Re: [PATCH v5 16/16] of: Add plumbing for restricted DMA pool

On Fri, Apr 23, 2021 at 9:35 PM Robin Murphy [off-list ref] wrote:

On 2021-04-22 09:15, Claire Chang wrote:

quoted

If a device is not behind an IOMMU, we look up the device node and set
up the restricted DMA when the restricted-dma-pool is presented.

Signed-off-by: Claire Chang <redacted>
---
  drivers/of/address.c    | 25 +++++++++++++++++++++++++
  drivers/of/device.c     |  3 +++
  drivers/of/of_private.h |  5 +++++
  3 files changed, 33 insertions(+)

diff --git a/drivers/of/address.c b/drivers/of/address.c
index 54f221dde267..fff3adfe4986 100644
--- a/drivers/of/address.c
+++ b/drivers/of/address.c

@@ -8,6 +8,7 @@
  #include <linux/logic_pio.h>
  #include <linux/module.h>
  #include <linux/of_address.h>
+#include <linux/of_reserved_mem.h>
  #include <linux/pci.h>
  #include <linux/pci_regs.h>
  #include <linux/sizes.h>

@@ -1109,6 +1110,30 @@ bool of_dma_is_coherent(struct device_node *np)
  }
  EXPORT_SYMBOL_GPL(of_dma_is_coherent);

+int of_dma_set_restricted_buffer(struct device *dev)
+{
+     struct device_node *node;
+     int count, i;
+
+     if (!dev->of_node)
+             return 0;
+
+     count = of_property_count_elems_of_size(dev->of_node, "memory-region",
+                                             sizeof(phandle));
+     for (i = 0; i < count; i++) {
+             node = of_parse_phandle(dev->of_node, "memory-region", i);
+             /* There might be multiple memory regions, but only one
+              * restriced-dma-pool region is allowed.
+              */

What's the use-case for having multiple regions if the restricted pool
is by definition the only one accessible?

There might be a device coherent pool (shared-dma-pool) and
dma_alloc_attrs might allocate memory from that pool [1].
I'm not sure if it makes sense to have another device coherent pool
while using restricted DMA pool though.

[1] https://elixir.bootlin.com/linux/v5.12/source/kernel/dma/mapping.c#L435

Robin.

quoted

+             if (of_device_is_compatible(node, "restricted-dma-pool") &&
+                 of_device_is_available(node))
+                     return of_reserved_mem_device_init_by_idx(
+                             dev, dev->of_node, i);
+     }
+
+     return 0;
+}
+
  /**
   * of_mmio_is_nonposted - Check if device uses non-posted MMIO
   * @np:     device node

diff --git a/drivers/of/device.c b/drivers/of/device.c
index c5a9473a5fb1..d8d865223e51 100644
--- a/drivers/of/device.c
+++ b/drivers/of/device.c

@@ -165,6 +165,9 @@ int of_dma_configure_id(struct device *dev, struct device_node *np,

      arch_setup_dma_ops(dev, dma_start, size, iommu, coherent);

+     if (!iommu)
+             return of_dma_set_restricted_buffer(dev);
+
      return 0;
  }
  EXPORT_SYMBOL_GPL(of_dma_configure_id);

diff --git a/drivers/of/of_private.h b/drivers/of/of_private.h
index d717efbd637d..e9237f5eff48 100644
--- a/drivers/of/of_private.h
+++ b/drivers/of/of_private.h

@@ -163,12 +163,17 @@ struct bus_dma_region;
  #if defined(CONFIG_OF_ADDRESS) && defined(CONFIG_HAS_DMA)
  int of_dma_get_range(struct device_node *np,
              const struct bus_dma_region **map);
+int of_dma_set_restricted_buffer(struct device *dev);
  #else
  static inline int of_dma_get_range(struct device_node *np,
              const struct bus_dma_region **map)
  {
      return -ENODEV;
  }
+static inline int of_dma_get_restricted_buffer(struct device *dev)
+{
+     return -ENODEV;
+}
  #endif

  #endif /* _LINUX_OF_PRIVATE_H */