Re: [PATCH v2 1/2] PCI: altera: refactor driver for supporting new platform

[Bjorn Helgaas]

On Fri, Sep 08, 2023 at 01:40:13PM +0000, D M, Sharath Kumar wrote:
> > -----Original Message-----
> > From: Bjorn Helgaas <helgaas@xxxxxxxxxx>
> > Sent: Friday, September 8, 2023 6:14 PM
> > To: D M, Sharath Kumar <sharath.kumar.d.m@xxxxxxxxx>
> > Cc: lpieralisi@xxxxxxxxxx; kw@xxxxxxxxx; robh@xxxxxxxxxx;
> > bhelgaas@xxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; dinguyen@xxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH v2 1/2] PCI: altera: refactor driver for supporting new
> > platform
> > 
> > On Fri, Sep 08, 2023 at 09:09:34AM +0000, D M, Sharath Kumar wrote:
> > > > -----Original Message-----
> > > > From: Bjorn Helgaas <helgaas@xxxxxxxxxx> ...
> > 
> > > > > +	int (*ep_read_cfg)(struct altera_pcie *pcie, u8 busno,
> > > > > +			unsigned int devfn, int where, int size, u32 *value);
> > > > > +	int (*ep_write_cfg)(struct altera_pcie *pcie, u8 busno,
> > > > > +			unsigned int devfn, int where, int size, u32 value);
> > > >
> > > > "ep_read_cfg" isn't the ideal name because it suggests "endpoint",
> > > > but it may be either an endpoint or a switch upstream port.  The
> > > > rockchip driver uses "other", which isn't super descriptive either but
> > might be better.
> > > >
> > > Ok will change to "nonrp_read_cfg" ?
> > 
> > I think the important point is not whether it's a Root Port or not, but whether
> > it's on the root *bus* or not.  In other words, I think the driver has to
> > determine whether to generate a Type 0 (targeting something on the root
> > bus) or a Type 1 (targeting something below a
> > bridge) config transaction (see PCI-to-PCI Bridge spec r1.2, sec 3.1.2.1).
> > 
> > There can be non-Root Ports on the root bus, so "nonrp" doesn't seem quite
> > right.  "Other" would be OK, since that's already used by other drivers.
> > Maybe "type0" and "type1" would be better and would fit well with the
> > root_bus_nr check you use to distinguish them?
>
> Situation is
> Root port configuration space  - memory mapped

I don't quite believe the idea that the access method is based on
whether it's a root port.  For one thing, you decide whether to use
the memory-mapped accessor or the indirect accessor based on whether
the read targets the *root bus*, not whether it targets a root port.
And obviously you don't *know* whether the device at a B/D/F address
is a root port until after you read the PCIe type.

I think using names similar to other drivers will be helpful.

These all work on the root bus:

  exynos_pcie_rd_own_conf
  meson_pcie_rd_own_conf
  rockchip_pcie_rd_own_conf

These work on non-root buses:

  dw_pcie_rd_other_conf
  rockchip_pcie_rd_other_conf

> Non root port configuration space - indirect access/proprietary access
>     Type 0 for devices directly connected to root port
>     Type 1 for others

> > > > > +static int _altera_pcie_cfg_read(struct altera_pcie *pcie, u8 busno,
> > > > > +				 unsigned int devfn, int where, int size,
> > > > > +				 u32 *value)
> > > > > +{
> > > > > +	if (busno == pcie->root_bus_nr && pcie->pcie_data->ops-
> > > > >rp_read_cfg)
> > > > > +		return pcie->pcie_data->ops->rp_read_cfg(pcie, busno,
> > > > devfn,
> > > > > +							where, size, value);
> > > > > +
> > > > > +	if (pcie->pcie_data->ops->ep_read_cfg)
> > > > > +		return pcie->pcie_data->ops->ep_read_cfg(pcie, busno,
> > > > devfn,
> > > > > +							where, size, value);
> > > > > +	return PCIBIOS_FUNC_NOT_SUPPORTED; }