Re: [PATCH v2] PCI: Add save and restore capability for TPH config space

[Lukas Wunner <lukas@wunner.de>]

On Thu, Jul 07, 2022 at 01:06:53PM -0600, Keith Busch wrote:
> On Thu, Jul 07, 2022 at 11:42:38AM -0400, Paul Luse wrote:
> > +config PCIE_TPH
> > +	bool "PCI TPH (Transaction Layer Packet Processing Hints) capability support"
> > +	help
> > +	  This enables PCI Express TPH (Transaction Layer Packet Processing Hints) by
> > +	  making sure they are saved and restored across resets. Enable this if your
> > +	  hardware uses the PCI Express TPH capabilities. For more information please
> > +	  refer to PCIe r6.0 sec 6.17.
> 
> I'm not sure this needs a config option. Even if hardware isn't supporting TPH,
> this state save code doesn't takes up much space, and saving this config
> capability seems to always be the right thing if hardware is supporting TPH.

We recently had a complaint from Christoph Hellwig that DOE support
should not "bloat every single kernel built with PCI support":

https://lore.kernel.org/linux-pci/YkVBJ+nRA2g%2FWDxa@infradead.org/

And OpenWRT folks once tried to make PCI quirks conditional on a
config option to reduce kernel footprint by 12 kBytes on space-
constrained Mips routers:

https://lore.kernel.org/linux-pci/1482306784-29224-1-git-send-email-john@phrozen.org/

Paul has followed your advice and did away with the config option
in his most recent version of the patch, but I'm not sure that's
actually the right thing to do.  I don't think TPH is a mainstream
PCIe feature and we need to cater to the needs of low-end devices
with PCIe as well.

Thanks,

Lukas