Re: [PATCH v3 02/25] printk: Add print format (%par) for struct range

[Petr Mladek <pmladek@suse.com>]

On Mon 2024-08-26 16:17:52, Ira Weiny wrote:
> Andy Shevchenko wrote:
> > On Mon, Aug 26, 2024 at 03:23:50PM +0200, Petr Mladek wrote:
> > > On Thu 2024-08-22 21:10:25, Andy Shevchenko wrote:
> > > > On Thu, Aug 22, 2024 at 12:53:32PM -0500, Ira Weiny wrote:
> > > > > Petr Mladek wrote:
> > > > > > On Fri 2024-08-16 09:44:10, Ira Weiny wrote:
> > 
> > ...
> > 
> > > > > > > +	%par	[range 0x60000000-0x6fffffff] or
> > > > > > 
> > > > > > It seems that it is always 64-bit. It prints:
> > > > > > 
> > > > > > struct range {
> > > > > > 	u64   start;
> > > > > > 	u64   end;
> > > > > > };
> > > > > 
> > > > > Indeed.  Thanks I should not have just copied/pasted.
> > > > 
> > > > With that said, I'm not sure the %pa is a good placeholder for this ('a' stands
> > > > to "address" AFAIU). Perhaps this should go somewhere under %pr/%pR?
> 
> I'm speaking a bit for Dan here but also the logical way I thought of
> things.
> 
> 1) %p does not dictate anything about the format of the data.  Rather
>    indicates that what is passed is a pointer.  Because we are passing a
>    pointer to a range struct %pXX makes sense.
> 2) %pa indicates what follows is 'address'.  This was a bit of creative
>    license because, as I said in the commit message most of the time
>    struct range contains an address range.  So for this narrow use case it
>    also makes sense.
> 3) %par r for range.

Yes. I got it.

Well, is struct range really used for addresses? It rather looks like
a range of any 64-bit values.

> %p[rR] is taken.  %pra confuses things IMO.

Another variants might be %pr64 or %prange.

IMHO, there is no good solution. We are trying to find the least
bad one. The meaning should be as obvious and as least confusing
as possible.

Honestly, I do not have a strong opinion. I kind of like %prange ;-)
But I could live with all other variants, except for %pn mentioned below.

> > > The r/R in %pr/%pR actually stands for "resource".
> > > 
> > > But "%ra" really looks like a better choice than "%par". Both
> > > "resource"  and "range" starts with 'r'. Also the struct resource
> > > is printed as a range of values.
> 
> %r could be used I think.  But this breaks with the convention of passing a
> pointer and how to interpret it.

How exactly does it break the convention, please?

Do you passing a pointer to struct range instead of a pointer to
struct resource?

It should not be a big problem as long as the vsprintf() code is
able to guess the right pointer type from the %pXX modifier.

> The other idea I had, mentioned in the commit
> message was %pn.  Meaning passed by pointer 'raNge'.

This looks like the worst variant to me.

> > Fine with me as long as it:
> > 1) doesn't collide with %pa namespace
> > 2) tries to deduplicate existing code as much as possible.
> 
> Andy, I'm not quite following how you expect to share the code between
> resource_string() and range_string()?
> 
> There is very little duplicated code.  In fact with Petr's suggestions and some
> more work range_string() is quite simple:
>
> +static noinline_for_stack
> +char *range_string(char *buf, char *end, const struct range *range,
> +                     struct printf_spec spec, const char *fmt)
> +{
> +#define RANGE_DECODED_BUF_SIZE         ((2 * sizeof(struct range)) + 4)
> +#define RANGE_PRINT_BUF_SIZE           sizeof("[range -]")
> +       char sym[RANGE_DECODED_BUF_SIZE + RANGE_PRINT_BUF_SIZE];
> +       char *p = sym, *pend = sym + sizeof(sym);
> +
> +       *p++ = '[';
> +       p = string_nocheck(p, pend, "range ", default_str_spec);
> +       p = special_hex_number(p, pend, range->start, sizeof(range->start));
> +       *p++ = '-';
> +       p = special_hex_number(p, pend, range->end, sizeof(range->end));
> +       *p++ = ']';
> +       *p = '\0';
> +
> +       return string_nocheck(buf, end, sym, spec);
> +}

I agree that there is not much duplicated code in the end.

> Also this is the bulk of the patch except for documentation and the new
> testing code.  [new patch below]
> 
> Am I missing your point somehow?  I considered cramming a struct range into a
> struct resource to let resource_string() process the data.  But that would
> involve creating a new IORESOURCE_* flag (not ideal) and also does not allow
> for the larger u64 data in struct range should this be a 32 bit physical
> address config.

This would be nasty. I believe that this is not what Andy meant.

Best Regards,
Petr

PS: I have vacation until the end of the week, so my next eventual
    reaction would be delayed.