linux-kernel - Re: [PATCH v4 3/5] clk: renesas: rcar-gen3: Support Z and Z2 clocks with high frequency parents

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Message-ID: <20190207152242.xzxfvojyyvsqhmtt@verge.net.au>
Date:   Thu, 7 Feb 2019 16:22:46 +0100
From:   Simon Horman <horms@...ge.net.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Fabrizio Castro <fabrizio.castro@...renesas.com>,
        Biju Das <biju.das@...renesas.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/5] clk: renesas: rcar-gen3: Support Z and Z2 clocks
 with high frequency parents

On Thu, Feb 07, 2019 at 04:05:58PM +0100, Geert Uytterhoeven wrote:
> Hi Simon,
> 
> On Thu, Feb 7, 2019 at 2:36 PM Simon Horman <horms+renesas@...ge.net.au> wrote:
> > Support Z and Z2 clocks with parent frequencies greater than UINT32_MAX Hz
> > (~4.29GHz).
> >
> > The DIV_ROUND_CLOSEST_ULL() macro accepts a 64bit dividend and 32bit
> > divisor. This leads to truncation of the dividend, which is the Z or Z2
> 
> truncation of the divisor.
> 
> > parent clock frequency in HZ, on platforms where frequency of that clock is
> > greater than UINT32_MAX Hz.
> >
> > To resolve this problem the DIV64_U64_ROUND_CLOSEST() macro, which an
> 
> which takes an
> 
> > unsigned 32bit dividend and divisor, is introduced.
> 
> 64-bit
> 
> > An earlier version of this patch made use of the existing
> > DIV_ROUND_CLOSEST() macro, which accepts the prevailing type of the
> > dividend and divisor. However, this does not compile on 32bit systems, such
> > as i386 and mips, when called with the types used at this callsite, an
> > unsigned long long dividend and unsigned long divisor.
> 
> Thanks for fixing this!
> 
> > This work is in preparation for supporting the Z2 cloco on the R-Car Gen3
> > E3 (r8a77990) SoC which has a 4.8GHz parent clock.
> >
> > Signed-off-by: Simon Horman <horms+renesas@...ge.net.au>
> 
> With the above fixed:
> Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
> 
> > ---
> > v4: Add and use DIV64_U64_ROUND_CLOSEST
> >
> > v2: New patch
> > ---
> >  drivers/clk/renesas/rcar-gen3-cpg.c |  4 ++--
> >  include/linux/math64.h              | 13 +++++++++++++
> 
> While I have no issue taking this change through the clk-renesas tree if
> Andrew provides his Acked-by, I think the introduction of
> DIV64_U64_ROUND_CLOSEST() should be a separate patch.
> 
> However, given commit 68600f623d69da42 ("mm: don't miss the last page
> because of round-off error") added both DIV64_U64_ROUND_UP() and its
> user under mm/, there's precedence for not splitting it off...

I also assumed a separate patch would be better,
but I found little evidence of that practice being
recently used in math64.h.

In any case, as I need to respin I'll break it out into
a separate patch.