linux-kernel - Re: [PATCH 13/15] cxl/acpi: Rework devm_cxl_enumerate

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Date:   Thu, 1 Sep 2022 09:50:33 +0200
From:   Robert Richter <rrichter@....com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.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>,
        Dan Williams <dan.j.williams@...el.com>,
        <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>
Subject: Re: [PATCH 13/15] cxl/acpi: Rework devm_cxl_enumerate_ports() to
 support RCD mode

On 31.08.22 13:11:19, Jonathan Cameron wrote:
> On Wed, 31 Aug 2022 10:16:01 +0200
> Robert Richter <rrichter@....com> wrote:
> 
> > RCD mode has a different enumeration scheme other than in CXL VH mode.
> > An RCD is directly connected to an RCH without downstream and upstream
> > ports showing up in between in the PCI hierarchy. Due to the direct
> > connection of RCD and RCH, the host bridge is always the RCD's parent
> > instead of the grandparent. 
> Mentioned earlier, but that's not quite true.  There is a reference in
> the spec to allowing it to be under a root port (some sort of virtual structure
> - I'm not sure of 'why' you would that though.)(

Yes, but software view is still the same, see other mail.

> 
> > Modify devm_cxl_enumerate_ports()
> > respectively.
> 
> Don't line wrap above.
> 
> > 
> > Implement this by introducing a function to determine the device's
> > downstream port. The 'for' loop is adjusted for RCD mode and in this
> > case find_cxl_port() will always find the host's associated port and
> > the loop iteration stops.
> > 
> > Signed-off-by: Robert Richter <rrichter@....com>
> > ---
> >  drivers/cxl/core/port.c | 36 ++++++++++++++++++++++++------------
> >  1 file changed, 24 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
> > index 61e9915162d5..08b99423dbf8 100644
> > --- a/drivers/cxl/core/port.c
> > +++ b/drivers/cxl/core/port.c
> > @@ -1084,6 +1084,22 @@ static struct device *grandparent(struct device *dev)
> >  	return NULL;
> >  }
> >  
> > +static struct device *cxl_mem_dport_dev(struct cxl_memdev *cxlmd)
> > +{
> > +	struct device *dev = cxlmd->dev.parent;
> > +	struct pci_dev *pdev = to_pci_dev(cxlmd->dev.parent);
> 
> to_pci_dev(dev);

Ok.

> 
> > +
> > +	/*
> > +	 * An RCiEP is directly connected to the root bridge without
> > +	 * any PCI bridges/ports in between. Reduce the parent level
> > +	 * for those.
> > +	 */
> > +	if (pci_pcie_type(pdev) == PCI_EXP_TYPE_RC_END)
> > +		return dev;
> > +
> > +	return dev->parent;
> Switching from grandparent to this is a little confusing because it's
> done in two steps.  Perhaps use
> 	return grandparent(cmlmd->dev);
> here to keep that connection and rename dev in this function to parent.
> 
> Far too many devices in here with similar names for it to be easy
> to read.

Can improve this a little.

> 
> 
> > +}
> > +
> >  static void delete_endpoint(void *data)
> >  {
> >  	struct cxl_memdev *cxlmd = data;
> > @@ -1339,7 +1355,7 @@ static int add_port_attach_ep(struct cxl_memdev *cxlmd,
> >  int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
> >  {
> >  	struct device *dev = &cxlmd->dev;
> > -	struct device *iter;
> > +	struct device *dport_dev;
> >  	int rc;
> >  
> >  	rc = devm_add_action_or_reset(&cxlmd->dev, cxl_detach_ep, cxlmd);
> > @@ -1352,25 +1368,21 @@ int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
> >  	 * attempt fails.
> >  	 */
> >  retry:
> > -	for (iter = dev; iter; iter = grandparent(iter)) {
> > -		struct device *dport_dev = grandparent(iter);
> > +	for (dport_dev = cxl_mem_dport_dev(cxlmd); dport_dev;
> > +	     dport_dev = grandparent(dport_dev)) {
> 
> I don't like looping for the RCD case as it relies a bit too
> much on subtle relationships between devices and parent.
> 
> Perhaps better to factor out the contents of the loop, then handle
> the RCD case separately from the main loop.
> I haven't tried it, so perhaps that looks even less readable.

I see your point here, will take a look.

Thanks,

-Robert