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

Ben Widawsky
2021-02-03T17:25:13Z

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?