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

Ben Widawsky
2021-02-11T16:57:05Z

On 21-02-11 10:06:46, Jonathan Cameron wrote:
> 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
>

A minor tweak while here...

diff --git a/include/uapi/linux/cxl_mem.h b/include/uapi/linux/cxl_mem.h
index 655fbfde97fd..dac0adb879ec 100644
--- a/include/uapi/linux/cxl_mem.h
+++ b/include/uapi/linux/cxl_mem.h
@@ -22,7 +22,7 @@
#define CXL_CMDS \
___C(INVALID, "Invalid Command"), \
___C(IDENTIFY, "Identify Command"), \
- ___C(MAX, "Last command")
+ ___C(MAX, "invalid / last command")

#define ___C(a, b) CXL_MEM_COMMAND_ID_##a
enum { CXL_CMDS };
@@ -32,6 +32,17 @@ enum { CXL_CMDS };
static const struct {
const char *name;
} cxl_command_names[] = { CXL_CMDS };
+
+/*
+ * Here's how this actually breaks out:
+ * cxl_command_names[] = {
+ * [CXL_MEM_COMMAND_ID_INVALID] = { "Invalid Command" },
+ * [CXL_MEM_COMMAND_ID_IDENTIFY] = { "Identify Comamnd" },
+ * ...
+ * [CXL_MEM_COMMAND_ID_MAX] = { "invalid / last command" },
+ * };
+ */
+