Re: [PATCH v3 3/4] x86/efi: print size in binary units in efi_print_memmap

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Sat, 2016-01-23 at 10:03 -0800, James Bottomley wrote:
> On Sat, 2016-01-23 at 19:18 +0200, Andy Shevchenko wrote:
> > On Sat, Jan 23, 2016 at 6:44 PM, James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:
> > > On Sat, 2016-01-23 at 16:55 +0200, Andy Shevchenko wrote:
> > 
> > > > +static char * __init efi_size_format(char *buf, size_t size,
> > > > u64
> > > > bytes)
> > > > +{
> > > > +     unsigned long i = bytes ? __ffs64(bytes) / 10 : 0;
> > > 
> > > What if size is zero, which might happen on a UEFI screw up?
> > 
> > size of what? Of input buffer?
> 
> I mean when bytes == 0 ffs is undefined.

Well, someone misread the above code ;-)

There is ternary operator exactly to serve this purpose.

> 
> > >  Also it gives really odd results for non power of two memory 
> > > sizes. 16384MB prints as 16GiB but 16385 prints as 16385MiB.
> > 
> > Adaptive precision. I don't think the idea is to print a nearby
> > numbers here.
> 
> Well either there's a point to reducing to the nearest exponent or we
> simply print everything in MB as the original did.  Doing it
> inconsistently is asking for trouble ... and lots of user queries.  I
> mean, supposing there's a range off by one ... now we print a huge
> number in B.

> I really advise against hacking around like this.  In any event if
> efi
> must have this, please don't involve the parts of the kernel that try
> to do this correctly, like lib/string_helpers.h

> 
> > > If the goal is to have a clean interface reporting only the
> > > first 
> > > four significant figures and a size exponent, then a helper
> > > would 
> > > be much better than trying to open code this ad hoc.
> > 
> > No. You get it wrong. The initial idea was (actually not mine, see
> > authorship) to print an exact number with units and reduce whenever
> > it's possible, i.e number is a multiplication of certain unit.
> 
> so you must implement the original idea no matter how inconsistent it
> leads us to be?  Is it wrong to try to do better?

For both comments I prefer to hear Matt's opinion as he is maintainer
of EFI stuff.

My role in this all is to reduce the code base by avoiding 'not
invented here' syndrome.

-- 
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy