region to multiple devices. The 
code supports that and it will be used on Apple silicon. Not necessary 
to extend and complicate the example for I wanted to mention it 
explicitly.

> 
> 		/*
> 		 * Creates a reservation in the IOVA space to prevent
> 		 * any buffers from being mapped to that region. Note
> 		 * that on Tegra the range is actually quite different
> 		 * from this, but it would conflict with the display
> 		 * driver that I tested this against, so this is just
> 		 * a dummy region for testing.
> 		 */
> 		adsp: reservation-adsp {
> 			iommu-addresses = <&dc0 0x0 0x90000000 0x0 0x00010000>;
> 		};
> 	};
> 
> 	host1x@50000000 {
> 		dc@54200000 {
> 			memory-region = <&fb>, <&adsp>;
> 		};
> 	};
> 
> This is abbreviated a little to focus on the essentials. Note also that
> the ADSP reservation is not actually used on this device and the driver
> for this doesn't exist yet, but I wanted to include this variant for
> testing, because we'll want to use these bindings for the reservation
> use-case as well at some point.
> 
> Adding Alyssa and Janne who have in the past tried to make these
> bindings work on Apple M1. Also adding Sameer from the Tegra audio team
> to look at the ADSP reservation and double-check that this is suitable
> for our needs.

The binding itself is sufficient for the needs of the display subsystem 
on Apple silicon. The device tree parsing code for reserved regions is 
of limited use in it's current form. We will have either to extend or 
duplicate it to retrieve the non-identity mappings. That's our problem 
to solve.

Janne

Re: [PATCH v5 1/5] dt-bindings: reserved-memory: Document iommu-addresses

[Janne Grunau]

On 2022-05-12 21:00:48 +0200, Thierry Reding wrote:
> From: Thierry Reding <treding@nvidia.com>
> 
> This adds the "iommu-addresses" property to reserved-memory nodes, which
> allow describing the interaction of memory regions with IOMMUs. Two use-
> cases are supported:
> 
>   1. Static mappings can be described by pairing the "iommu-addresses"
>      property with a "reg" property. This is mostly useful for adopting
>      firmware-allocated buffers via identity mappings. One common use-
>      case where this is required is if early firmware or bootloaders
>      have set up a bootsplash framebuffer that a display controller is
>      actively scanning out from during the operating system boot
>      process.
> 
>   2. If an "iommu-addresses" property exists without a "reg" property,
>      the reserved-memory node describes an IOVA reservation. Such memory
>      regions are excluded from the IOVA space available to operating
>      system drivers and can be used for regions that must not be used to
>      map arbitrary buffers.
> 
> Each mapping or reservation is tied to a specific device via a phandle
> to the device's device tree node. This allows a reserved-memory region
> to be reused across multiple devices.
> 
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
>  .../reserved-memory/reserved-memory.txt       |  1 -
>  .../reserved-memory/reserved-memory.yaml      | 62 +++++++++++++++++++
>  include/dt-bindings/reserved-memory.h         |  8 +++
>  3 files changed, 70 insertions(+), 1 deletion(-)
>  delete mode 100644 Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
>  create mode 100644 include/dt-bindings/reserved-memory.h
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> deleted file mode 100644
> index 1810701a8509..000000000000
> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> +++ /dev/null
> @@ -1 +0,0 @@
> -This file has been moved to reserved-memory.yaml.
> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> index 7a0744052ff6..3a769aa66e1c 100644
> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> @@ -52,6 +52,30 @@ properties:
>        Address and Length pairs. Specifies regions of memory that are
>        acceptable to allocate from.
>  
> +  iommu-addresses:
> +    $ref: /schemas/types.yaml#/definitions/phandle-array
> +    description: >
> +      A list of phandle and specifier pairs that describe static IO virtual
> +      address space mappings and carveouts associated with a given reserved
> +      memory region. The phandle in the first cell refers to the device for
> +      which the mapping or carveout is to be created.
> +
> +      The specifier consists of an address/size pair and denotes the IO
> +      virtual address range of the region for the given device. The exact
> +      format depends on the values of the "#address-cells" and "#size-cells"
> +      properties of the device referenced via the phandle.
> +
> +      When used in combination with a "reg" property, an IOVA mapping is to
> +      be established for this memory region. One example where this can be
> +      useful is to create an identity mapping for physical memory that the
> +      firmware has configured some hardware to access (such as a bootsplash
> +      framebuffer).
> +
> +      If no "reg" property is specified, the "iommu-addresses" property
> +      defines carveout regions in the IOVA space for the given device. This
> +      can be useful if a certain memory region should not be mapped through
> +      the IOMMU.
> +
>    no-map:
>      type: boolean
>      description: >
> @@ -97,4 +121,42 @@ oneOf:
>  
>  additionalProperties: true
>  
> +examples:
> +  - |
> +    reserved-memory {
> +      #address-cells = <2>;
> +      #size-cells = <2>;
> +      ranges;
> +
> +      adsp: reservation-adsp {
> +        /*
> +         * Restrict IOVA mappings for ADSP buffers to the 512 MiB region
> +         * from 0x40000000 - 0x5fffffff. Anything outside is reserved by
> +         * the ADSP for I/O memory and private memory allocations.
> +         */
> +        iommu-addresses = <0x0 0x00000000 0x00 0x40000000>,
> +                          <0x0 0x60000000 0xff 0xa0000000>;

