Re: [PATCH v12 05/13] PCI: dwc: Add dw_pcie_parent_bus_offset()

[Manivannan Sadhasivam]

On Mon, Mar 24, 2025 at 01:28:27PM -0500, Bjorn Helgaas wrote:
> On Mon, Mar 24, 2025 at 10:48:23PM +0530, Manivannan Sadhasivam wrote:
> > On Sat, Mar 15, 2025 at 03:15:40PM -0500, Bjorn Helgaas wrote:
> > > From: Frank Li <Frank.Li@xxxxxxx>
> > > 
> > > Return the offset from CPU physical address to the parent bus address of
> > > the specified element of the devicetree 'reg' property.
> 
> > > +resource_size_t dw_pcie_parent_bus_offset(struct dw_pcie *pci,
> > > +					  const char *reg_name,
> > > +					  resource_size_t cpu_phy_addr)
> > > +{
> > 
> > s/cpu_phy_addr/cpu_phys_addr/g
> 
> Fixed, thanks!
> 
> > > +	struct device *dev = pci->dev;
> > > +	struct device_node *np = dev->of_node;
> > > +	int index;
> > > +	u64 reg_addr;
> > > +
> > > +	/* Look up reg_name address on parent bus */
> > 
> > 'parent bus' is not accurate as the below code checks for the 'reg_name' in
> > current PCI controller node.
> 
> We want the address of "reg_name" on the node's primary side.  We've
> been calling that the "parent bus address", I guess because it's the
> address on the "parent bus" of the node.
> 

Yeah, 'parent bus address' sounds bogus to me. 'ranges' property is described
as:

	ranges = <child_addr parent_addr child_size>

Here, child_addr refers to the PCIe host controller's view of the address space
and parent_addr refers to the CPU's view of the address space.

So the register address described in the PCIe controller node is not a 'parent
bus address'.

> I'm not sure what the best term is for this.  Do you have a
> suggestion?
> 

We are just extracting the offset between translated (cpu_phy_addr) and
untranslated (reg_addr) addresses of a specific register. Maybe the function
should just return the 'untranslated address' and the caller should compute the
offset to make it simple?

> If "parent bus address" is the wrong term, maybe we need to rename
> dw_pcie_parent_bus_offset() itself?  
> 

Yes!

> Currently we pass in cpu_phys_addr, but this function doesn't need it
> except for the debug code added later.  I would really rather have
> something like this in the callers:
> 
>   pci->parent_bus_offset = pp->cfg0_base -
>       dw_pcie_parent_bus_addr(pci, "config");
> 

I agree. This should become, dw_pcie_get_untranslated_addr().

> because then the offset is computed sort of at the same level where
> it's used, and a grep for "cfg0_base" would find both the set and the
> use and they would be easy to match up.
> 
> > > +	index = of_property_match_string(np, "reg-names", reg_name);
> > > +
> > > +	if (index < 0) {
> > > +		dev_err(dev, "No %s in devicetree \"reg\" property\n", reg_name);
> > 
> > Both of these callers are checking for the existence of the
> > 'reg_name' property before calling this API. So this check seems to
> > be redundant (for now).
> 
> True, but I don't see a way to enforce the caller checks.  I don't
> like the idea of calling of_property_read_reg(np, index, ...) where we
> have to look the caller to verify that "index" is valid.
> 

Ok.

- Mani

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