linux-kernel - Re: [PATCH 03/14] cxl/mem: Find device capabilities

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Message-ID: <CAPcyv4hvFjs=QqmUYqPipuaLoFiZ-dr6qVhqbDupWuKTw3QDkg@mail.gmail.com>
Date:   Wed, 3 Feb 2021 13:23:31 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ben Widawsky <ben.widawsky@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-cxl@...r.kernel.org,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Chris Browy <cbrowy@...ry-design.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Jon Masters <jcm@...masters.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        daniel.lll@...baba-inc.com,
        "John Groves (jgroves)" <jgroves@...ron.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>
Subject: Re: [PATCH 03/14] cxl/mem: Find device capabilities

On Wed, Feb 3, 2021 at 9:23 AM Ben Widawsky <ben.widawsky@...el.com> wrote:
>
> On 21-02-03 17:15:34, Christoph Hellwig wrote:
> > On Tue, Feb 02, 2021 at 10:24:18AM -0800, Ben Widawsky wrote:
> > > > > +       /* Cap 4000h - CXL_CAP_CAP_ID_MEMDEV */
> > > > > +       struct {
> > > > > +               void __iomem *regs;
> > > > > +       } mem;
> > > >
> > > > This style looks massively obsfucated.  For one the comments look like
> > > > absolute gibberish, but also what is the point of all these anonymous
> > > > structures?
> > >
> > > They're not anonymous, and their names are for the below register functions. The
> > > comments are connected spec reference 'Cap XXXXh' to definitions in cxl.h. I can
> > > articulate that if it helps.
> >
> > But why no simply a
> >
> >       void __iomem *mem_regs;
> >
> > field vs the extra struct?
> >
> > > The register space for CXL devices is a bit weird since it's all subdivided
> > > under 1 BAR for now. To clearly distinguish over the different subregions, these
> > > helpers exist. It's really easy to mess this up as the developer and I actually
> > > would disagree that it makes debugging quite a bit easier. It also gets more
> > > convoluted when you add the other 2 BARs which also each have their own
> > > subregions.
> > >
> > > For example. if my mailbox function does:
> > > cxl_read_status_reg32(cxlm, CXLDEV_MB_CAPS_OFFSET);
> > >
> > > instead of:
> > > cxl_read_mbox_reg32(cxlm, CXLDEV_MB_CAPS_OFFSET);
> > >
> > > It's easier to spot than:
> > > readl(cxlm->regs + cxlm->status_offset + CXLDEV_MB_CAPS_OFFSET)
> >
> > Well, what I think would be the most obvious is:
> >
> > readl(cxlm->status_regs + CXLDEV_MB_CAPS_OFFSET);
> >
>
> Right, so you wrote the buggy version. Should be.
> readl(cxlm->mbox_regs + CXLDEV_MB_CAPS_OFFSET);
>
> Admittedly, "MB" for mailbox isn't super obvious. I think you've convinced me to
> rename these register definitions
> s/MB/MBOX.
>
> I'd prefer to keep the helpers for now as I do find them helpful, and so far
> nobody else who has touched the code has complained. If you feel strongly, I
> will change it.

After seeing the options, I think I'd prefer to not have to worry what
extra magic is happening with cxl_read_mbox_reg32()

cxl_read_mbox_reg32(cxlm, CXLDEV_MB_CAPS_OFFSET);

readl(cxlm->mbox_regs + CXLDEV_MB_CAPS_OFFSET);

The latter is both shorter and more idiomatic.


>
> > > > > +       /* 8.2.8.4.3 */
> > > >
> > > > ????
> > > >
> > >
> > > I had been trying to be consistent with 'CXL2.0 - ' in front of all spec
> > > reference. I obviously missed this one.
> >
> > FYI, I generally find section names much easier to find than section
> > numbers.  Especially as the numbers change very frequently, some times
> > even for very minor updates to the spec.  E.g. in NVMe the numbers might
> > even change from NVMe 1.X to NVMe 1.Xa because an errata had to add
> > a clarification as its own section.
>
> Why not both?
>
> I ran into this in fact going from version 0.7 to 1.0 of the spec. I did call
> out the spec version to address this, but you're right. Section names can change
> too in theory.
>
> /*
>  * CXL 2.0 8.2.8.4.3
>  * Mailbox Capabilities Register
>  */
>
> Too much?

That seems like too many lines.

/* CXL 2.0 8.2.8.4.3 Mailbox Capabilities Register */

...this looks ok.