linux-kernel - Re: [PATCH v3 3/4] x86/efi: print size in binary units in efi_print

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Message-ID: <CAHp75VeXHoyOgFSb4vzWBmELH_yJx1mSN+quxue=tSZCh5sTXw@mail.gmail.com>
Date:	Mon, 25 Jan 2016 22:01:11 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	"Elliott, Robert (Persistent Memory)" <elliott@....com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel @ vger . kernel . org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/4] x86/efi: print size in binary units in efi_print_memmap

On Mon, Jan 25, 2016 at 9:45 PM, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
> On Mon, 2016-01-25 at 21:28 +0200, Andy Shevchenko wrote:
>> On Mon, Jan 25, 2016 at 8:56 PM, James Bottomley
>> <James.Bottomley@...senpartnership.com> wrote:
>> > On Mon, 2016-01-25 at 18:02 +0000, Elliott, Robert (Persistent
>> > Memory)
>> > wrote:
>>
>> >  Using ffs leads to precision runaway
>>
>> How exactly?!
>
> Off by one.  A size of 0xffffffffffffffff prints 18446744073709551615 B
> rather than 20 GiB.

Because it's not a 20 GiB. It's exactly 20 GiB - 1 B.

AFAIU, the intention was to show _exact_ size.

>> It's good you are doing this better, but I still vote for __ffs64(),
>> since it would be faster on binary units.
>
> Is speed of a start of day print a particular concern?

If it's cheap to do, why not to do?

>> Also, in one version I tried to convert couple of other users which
>> are using only KM (in general whatever range it would be) units. Any
>> ideas how to modify to support them?
>
> You mean units in odd increments of 6 digits (so K, M, T ...)? no.  The
> logarithmic reduction is done to the base of the unit increment (1000
> or 1024) so it doesn't really fit this case and it would be hard to
> adjust because we don't have enough precision in the remainder.
>  However, unless there's a huge need to keep it, I'd just fit to the
> closest 3 digit increment and then everything would work.

KM case:
K) if 1 MiB > value >= 0 — prints in KiB
M) if ∞ > value >= 1 MiB — prints in MiB.

-- 
With Best Regards,
Andy Shevchenko