linux-kernel - Re: [PATCH v3 4/9] cxl/mem: Skip intermediate port enumeration of restricted endpoints (RCDs)

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Message-ID: <Y3ORTv/1YbTFJQzq@rric.localdomain>
Date:   Tue, 15 Nov 2022 14:17:02 +0100
From:   Robert Richter <rrichter@....com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH v3 4/9] cxl/mem: Skip intermediate port enumeration of
 restricted endpoints (RCDs)

On 14.11.22 16:07:58, Dan Williams wrote:
> Robert Richter wrote:
> > When an endpoint is found, all ports in beetween the endpoint and the
> > CXL host bridge need to be created. In the RCH case there are no ports
> > in between a host bridge and the endpoint. Skip the enumeration of
> > intermediate ports.
> > 
> > The port enumeration does not only create all ports, it also
> > initializes the endpoint chain by adding the endpoint to every
> > downstream port up to the root bridge. This must be done also in RCD
> > mode, but is much more simple as the endpoint only needs to be added
> > to the host bridge's dport.
> > 
> > Note: For endpoint removal the cxl_detach_ep() is not needed as it is
> > released in cxl_port_release().
> > 
> > Signed-off-by: Robert Richter <rrichter@....com>
> > ---
> >  drivers/cxl/core/port.c | 18 ++++++++++++++++--
> >  1 file changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
> > index d10c3580719b..e21a9c3fe4da 100644
> > --- a/drivers/cxl/core/port.c
> > +++ b/drivers/cxl/core/port.c
> > @@ -1366,8 +1366,24 @@ int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
> >  {
> >  	struct device *dev = &cxlmd->dev;
> >  	struct device *iter;
> > +	struct cxl_dport *dport;
> > +	struct cxl_port *port;
> >  	int rc;
> >  
> > +	/*
> > +	 * Skip intermediate port enumeration in the RCH case, there
> > +	 * are no ports in between a host bridge and an endpoint. Only
> > +	 * initialize the EP chain.
> > +	 */
> > +	if (is_cxl_restricted(cxlmd)) {
> 
> I changed this to:
> 
> 	if (cxlmd->cxlds->rcd) {

I a mail to Bjorn I suggested to have cxl_dev_cap and a cxl_port_cap
in the pci_dev struct that could be looked up too including RCD mode.
Checking the pci_dev looks more reasonable to me here, though we could
have a flag of it in cxlds too.

-Robert

> 
> ...where the cxl_pci driver sets that rcd flag. Aside from keeping some
> PCI specifics out of this helper it also allows RCH/RCD configurations
> to be mocked with cxl_test.
> 
> > +		port = cxl_mem_find_port(cxlmd, &dport);
> > +		if (!port)
> > +			return -ENXIO;
> > +		rc = cxl_add_ep(dport, &cxlmd->dev);
> 
> Ah, good catch, I had missed this detail previously.
> 
> > +		put_device(&port->dev);
> > +		return rc;
> > +	}
> > +
> >  	rc = devm_add_action_or_reset(&cxlmd->dev, cxl_detach_ep, cxlmd);
> >  	if (rc)
> >  		return rc;
> > @@ -1381,8 +1397,6 @@ int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
> >  	for (iter = dev; iter; iter = grandparent(iter)) {
> >  		struct device *dport_dev = grandparent(iter);
> >  		struct device *uport_dev;
> > -		struct cxl_dport *dport;
> > -		struct cxl_port *port;
> >  
> >  		if (!dport_dev)
> >  			return 0;
> > -- 
> > 2.30.2
> > 
> 
>