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

[Claire Chang <tientzu@chromium.org>]

On Fri, Apr 23, 2021 at 9:35 PM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2021-04-22 09:15, Claire Chang wrote:
> > 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 <tientzu@chromium.org>
> > ---
> >   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.
>
> > +             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 */
> >