This misses the device's phandle. One could argue it's not necessary for 
reservations but it will complicate the parsing code and the current 
parsing code is not prepared for it.

> +      };
> +
> +      fb: framebuffer@90000000 {
> +        reg = <0x0 0x90000000 0x0 0x00800000>;
> +        iommu-addresses = <&dc0 0x0 0x90000000 0x0 0x00800000>;
> +      };
> +    };
> +
> +    bus@0 {
> +      #address-cells = <2>;
> +      #size-cells = <2>;
> +
> +      adsp@2990000 {
> +        reg = <0x0 0x2990000 0x0 0x2000>;
> +        memory-region = <&adsp>;
> +      };
> +
> +      display@15200000 {
> +        reg = <0x0 0x15200000 0x0 0x10000>;
> +        memory-region = <&fb>;
> +      };
> +    };
> +
>  ...
> diff --git a/include/dt-bindings/reserved-memory.h b/include/dt-bindings/reserved-memory.h
> new file mode 100644
> index 000000000000..174ca3448342
> --- /dev/null
> +++ b/include/dt-bindings/reserved-memory.h
> @@ -0,0 +1,8 @@
> +/* SPDX-License-Identifier: (GPL-2.0+ or MIT) */
> +
> +#ifndef _DT_BINDINGS_RESERVED_MEMORY_H
> +#define _DT_BINDINGS_RESERVED_MEMORY_H
> +
> +#define MEMORY_REGION_IDENTITY_MAPPING 0x1
> +
> +#endif

This appears to be an unused leftover from a previous version.

Janne

Re: [PATCH v5 2/5] iommu: Implement of_iommu_get_resv_regions()

[Janne Grunau]

On 2022-05-12 21:00:49 +0200, Thierry Reding wrote:
> From: Thierry Reding <treding@nvidia.com>
> 
> This is an implementation that IOMMU drivers can use to obtain reserved
> memory regions from a device tree node. It uses the reserved-memory DT
> bindings to find the regions associated with a given device. If these
> regions are marked accordingly, identity mappings will be created for
> them in the IOMMU domain that the devices will be attached to.
> 
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
> Changes in v5:
> - update for new "iommu-addresses" device tree bindings
> 
> Changes in v4:
> - fix build failure on !CONFIG_OF_ADDRESS
> 
> Changes in v3:
> - change "active" property to identity mapping flag that is part of the
>   memory region specifier (as defined by #memory-region-cells) to allow
>   per-reference flags to be used
> 
> Changes in v2:
> - use "active" property to determine whether direct mappings are needed
> 
>  drivers/iommu/of_iommu.c | 90 ++++++++++++++++++++++++++++++++++++++++
>  include/linux/of_iommu.h |  8 ++++
>  2 files changed, 98 insertions(+)
> 
> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> index 5696314ae69e..9e341b5e307f 100644
> --- a/drivers/iommu/of_iommu.c
> +++ b/drivers/iommu/of_iommu.c
> @@ -11,12 +11,15 @@
>  #include <linux/module.h>
>  #include <linux/msi.h>
>  #include <linux/of.h>
> +#include <linux/of_address.h>
>  #include <linux/of_iommu.h>
>  #include <linux/of_pci.h>
>  #include <linux/pci.h>
>  #include <linux/slab.h>
>  #include <linux/fsl/mc.h>
>  
> +#include <dt-bindings/reserved-memory.h>
> +
>  #define NO_IOMMU	1
>  
>  static int of_iommu_xlate(struct device *dev,
> @@ -172,3 +175,90 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
>  
>  	return ops;
>  }
> +
> +/**
> + * of_iommu_get_resv_regions - reserved region driver helper for device tree
> + * @dev: device for which to get reserved regions
> + * @list: reserved region list
> + *
> + * IOMMU drivers can use this to implement their .get_resv_regions() callback
> + * for memory regions attached to a device tree node. See the reserved-memory
> + * device tree bindings on how to use these:
> + *
> + *   Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> + */
> +void of_iommu_get_resv_regions(struct device *dev, struct list_head *list)
> +{
> +#if IS_ENABLED(CONFIG_OF_ADDRESS)
> +	struct of_phandle_iterator it;
> +	int err;
> +
> +	of_for_each_phandle(&it, err, dev->of_node, "memory-region", NULL, 0) {
> +		struct iommu_resv_region *region;
> +		struct resource res;
> +		const __be32 *maps;
> +		int size;

Adding 'if (!of_device_is_available(it.node)) continue;' here would help 
backwards compatibility. My plan was to add the reserved regions with 
"iommu-addresses" with all zero adresses and sizes with status = 
"disabled" to the devicetree. A bootloader update is required to fill 
those.

> +
> +		memset(&res, 0, sizeof(res));
> +
> +		/*
> +		 * The "reg" property is optional and can be omitted by reserved-memory regions
> +		 * that represent reservations in the IOVA space, which are regions that should
> +		 * not be mapped.
> +		 */
> +		if (of_find_property(it.node, "reg", NULL)) {
> +			err = of_address_to_resource(it.node, 0, &res);
> +			if (err < 0) {
> +				dev_err(dev, "failed to parse memory region %pOF: %d\n",
> +					it.node, err);
> +				continue;
> +			}
> +		}
> +
> +		maps = of_get_property(it.node, "iommu-addresses", &size);
> +		if (maps) {
> +			const __be32 *end = maps + size / sizeof(__be32);
> +			struct device_node *np;
> +			unsigned int index = 0;
> +			u32 phandle;
> +			int na, ns;
> +
> +			while (maps < end) {
> +				phys_addr_t start, end;
> +				size_t length;
> +
> +				phandle = be32_to_cpup(maps++);
> +				np = of_find_node_by_phandle(phandle);
> +				na = of_n_addr_cells(np);
> +				ns = of_n_size_cells(np);
> +
> +				start = of_translate_dma_address(np, maps);
> +				length = of_read_number(maps + na, ns);

alternatively we could handle mappings/reservations with length 0 as 
error and skip them.

> +				end = start + length - 1;
> +
> +				if (np == dev->of_node) {
> +					int prot = IOMMU_READ | IOMMU_WRITE;
> +					enum iommu_resv_type type;
> +
> +					/*
> +					 * IOMMU regions without an associated physical region
> +					 * cannot be mapped and are simply reservations.
> +					 */
> +					if (res.end > res.start)
> +						type = IOMMU_RESV_DIRECT_RELAXABLE;
> +					else
> +						type = IOMMU_RESV_RESERVED;
> +
> +					region = iommu_alloc_resv_region(start, length, prot, type);
> +					if (region)
> +						list_add_tail(&region->list, list);
> +				}
> +
> +				maps += na + ns;
> +				index++;
> +			}
> +		}
> +	}
> +#endif
> +}
> +EXPORT_SYMBOL(of_iommu_get_resv_regions);
> diff --git a/include/linux/of_iommu.h b/include/linux/of_iommu.h
> index 55c1eb300a86..9a5e6b410dd2 100644
> --- a/include/linux/of_iommu.h
> +++ b/include/linux/of_iommu.h
> @@ -12,6 +12,9 @@ extern const struct iommu_ops *of_iommu_configure(struct device *dev,
>  					struct device_node *master_np,
>  					const u32 *id);
>  
> +extern void of_iommu_get_resv_regions(struct device *dev,
> +				      struct list_head *list);
> +
>  #else
>  
>  static inline const struct iommu_ops *of_iommu_configure(struct device *dev,
> @@ -21,6 +24,11 @@ static inline const struct iommu_ops *of_iommu_configure(struct device *dev,
>  	return NULL;
>  }
>  
> +static inline void of_iommu_get_resv_regions(struct device *dev,
> +					     struct list_head *list)
> +{
> +}
> +
>  #endif	/* CONFIG_OF_IOMMU */
>  
>  #endif /* __OF_IOMMU_H */
> -- 
> 2.36.1
> 

Janne

Re: [PATCH v5 2/5] iommu: Implement of_iommu_get_resv_regions()

[Thierry Reding]

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On Sun, May 15, 2022 at 01:10:38PM +0200, Janne Grunau wrote:
> On 2022-05-12 21:00:49 +0200, Thierry Reding wrote:
> > From: Thierry Reding <treding@nvidia.com>
> > 
> > This is an implementation that IOMMU drivers can use to obtain reserved
> > memory regions from a device tree node. It uses the reserved-memory DT
> > bindings to find the regions associated with a given device. If these
> > regions are marked accordingly, identity mappings will be created for
> > them in the IOMMU domain that the devices will be attached to.
> > 
> > Signed-off-by: Thierry Reding <treding@nvidia.com>
> > ---
> > Changes in v5:
> > - update for new "iommu-addresses" device tree bindings
> > 
> > Changes in v4:
> > - fix build failure on !CONFIG_OF_ADDRESS
> > 
> > Changes in v3:
> > - change "active" property to identity mapping flag that is part of the
> >   memory region specifier (as defined by #memory-region-cells) to allow
> >   per-reference flags to be used
> > 
> > Changes in v2:
> > - use "active" property to determine whether direct mappings are needed
> > 
> >  drivers/iommu/of_iommu.c | 90 ++++++++++++++++++++++++++++++++++++++++
> >  include/linux/of_iommu.h |  8 ++++
> >  2 files changed, 98 insertions(+)
> > 
> > diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> > index 5696314ae69e..9e341b5e307f 100644
> > --- a/drivers/iommu/of_iommu.c
> > +++ b/drivers/iommu/of_iommu.c
> > @@ -11,12 +11,15 @@
> >  #include <linux/module.h>
> >  #include <linux/msi.h>
> >  #include <linux/of.h>
> > +#include <linux/of_address.h>
> >  #include <linux/of_iommu.h>
> >  #include <linux/of_pci.h>
> >  #include <linux/pci.h>
> >  #include <linux/slab.h>
> >  #include <linux/fsl/mc.h>
> >  
> > +#include <dt-bindings/reserved-memory.h>
> > +
> >  #define NO_IOMMU	1
> >  
> >  static int of_iommu_xlate(struct device *dev,
> > @@ -172,3 +175,90 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
> >  
> >  	return ops;
> >  }
> > +
> > +/**
> > + * of_iommu_get_resv_regions - reserved region driver helper for device tree
> > + * @dev: device for which to get reserved regions
> > + * @list: reserved region list
> > + *
> > + * IOMMU drivers can use this to implement their .get_resv_regions() callback
> > + * for memory regions attached to a device tree node. See the reserved-memory
> > + * device tree bindings on how to use these:
> > + *
> > + *   Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > + */
> > +void of_iommu_get_resv_regions(struct device *dev, struct list_head *list)
> > +{
> > +#if IS_ENABLED(CONFIG_OF_ADDRESS)
> > +	struct of_phandle_iterator it;
> > +	int err;
> > +
> > +	of_for_each_phandle(&it, err, dev->of_node, "memory-region", NULL, 0) {
> > +		struct iommu_resv_region *region;
> > +		struct resource res;
> > +		const __be32 *maps;
> > +		int size;
> 
> Adding 'if (!of_device_is_available(it.node)) continue;' here would help 
> backwards compatibility. My plan was to add the reserved regions with 
> "iommu-addresses" with all zero adresses and sizes with status = 
> "disabled" to the devicetree. A bootloader update is required to fill 
> those.

Yes, good point. My plan was originally to have the bootloader/firmware
generate these nodes in their entirety, but yeah, prepopulating them and
having firmware just fill in updated values and setting status = "okay"
seems reasonable to me.

