Re: [PATCH v3 3/4] x86/efi: print size in binary units in efi_print_memmap
From: Andy Shevchenko
Date: Mon Jan 25 2016 - 15:01:18 EST
On Mon, Jan 25, 2016 at 9:45 PM, James Bottomley
> 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@xxxxxxxxxxxxxxxxxxxxx> 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.
K) if 1 MiB > value >= 0 â prints in KiB
M) if â > value >= 1 MiB â prints in MiB.
With Best Regards,