Re: [PATCH v2 4/8] cxl/mem: Add basic IOCTL interface

Jonathan Cameron
2021-02-11T10:10:35Z

On Wed, 10 Feb 2021 20:40:52 -0800
Dan Williams <dan.j.williams@intel.com> wrote:

> On Wed, Feb 10, 2021 at 10:47 AM Jonathan Cameron
> <Jonathan.Cameron@huawei.com> wrote:
> [..]
> > > +#define CXL_CMDS \
> > > + ___C(INVALID, "Invalid Command"), \
> > > + ___C(IDENTIFY, "Identify Command"), \
> > > + ___C(MAX, "Last command")
> > > +
> > > +#define ___C(a, b) CXL_MEM_COMMAND_ID_##a
> > > +enum { CXL_CMDS };
> > > +
> > > +#undef ___C
> > > +#define ___C(a, b) { b }
> > > +static const struct {
> > > + const char *name;
> > > +} cxl_command_names[] = { CXL_CMDS };
> > > +#undef ___C
> >
> > Unless there are going to be a lot of these, I'd just write them out long hand
> > as much more readable than the macro magic.
>
> This macro magic isn't new to Linux it was introduced with ftrace:
>
> See "cpp tricks and treats": https://lwn.net/Articles/383362/

Yeah. I've dealt with that one a few times. It's very cleaver and compact
but a PITA to debug build errors related to it.

>
> >
> > enum {
> > CXL_MEM_COMMAND_ID_INVALID,
> > CXL_MEM_COMMAND_ID_IDENTIFY,
> > CXL_MEM_COMMAND_ID_MAX
> > };
> >
> > static const struct {
> > const char *name;
> > } cxl_command_names[] = {
> > [CXL_MEM_COMMAND_ID_INVALID] = { "Invalid Command" },
> > [CXL_MEM_COMMAND_ID_IDENTIFY] = { "Identify Comamnd" },
> > /* I hope you never need the Last command to exist in here as that sounds like a bug */
> > };
> >
> > That's assuming I actually figured the macro fun out correctly.
> > To my mind it's worth doing this stuff for 'lots' no so much for 3.
>
> The list will continue to expand, and it eliminates the "did you
> remember to update cxl_command_names" review burden permanently.

How about a compromise. Add a comment giving how the first entry expands to
avoid people (me at least :) having to think their way through it every time?

Jonathan