linux-kernel - Re: [RFC PATCH 4/9] cxl/mem: Map memory device registers

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Date:   Mon, 16 Nov 2020 18:23:21 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
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 4/9] cxl/mem: Map memory device registers

On Mon, Nov 16, 2020 at 03:19:41PM -0800, Dan Williams wrote:
> On Fri, Nov 13, 2020 at 5:12 PM Ben Widawsky <ben.widawsky@...el.com> wrote:
> > On 20-11-13 12:17:32, Bjorn Helgaas wrote:
> > > On Tue, Nov 10, 2020 at 09:43:51PM -0800, Ben Widawsky wrote:

> > > >  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_mem *cxlm;
> > >
> > > The order was better before ("dev", then "clxm").  Oh, I suppose this
> > > is a "reverse Christmas tree" thing.
> > >
> >
> > I don't actually care either way as long as it's consistent. I tend to do
> > reverse Christmas tree for no particular reason.
> 
> Yeah, reverse Christmas tree for no particular reason.

FWIW, the usual drivers/pci style is to order the decls in the order
the variables are used in the code.  But this isn't drivers/pci, so
it's up to you.  I only noticed because changing the order made the
diff bigger than it needed to be.

> > > I think this would be easier to read if cxl_mem_create() returned NULL
> > > on failure (it prints error messages and we throw away
> > > -ENXIO/-ENOMEM distinction here anyway) so you could do:
> > >
> > >   struct cxl_mem *cxlm = NULL;
> > >
> > >   for (...) {
> > >     if (...) {
> > >       cxlm = cxl_mem_create(pdev, reg_lo, reg_hi);
> > >       break;
> > >     }
> > >   }
> > >
> > >   if (!cxlm)
> > >     return -ENXIO;  /* -ENODEV might be more natural? */
> > >
> >
> > I agree on both counts. Both of these came from Dan, so I will let him explain.
> 
> I'm not attached to differentiating -ENOMEM from -ENXIO and am ok to
> drop the ERR_PTR() return. I do tend to use -ENXIO for failure to
> perform an initialization action vs failure to even find the device,
> but if -ENODEV seems more idiomatic to Bjorn, I won't argue.

-ENXIO is fine with me.  I just don't see it as often so I don't
really know what it is.

Bjorn