Re: [PATCH V2] PCI: tegra: Fix building Tegra194 PCIe driver

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Jun 08, 2021 at 08:44:49AM +0100, Jon Hunter wrote:
> On 08/06/2021 00:50, Bjorn Helgaas wrote:
> 
> ...
> 
> > My understanding is that we want pcie-tegra194.c to be:
> > 
> >   - Built into the kernel when CONFIG_PCIE_TEGRA194=m or =y and
> >     CONFIG_ACPI=y and CONFIG_PCI_QUIRKS=y.  If we're using the ACPI
> >     pci_root.c driver, we must have the MCFG quirk built-in, and this
> >     case worked as I expected (this is on x86):
> > 
> >       $ grep -E "CONFIG_(ACPI\>|PCI_QUIRKS|PCIE_TEGRA194)" .config
> >       CONFIG_ACPI=y
> >       CONFIG_PCI_QUIRKS=y
> >       CONFIG_PCIE_TEGRA194=y
> >       CONFIG_PCIE_TEGRA194_HOST=m
> >       CONFIG_PCIE_TEGRA194_EP=y
> > 
> >       $ rm drivers/pci/controller/dwc/pcie-tegra194.*o
> >       $ make drivers/pci/controller/dwc/
> > 	...
> > 	CC      drivers/pci/controller/dwc/pcie-tegra194.o
> > 	AR      drivers/pci/controller/dwc/built-in.a
> > 
> >   - Built as a module when CONFIG_PCIE_TEGRA194=m and CONFIG_ACPI is
> >     not set.  In this case, we're not using the ACPI pci_root.c
> >     driver, and we don't need the MCFG quirk built-in, so it should be
> >     OK to build a module, and IIUC this patch is supposed to *allow*
> >     that.  But in my testing, it did *not* build a module.  Am I
> >     missing something?
> > 
> >       $ grep -E "CONFIG_(ACPI\>|PCI_QUIRKS|PCIE_TEGRA194)" .config
> >       # CONFIG_ACPI is not set
> >       # CONFIG_PCI_QUIRKS is not set
> >       CONFIG_PCIE_TEGRA194=y
> >       CONFIG_PCIE_TEGRA194_HOST=m
> >       CONFIG_PCIE_TEGRA194_EP=y
> 
> The problem appears to be that you still have CONFIG_PCIE_TEGRA194=y and
> CONFIG_PCIE_TEGRA194_EP=y above. If I have ...

Huh.  I can't set CONFIG_PCIE_TEGRA194 directly; it's only selectable
by PCIE_TEGRA194_HOST and PCIE_TEGRA194_EP.  PCIE_TEGRA194 is
tristate, but apparently kconfig sets it to the most restrictive,
which I guess makes sense.

So I would expect the shared infrastructure to be built-in if either
driver is built-in, but it's somewhat confusing that
CONFIG_PCIE_TEGRA194_HOST=m results in a builtin driver.  If I can set
CONFIG_PCIE_TEGRA194_HOST and CONFIG_PCIE_TEGRA194_EP independently,
it seems like they should *be* independent.

What is the purpose of PCIE_TEGRA194_EP (added by c57247f940e8 ("PCI:
tegra: Add support for PCIe endpoint mode in Tegra194") [1])?  I don't
see any reference to it in a makefile or a source file.

It looks like one can build a single driver that works in either host
or endpoint mode, depending on whether a DT node matches
"nvidia,tegra194-pcie" or "nvidia,tegra194-pcie-ep".

So I think PCIE_TEGRA194_EP is superfluous and should be removed and
you should have a single tristate Kconfig option.

Tangentially related, this cast in tegra_pcie_dw_probe() looks
unnecessary:

  pcie->mode = (enum dw_pcie_device_mode)data->mode;

Looks like it was copied from similar code in dra7xx_pcie_probe(),
artpec6_pcie_probe(), and dw_plat_pcie_probe(), which are all littered
with similar unnecessary casts.  But that should be solved separately
from the Kconfig question.

[1] https://git.kernel.org/linus/c57247f940e8

> $ grep -E "CONFIG_(ACPI\>|PCI_QUIRKS|PCIE_TEGRA194)" .config
> # CONFIG_ACPI is not set
> CONFIG_PCI_QUIRKS=y
> CONFIG_PCIE_TEGRA194=m
> CONFIG_PCIE_TEGRA194_HOST=m
> # CONFIG_PCIE_TEGRA194_EP is not set
> 
> 
> > 
> >       $ rm drivers/pci/controller/dwc/pcie-tegra194.*o
> >       $ make drivers/pci/controller/dwc/
> > 	...
> > 	CC      drivers/pci/controller/dwc/pcie-tegra194.o
> > 	AR      drivers/pci/controller/dwc/built-in.a
> 
> Then I see ...
> 
> $ rm drivers/pci/controller/dwc/pcie-tegra194.*o
> $ make drivers/pci/controller/dwc/
>   ...
>   CC [M]  drivers/pci/controller/dwc/pcie-tegra194.o
> 
> Cheers
> Jon
> 
> -- 
> nvpublic