> > +
> > +		memset(&res, 0, sizeof(res));
> > +
> > +		/*
> > +		 * The "reg" property is optional and can be omitted by reserved-memory regions
> > +		 * that represent reservations in the IOVA space, which are regions that should
> > +		 * not be mapped.
> > +		 */
> > +		if (of_find_property(it.node, "reg", NULL)) {
> > +			err = of_address_to_resource(it.node, 0, &res);
> > +			if (err < 0) {
> > +				dev_err(dev, "failed to parse memory region %pOF: %d\n",
> > +					it.node, err);
> > +				continue;
> > +			}
> > +		}
> > +
> > +		maps = of_get_property(it.node, "iommu-addresses", &size);
> > +		if (maps) {
> > +			const __be32 *end = maps + size / sizeof(__be32);
> > +			struct device_node *np;
> > +			unsigned int index = 0;
> > +			u32 phandle;
> > +			int na, ns;
> > +
> > +			while (maps < end) {
> > +				phys_addr_t start, end;
> > +				size_t length;
> > +
> > +				phandle = be32_to_cpup(maps++);
> > +				np = of_find_node_by_phandle(phandle);
> > +				na = of_n_addr_cells(np);
> > +				ns = of_n_size_cells(np);
> > +
> > +				start = of_translate_dma_address(np, maps);
> > +				length = of_read_number(maps + na, ns);
> 
> alternatively we could handle mappings/reservations with length 0 as 
> error and skip them.

I think we could do both.

Thanks for the feedback,
Thierry

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Re: [PATCH v5 1/5] dt-bindings: reserved-memory: Document iommu-addresses

[Thierry Reding]

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On Sun, May 15, 2022 at 12:45:54PM +0200, Janne Grunau wrote:
> On 2022-05-12 21:00:48 +0200, Thierry Reding wrote:
> > From: Thierry Reding <treding@nvidia.com>
> > 
> > This adds the "iommu-addresses" property to reserved-memory nodes, which
> > allow describing the interaction of memory regions with IOMMUs. Two use-
> > cases are supported:
> > 
> >   1. Static mappings can be described by pairing the "iommu-addresses"
> >      property with a "reg" property. This is mostly useful for adopting
> >      firmware-allocated buffers via identity mappings. One common use-
> >      case where this is required is if early firmware or bootloaders
> >      have set up a bootsplash framebuffer that a display controller is
> >      actively scanning out from during the operating system boot
> >      process.
> > 
> >   2. If an "iommu-addresses" property exists without a "reg" property,
> >      the reserved-memory node describes an IOVA reservation. Such memory
> >      regions are excluded from the IOVA space available to operating
> >      system drivers and can be used for regions that must not be used to
> >      map arbitrary buffers.
> > 
> > Each mapping or reservation is tied to a specific device via a phandle
> > to the device's device tree node. This allows a reserved-memory region
> > to be reused across multiple devices.
> > 
> > Signed-off-by: Thierry Reding <treding@nvidia.com>
> > ---
> >  .../reserved-memory/reserved-memory.txt       |  1 -
> >  .../reserved-memory/reserved-memory.yaml      | 62 +++++++++++++++++++
> >  include/dt-bindings/reserved-memory.h         |  8 +++
> >  3 files changed, 70 insertions(+), 1 deletion(-)
> >  delete mode 100644 Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> >  create mode 100644 include/dt-bindings/reserved-memory.h
> > 
> > diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > deleted file mode 100644
> > index 1810701a8509..000000000000
> > --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > +++ /dev/null
> > @@ -1 +0,0 @@
> > -This file has been moved to reserved-memory.yaml.
> > diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> > index 7a0744052ff6..3a769aa66e1c 100644
> > --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> > +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> > @@ -52,6 +52,30 @@ properties:
> >        Address and Length pairs. Specifies regions of memory that are
> >        acceptable to allocate from.
> >  
> > +  iommu-addresses:
> > +    $ref: /schemas/types.yaml#/definitions/phandle-array
> > +    description: >
> > +      A list of phandle and specifier pairs that describe static IO virtual
> > +      address space mappings and carveouts associated with a given reserved
> > +      memory region. The phandle in the first cell refers to the device for
> > +      which the mapping or carveout is to be created.
> > +
> > +      The specifier consists of an address/size pair and denotes the IO
> > +      virtual address range of the region for the given device. The exact
> > +      format depends on the values of the "#address-cells" and "#size-cells"
> > +      properties of the device referenced via the phandle.
> > +
> > +      When used in combination with a "reg" property, an IOVA mapping is to
> > +      be established for this memory region. One example where this can be
> > +      useful is to create an identity mapping for physical memory that the
> > +      firmware has configured some hardware to access (such as a bootsplash
> > +      framebuffer).
> > +
> > +      If no "reg" property is specified, the "iommu-addresses" property
> > +      defines carveout regions in the IOVA space for the given device. This
> > +      can be useful if a certain memory region should not be mapped through
> > +      the IOMMU.
> > +
> >    no-map:
> >      type: boolean
> >      description: >
> > @@ -97,4 +121,42 @@ oneOf:
> >  
> >  additionalProperties: true
> >  
> > +examples:
> > +  - |
> > +    reserved-memory {
> > +      #address-cells = <2>;
> > +      #size-cells = <2>;
> > +      ranges;
> > +
> > +      adsp: reservation-adsp {
> > +        /*
> > +         * Restrict IOVA mappings for ADSP buffers to the 512 MiB region
> > +         * from 0x40000000 - 0x5fffffff. Anything outside is reserved by
> > +         * the ADSP for I/O memory and private memory allocations.
> > +         */
> > +        iommu-addresses = <0x0 0x00000000 0x00 0x40000000>,
> > +                          <0x0 0x60000000 0xff 0xa0000000>;
> 
> This misses the device's phandle. One could argue it's not necessary for 
> reservations but it will complicate the parsing code and the current 
> parsing code is not prepared for it.

