Re: [PATCH v9 2/3] of: factor arguments passed to of_map_id() into a struct

[Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>]

On Sun, Mar 01, 2026 at 10:02:49AM +0000, Marc Zyngier wrote:
> On Sun, 01 Mar 2026 08:34:20 +0000,
> Vijayanand Jitta <vijayanand.jitta@xxxxxxxxxxxxxxxx> wrote:
> > 
> > From: Charan Teja Kalla <charan.kalla@xxxxxxxxxxxxxxxx>
> > 
> > Change of_map_id() to take a pointer to struct of_phandle_args
> > instead of passing target device node and translated IDs separately.
> > Update all callers accordingly.
> > 
> > Subsequent patch will make use of the args_count field in
> > struct of_phandle_args.
> > 
> > Suggested-by: Rob Herring (Arm) <robh@xxxxxxxxxx>
> > Signed-off-by: Charan Teja Kalla <charan.kalla@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Vijayanand Jitta <vijayanand.jitta@xxxxxxxxxxxxxxxx>
> > ---
> >  drivers/iommu/of_iommu.c              |  2 +-
> >  drivers/of/base.c                     | 37 
> > +++++++++++++++++------------------
> >  drivers/pci/controller/dwc/pci-imx6.c |  8 +++++++-
> >  drivers/pci/controller/pcie-apple.c   |  4 +++-
> >  drivers/xen/grant-dma-ops.c           |  2 +-
> >  include/linux/of.h                    | 21 +++++++++++++-------
> >  6 files changed, 44 insertions(+), 30 deletions(-)
> > 
> > diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> > index a511ecf21fcd..d255d0f58e8c 100644
> > --- a/drivers/iommu/of_iommu.c
> > +++ b/drivers/iommu/of_iommu.c
> > @@ -48,7 +48,7 @@ static int of_iommu_configure_dev_id(struct device_node 
> > *master_np,
> >     struct of_phandle_args iommu_spec = { .args_count = 1 };
> >     int err;
> >  
> > -   err = of_map_iommu_id(master_np, *id, &iommu_spec.np, iommu_spec.args);
> > +   err = of_map_iommu_id(master_np, *id, &iommu_spec);
> >     if (err)
> >             return err;
> >  
> > diff --git a/drivers/of/base.c b/drivers/of/base.c
> > index 57420806c1a2..6c3628255908 100644
> > --- a/drivers/of/base.c
> > +++ b/drivers/of/base.c
> > @@ -2102,8 +2102,11 @@ int of_find_last_cache_level(unsigned int cpu)
> >   * @id: device ID to map.
> >   * @map_name: property name of the map to use.
> >   * @map_mask_name: optional property name of the mask to use.
> > - * @target: optional pointer to a target device node.
> > - * @id_out: optional pointer to receive the translated ID.
> > + * @arg: of_phandle_args structure,
> > + * which includes:
> > + * np: pointer to the target device node
> > + * args_count: number of arguments
> 
> Number of arguments *to what*? Isn't that the size of args[] instead?

It is a number of values corresponding to the phandle in the DT
property. It might be not obvious here for iommu-maps, but the struct is
idiomatic in OF world. Let me quote a (trimmed) example from
qcom/sm8650.dtsi (for a different property, but it explains the meaning
of the values here):

gem_noc: interconnect@24100000 {
        #interconnect-cells = <2>;
};

epss_l3: interconnect@17d90000 {
        #interconnect-cells = <1>;
};

interconnects = <&gem_noc MASTER_APPSS_PROC QCOM_ICC_TAG_ACTIVE_ONLY
                 &gem_noc SLAVE_LLCC QCOM_ICC_TAG_ACTIVE_ONLY>,
                <&epss_l3 MASTER_EPSS_L3_APPS
                 &epss_l3 SLAVE_EPSS_L3_SHARED>;
/* I skipped the second pair, it adds nothing here */

Here the parsing function for this property (of_icc_get_by_index()) will
call of_parse_phandle_with_args() 4 times and it expects to return the
following values in the of_phandle_args:

1. { .np = gem_noc, .args_count = 2, .args = [MASTER_APPSS_PROC,
                                              QCOM_ICC_TAG_ACTIVE_ONLY] }
2. { .np = gem_noc, .args_count = 2, .args = [SLAVE_LLCC,
                                              QCOM_ICC_TAG_ACTIVE_ONLY] }
3. { .np = epss_l3, .args_count = 1, .args = [MASTER_EPSS_L3_APPS] }
4. { .np = epss_l3, .args_count = 1, .args = [SLAVE_EPSS_L3_SHARED] }

The whole of_phandle_args is then typically passed to the corresponding
xlate function, specific to the paricular .np ('provider'), which will
use #args_count values from the #args array to return the object from
the provider.

Now let's see iommu-maps (again, qcom/sm8650.dtsi):

apps_smmu: iommu@15000000 {
        #iommu-cells = <2>;
};

iommu-map = <0     &apps_smmu 0x1400 0x1>,
            <0x100 &apps_smmu 0x1401 0x1>;

The property matches current definition at [1], however this spec
doesn't match the DT practice. It forces that the property should use 1
cell for identifying the "object" in the IOMMU provider, even if the
provider expects to use 2 cells (two args).

