Re: [PATCH v3 2/2] PCI: dwc: add support for 64-bit MSI target address

[Bjorn Helgaas]

No need to rush the versions (I see v1 on 8/9, v2 and v3 on 8/10).

But if/when you update this, capitalize the subject lines
("PCI: dwc: Add support ...") to match previous history.

On Wed, Aug 10, 2022 at 11:14:44PM +0000, Will McVicker wrote:
> Since not all devices require a 32-bit MSI address, add support to the
> PCIe host driver to allow setting the DMA mask to 64-bits. This allows
> kernels to disable ZONE_DMA32 and bounce buffering (swiotlb) without
> risking not being able to get a 32-bit address during DMA allocation.
> Basically, in the slim chance that there are no 32-bit allocations
> available, the current PCIe host driver will fail to allocate the
> msi_msg page due to a DMA address overflow (seen in [1]).

> With this patch, the PCIe driver can advertise 64-bit support via
> it's MSI capabilities to hint to the PCIe host driver to set the DMA
> mask to 64-bits.

s/via it's/via its/

I'm not quite sure what this sentence is saying.  I think it's
actually the *device* (not the PCIe driver) that advertises 64-bit
support in its MSI capability.

> +u16 dw_pcie_msi_capabilities(struct dw_pcie *pci)
> +{
> +	u8 offset;
> +
> +	offset = dw_pcie_find_capability(pci, PCI_CAP_ID_MSI);
> +	return dw_pcie_readw_dbi(pci, offset + PCI_MSI_FLAGS);
> +}
> +EXPORT_SYMBOL_GPL(dw_pcie_msi_capabilities);

Why does this need to be exported?  CONFIG_PCIE_DW and
CONFIG_PCIE_DW_HOST are both bool, so I don't see any callers from
modules.

I see there are some other functions in pcie-designware.c that are
exported, but I'm a little dubious about those, too.  There are
several DWC drivers that are tristate (PCI_DRA7XX, PCI_EXYNOS, etc),
but they select PCIE_DW_HOST and PCIE_DW, which are bool.  I guess
this means the DWC core code gets built-in while the dra7xx, exynos,
etc code is a module.

If we want these to be modules, it would make more sense to me to have
the module include both the DWC core code and the specific driver.
I.e., the DWC core code would not be statically included at all, and
the dra7xx module would contain DWC core and dra7xx, the exynos module
would contain DWC core and exynos, etc.

Maybe Kconfig isn't expressive enough for that?  I don't know Kconfig
well enough.

Bjorn