Ugh... I evidently messed this one up. It's not ever close to what I had
in my device trees for testing. I'll make sure to fix those up properly
and also that they pass validation.

> > +      };
> > +
> > +      fb: framebuffer@90000000 {
> > +        reg = <0x0 0x90000000 0x0 0x00800000>;
> > +        iommu-addresses = <&dc0 0x0 0x90000000 0x0 0x00800000>;
> > +      };
> > +    };
> > +
> > +    bus@0 {
> > +      #address-cells = <2>;
> > +      #size-cells = <2>;
> > +
> > +      adsp@2990000 {
> > +        reg = <0x0 0x2990000 0x0 0x2000>;
> > +        memory-region = <&adsp>;
> > +      };
> > +
> > +      display@15200000 {
> > +        reg = <0x0 0x15200000 0x0 0x10000>;
> > +        memory-region = <&fb>;
> > +      };
> > +    };
> > +
> >  ...
> > diff --git a/include/dt-bindings/reserved-memory.h b/include/dt-bindings/reserved-memory.h
> > new file mode 100644
> > index 000000000000..174ca3448342
> > --- /dev/null
> > +++ b/include/dt-bindings/reserved-memory.h
> > @@ -0,0 +1,8 @@
> > +/* SPDX-License-Identifier: (GPL-2.0+ or MIT) */
> > +
> > +#ifndef _DT_BINDINGS_RESERVED_MEMORY_H
> > +#define _DT_BINDINGS_RESERVED_MEMORY_H
> > +
> > +#define MEMORY_REGION_IDENTITY_MAPPING 0x1
> > +
> > +#endif
> 
> This appears to be an unused leftover from a previous version.

Good catch. I'll drop this file from the patch set. It's no longer
needed.

Thanks,
Thierry

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Re: [PATCH v5 0/5] iommu: Support mappings/reservations in reserved-memory regions

[Thierry Reding]