The correct property should look like:

iommu-map = <0     &apps_smmu 0x1400 0x0 0x1>,
            <0x100 &apps_smmu 0x1401 0x0 0x1>;

[1] 
https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/pci/pci-iommu.yaml

> 
> > + * args[]: array to receive the translated ID(s).
> >   *
> >   * Given a device ID, look up the appropriate implementation-defined
> >   * platform ID and/or the target device which receives transactions on that
> > @@ -2117,21 +2120,21 @@ int of_find_last_cache_level(unsigned int cpu)
> >   */
> >  int of_map_id(const struct device_node *np, u32 id,
> >            const char *map_name, const char *map_mask_name,
> > -          struct device_node **target, u32 *id_out)
> > +          struct of_phandle_args *arg)
> >  {
> >     u32 map_mask, masked_id;
> >     int map_len;
> >     const __be32 *map = NULL;
> >  
> > -   if (!np || !map_name || (!target && !id_out))
> > +   if (!np || !map_name || !arg)
> >             return -EINVAL;
> >  
> >     map = of_get_property(np, map_name, &map_len);
> >     if (!map) {
> > -           if (target)
> > +           if (arg->np)
> >                     return -ENODEV;
> >             /* Otherwise, no map implies no translation */
> > -           *id_out = id;
> > +           arg->args[0] = id;
> 
> What if args_count is 0? Given that you place no restriction on the
> way this can be called, that'd be entirely legitimate, and you'd
> corrupt something you're not supposed to touch.

args is an array (not a pointer) in of_phandle_args. As such we know
that args[0] is legit.

> 
> >             return 0;
> >     }
> >  
> > @@ -2173,18 +2176,15 @@ int of_map_id(const struct device_node *np, u32 id,
> >             if (!phandle_node)
> >                     return -ENODEV;
> >  
> > -           if (target) {
> > -                   if (*target)
> > -                           of_node_put(phandle_node);
> > -                   else
> > -                           *target = phandle_node;
> > +           if (arg->np)
> > +                   of_node_put(phandle_node);
> > +           else
> > +                   arg->np = phandle_node;
> >  
> > -                   if (*target != phandle_node)
> > -                           continue;
> > -           }
> > +           if (arg->np != phandle_node)
> > +                   continue;
> >  
> > -           if (id_out)
> > -                   *id_out = masked_id - id_base + out_base;
> > +           arg->args[0] = masked_id - id_base + out_base;
> >  
> >             pr_debug("%pOF: %s, using mask %08x, id-base: %08x, out-base: 
> > %08x, length: %08x, id: %08x -> %08x\n",
> >                     np, map_name, map_mask, id_base, out_base,
> > @@ -2193,11 +2193,10 @@ int of_map_id(const struct device_node *np, u32 id,
> >     }
> >  
> >     pr_info("%pOF: no %s translation for id 0x%x on %pOF\n", np, map_name,
> > -           id, target && *target ? *target : NULL);
> > +           id, arg->np ? arg->np : NULL);
> >  
> >     /* Bypasses translation */
> > -   if (id_out)
> > -           *id_out = id;
> > +   arg->args[0] = id;
> >     return 0;
> >  }
> >  EXPORT_SYMBOL_GPL(of_map_id);
> > diff --git a/drivers/pci/controller/dwc/pci-imx6.c 
> > b/drivers/pci/controller/dwc/pci-imx6.c
> > index bff8289f804a..74fc603b3f84 100644
> > --- a/drivers/pci/controller/dwc/pci-imx6.c
> > +++ b/drivers/pci/controller/dwc/pci-imx6.c
> > @@ -1139,12 +1139,18 @@ static int imx_pcie_add_lut_by_rid(struct imx_pcie 
> > *imx_pcie, u32 rid)
> >  {
> >     struct device *dev = imx_pcie->pci->dev;
> >     struct device_node *target;
> > +   struct of_phandle_args iommu_spec = { .args_count = 1 };
> >     u32 sid_i, sid_m;
> >     int err_i, err_m;
> >     u32 sid = 0;
> >  
> >     target = NULL;
> > -   err_i = of_map_iommu_id(dev->of_node, rid, &target, &sid_i);
> > +   err_i = of_map_iommu_id(dev->of_node, rid, &iommu_spec);
> > +   if (!err_i) {
> > +           target = iommu_spec.np;
> > +           sid_i = iommu_spec.args[0];
> > +   }
> > +
> >     if (target) {
> >             of_node_put(target);
> >     } else {
> > diff --git a/drivers/pci/controller/pcie-apple.c 
> > b/drivers/pci/controller/pcie-apple.c
> > index a0937b7b3c4d..e1d4b37d200d 100644
> > --- a/drivers/pci/controller/pcie-apple.c
> > +++ b/drivers/pci/controller/pcie-apple.c
> > @@ -755,6 +755,7 @@ static int apple_pcie_enable_device(struct 
> > pci_host_bridge *bridge, struct pci_d
> >  {
> >     u32 sid, rid = pci_dev_id(pdev);
> >     struct apple_pcie_port *port;
> > +   struct of_phandle_args iommu_spec = { .args_count = 1 };
> >     int idx, err;
> >  
> >     port = apple_pcie_get_port(pdev);
> > @@ -764,10 +765,11 @@ static int apple_pcie_enable_device(struct 
> > pci_host_bridge *bridge, struct pci_d
> >     dev_dbg(&pdev->dev, "added to bus %s, index %d\n",
> >             pci_name(pdev->bus->self), port->idx);
> >  
> > -   err = of_map_iommu_id(port->pcie->dev->of_node, rid, NULL, &sid);
> > +   err = of_map_iommu_id(port->pcie->dev->of_node, rid, &iommu_spec);
> >     if (err)
> >             return err;
> >  
> > +   sid = iommu_spec.args[0];
> 
> I find this stuff absolutely unreadable. You're fishing the SID in a
> random position of a random structure, without any documentation of
> how this is supposed to work. This really needs a wrapper that hides
> this implementation detail.
> 
> Thanks,
> 
>       M.
> 
> -- 
> Without deviation from the norm, progress is not possible.

-- 
With best wishes
Dmitry