linux-kernel - Re: [RFC PATCH 8/9] cxl/mem: Register CXL memX devices

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Date:   Fri, 20 Nov 2020 15:20:18 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Ben Widawsky <ben.widawsky@...el.com>, <linux-cxl@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        "Ira Weiny" <ira.weiny@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [RFC PATCH 8/9] cxl/mem: Register CXL memX devices

On Thu, 19 Nov 2020 18:16:19 -0800
Dan Williams <dan.j.williams@...el.com> wrote:

> On Tue, Nov 17, 2020 at 7:57 AM Jonathan Cameron
> <Jonathan.Cameron@...wei.com> wrote:
> >
> > On Tue, 10 Nov 2020 21:43:55 -0800
> > Ben Widawsky <ben.widawsky@...el.com> wrote:
> >  
> > > From: Dan Williams <dan.j.williams@...el.com>
> > >
> > > Create the /sys/bus/cxl hierarchy to enumerate memory devices
> > > (per-endpoint control devices), memory address space devices (platform
> > > address ranges with interleaving, performance, and persistence
> > > attributes), and memory regions (active provisioned memory from an
> > > address space device that is in use as System RAM or delegated to
> > > libnvdimm as Persistent Memory regions).
> > >
> > > For now, only the per-endpoint control devices are registered on the
> > > 'cxl' bus.  
> >
> > Reviewing ABI without documentation is challenging even when it's simple
> > so please add that for v2.
> >
> > This patch feels somewhat unpolished, but I guess it is mainly here to
> > give an illustration of how stuff might fit together rather than
> > any expectation of detailed review.  
> 
> Yeah, this is definitely an early look in the spirit of "Release early
> / release often".
> 

Definitely a good idea.


...

> >  
> > >  static int cxl_mem_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> > >  {
> > >       struct cxl_mem *cxlm = ERR_PTR(-ENXIO);
> > >       struct device *dev = &pdev->dev;
> > > +     struct cxl_memdev *cxlmd;
> > >       int rc, regloc, i;
> > >
> > >       rc = cxl_bus_prepared(pdev);
> > > @@ -319,20 +545,31 @@ static int cxl_mem_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> > >       if (rc)
> > >               return rc;
> > >
> > > -     /* Check that hardware "looks" okay. */
> > > -     rc = cxl_mem_mbox_get(cxlm);
> > > +     rc = cxl_mem_identify(cxlm);
> > >       if (rc)
> > >               return rc;
> > > -
> > > -     cxl_mem_mbox_put(cxlm);  
> >
> > It was kind of nice to see the flow earlier, but I'm also thinking it made
> > a slightly harder to read patch.  Hmm.  Maybe just drop the version earlier
> > in favour of a todo comment that you then do here?  
> 
> Not sure I follow, but I think you're saying don't bother with an
> initial patch introducing just doing the raw cxl_mem_mbox_get() in
> this path, jump straight to cxl_mem_identify()?

Exactly.

> 
> >  
> > >       dev_dbg(&pdev->dev, "CXL Memory Device Interface Up\n");
> > > +  
> >
> > Nice to tidy that up by moving to earlier patch.  
> 
> Sure.
> 
> >  
> > >       pci_set_drvdata(pdev, cxlm);
> > >
> > > +     cxlmd = cxl_mem_add_memdev(cxlm);
> > > +     if (IS_ERR(cxlmd))
> > > +             return PTR_ERR(cxlmd);  
> >
> > Given we don't actually use cxlmd perhaps a simple return value
> > of 0 or error would be better from cxl_mem_add_memdev()
> >
> > (I guess you may have follow up patches that do something with it
> >  here, though it feels wrong to ever do so given it is now registered
> >  and hence exposed to the system).  
> 
> It's not added if IS_ERR() is true, but it would be simpler to just
> have cxl_mem_add_memdev() return an int since ->probe() doesn't use
> it.

Agreed.

> 
> >  
> > > +
> > >       return 0;
> > >  }
> > >

...