Re: [RFC PATCH v2 08/28] cxl/port: Introduce a port driver

[Ben Widawsky <ben.widawsky@intel.com>]

On 21-11-04 12:10:09, Dan Williams wrote:
> On Tue, Nov 2, 2021 at 9:58 AM Ben Widawsky <ben.widawsky@intel.com> wrote:
> >
> > On 21-11-01 20:31:03, Dan Williams wrote:
> > > On Mon, Nov 1, 2021 at 10:53 AM Ben Widawsky <ben.widawsky@intel.com> wrote:
> > > >
> > > > On 21-10-29 18:37:36, Dan Williams wrote:
> > > > > On Fri, Oct 22, 2021 at 11:37 AM Ben Widawsky <ben.widawsky@intel.com> wrote:
> > > > > >
> > > >
> > > > [snip]
> > > >
> > > > > > diff --git a/drivers/cxl/Makefile b/drivers/cxl/Makefile
> > > > > > index cf07ae6cea17..40b386aaedf7 100644
> > > > > > --- a/drivers/cxl/Makefile
> > > > > > +++ b/drivers/cxl/Makefile
> > > > > > @@ -1,5 +1,6 @@
> > > > > >  # SPDX-License-Identifier: GPL-2.0
> > > > > >  obj-$(CONFIG_CXL_BUS) += core/
> > > > > > +obj-$(CONFIG_CXL_MEM) += cxl_port.o
> > > > >
> > > > > It feel odd that CONFIG_CXL_MEM builds cxl_port, why not have a
> > > > > CONFIG_CXL_PORT that is simply selected by CONFIG_CXL_MEM, or a
> > > > > CONFIG_CXL_PORT that defaults to the value of CONFIG_CXL_BUS?
> > > > >
> > > >
> > > > Can you help me understand when CONFIG_CXL_MEM is useful when
> > > > #CONFIG_CXL_PORT=n? I was unable to figure out such a case and so I tied the two
> > > > together.
> > >
> > > With a 'select' dependency it's impossible to have the
> > > CONFIG_CXL_PORT=n and CONFIG_CXL_MEM=m combination. The extra config
> > > symbol is for idiomatic (one config-symbol per module .ko) reasons to
> > > reflect the module dependency in the Kconfig.
> > >
> >
> > Should CXL_MEM also select CXL_PCI? I don't see a way CXL_MEM can work without
> > CXL_PCI.
> 
> The "select" dependency should be an "I use services from module X"
> relationship. CXL_MEM uses port services and can't operate unless that
> module is present. CXL_MEM does not strictly need CXL_PCI especially
> since drivers/cxl/core/memdev.c abandoned all PCI refernces.

The problem I see is cxl_mem devices have a soft dependency on cxl_pci. For
example what we discussed regard DVSEC range registers. My plan was if the
endpoint driver can't determine range registers aren't in use, it won't bind. It
can't make this determination without cxl_pci. DOE has a similar implication
though not fatal.