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

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Message-ID: <20210203172342.fpn5vm4xj2xwh6fq@intel.com>
Date:   Wed, 3 Feb 2021 09:23:42 -0800
From:   Ben Widawsky <ben.widawsky@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-cxl@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-pci@...r.kernel.org, Bjorn Helgaas <helgaas@...nel.org>,
        Chris Browy <cbrowy@...ry-design.com>,
        Dan Williams <dan.j.williams@...el.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 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.

> > > > +	/* 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?