linux-kernel - Re: [PATCH v2 02/24] EDAC, ghes: Fix grain calculation

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Date:   Mon, 12 Aug 2019 12:05:25 +0000
From:   Robert Richter <rrichter@...vell.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     James Morse <james.morse@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 02/24] EDAC, ghes: Fix grain calculation

On 12.08.19 09:32:22, Borislav Petkov wrote:
> On Mon, Aug 12, 2019 at 06:42:00AM +0000, Robert Richter wrote:

> > With the grain_bits calculation the mask is rounded up to the next
> > power of 2 value.
> 
> mask	  = 0xffffffffff00ff00

grain = ~mask + 1

> ~mask	  = 0x0000000000ff00ff
> ~mask + 1 = 0x0000000000ff0100

grain_bits = fls_long(e->grain - 1);
grain_bits = 24

grain = 1 << grain_bits
grain = 0x1000000

So for masks in the range from 0xffffffffff000000 to
0xffffffffff7fffff we have grain_bits set to 24, which corresponds to
a grain of 0x1000000. Looks good to me.

> 
> Your "trick" of adding a 1 to get to the most significant bit simply
> doesn't work here. Thus:
> 
> "I guess we can leave it like that for now until some "inventive"
> firmware actually does it."

Fine to me.

-Robert