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On Sun, May 15, 2022 at 12:35:44PM +0200, Janne Grunau wrote:
> Hej,
> 
> I'm working on the display controller for Apple silicon SoCs and will 
> add some comments with support for it in mind.
> 
> added asahi@lists.linux.dev to CC for the Apple silicon related aspects
> 
> On 2022-05-12 21:00:47 +0200, Thierry Reding wrote:
> > 
> > this is another attempt at solving the problem of passing IOMMU
> > configuration via device tree. It has significantly evolved since the
> > last attempt, based on the discussion that followed. The discussion can
> > be found here:
> > 
> >   https://lore.kernel.org/all/20210423163234.3651547-1-thierry.reding@gmail.com/
> > 
> > Rather than using a memory-region specifier, this new version introduces
> > a new "iommu-addresses" property for the reserved-memory regions
> > themselves.
> 
> If experimented with both proposed bindings for dcp and I think this 
> binding is easer to understand and to work with.
> 
> > These are used to describe either a static mapping or
> > reservation that should be created for a given device. If both "reg" and
> > "iommu-addresses" properties are given, a mapping will be created
> > (typically this would be an identity mapping)
> 
> dcp on Apple silicon will not use identity mappings. The IOMMU supports 
> identity mapping but the pre-configured mappings setup by Apple's system 
> firmware will not work with identity mapping. It maps multiple regions 
> which are incompatible with a linear identity mapping. In addition the 
> embbeded aarch64 micro controllers used in the display subsystem appears 
> to use a heap mapped at low IOVA space starting at 0.
> 
> > whereas if only an "iommu-addresses" property is specified, a 
> > reservation for the specified range will be installed.
> > 
> > An example is included in the DT bindings, but here is an extract of
> > what I've used to test this:
> > 
> > 	reserved-memory {
> > 		#address-cells = <2>;
> > 		#size-cells = <2>;
> > 		ranges;
> > 
> > 		/*
> > 		 * Creates an identity mapping for the framebuffer that
> > 		 * the firmware has setup to scan out a bootsplash from.
> > 		 */
> > 		fb: framebuffer@92cb2000 {
> > 			reg = <0x0 0x92cb2000 0x0 0x00800000>;
> > 			iommu-addresses = <&dc0 0x0 0x92cb2000 0x0 0x00800000>;
> > 		};
> 
> The binding supports mapping the same region to multiple devices. The 
> code supports that and it will be used on Apple silicon. Not necessary 
> to extend and complicate the example for I wanted to mention it 
> explicitly.
> 
> > 
> > 		/*
> > 		 * Creates a reservation in the IOVA space to prevent
> > 		 * any buffers from being mapped to that region. Note
> > 		 * that on Tegra the range is actually quite different
> > 		 * from this, but it would conflict with the display
> > 		 * driver that I tested this against, so this is just
> > 		 * a dummy region for testing.
> > 		 */
> > 		adsp: reservation-adsp {
> > 			iommu-addresses = <&dc0 0x0 0x90000000 0x0 0x00010000>;
> > 		};
> > 	};
> > 
> > 	host1x@50000000 {
> > 		dc@54200000 {
> > 			memory-region = <&fb>, <&adsp>;
> > 		};
> > 	};
> > 
> > This is abbreviated a little to focus on the essentials. Note also that
> > the ADSP reservation is not actually used on this device and the driver
> > for this doesn't exist yet, but I wanted to include this variant for
> > testing, because we'll want to use these bindings for the reservation
> > use-case as well at some point.
> > 
> > Adding Alyssa and Janne who have in the past tried to make these
> > bindings work on Apple M1. Also adding Sameer from the Tegra audio team
> > to look at the ADSP reservation and double-check that this is suitable
> > for our needs.
> 
> The binding itself is sufficient for the needs of the display subsystem 
> on Apple silicon. The device tree parsing code for reserved regions is 
> of limited use in it's current form. We will have either to extend or 
> duplicate it to retrieve the non-identity mappings. That's our problem 
> to solve.

I had looked at it a bit to see if I could easily implement that, but
the direct mapping support in the IOMMU subsystem currently only
supports either reservations or identity mappings, so arbitrary mappings
would either have to be added to that code, or it would have to take a
different code path that basically goes through the same steps, except
that it uses different physical and I/O virtual addresses.

The easiest, I think, would be for struct iommu_resv_region to be
extended with a pair of start/length fields for the I/O virtual address
and then the rest of the code should mostly work. This shouldn't even be
very invasive, maybe just adding a version of iommu_alloc_resv_region()
that takes the I/O virtual addresses as additional parameters.

Come to think of it, the current code could probably be improved a bit
by checking if the addresses in the reg and iommu-addresses properties
match. Currently the code just ignores the reserved memory region's
"reg" property, so one could technically set up a mapping that points to
physical memory that the device doesn't "own".

Thierry

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