linux-kernel - Re: [PATCH 2/2] net: macb: Add support for SiFive FU540-C000

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Message-ID: <mhng-c7fba225-8035-4808-bdd6-bc05da5d2674@palmer-si-x1e>
Date:   Wed, 29 May 2019 19:42:32 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     andrew@...n.ch
CC:     yash.shah@...ive.com, mark.rutland@....com,
        devicetree@...r.kernel.org, aou@...s.berkeley.edu,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        nicolas.ferre@...rochip.com,
        Sachin Ghadi <sachin.ghadi@...ive.com>, robh+dt@...nel.org,
        Paul Walmsley <paul.walmsley@...ive.com>, ynezz@...e.cz,
        linux-riscv@...ts.infradead.org, davem@...emloft.net
Subject:     Re: [PATCH 2/2] net: macb: Add support for SiFive FU540-C000

On Fri, 24 May 2019 06:48:47 PDT (-0700), andrew@...n.ch wrote:
> On Fri, May 24, 2019 at 10:22:06AM +0530, Yash Shah wrote:
>> On Thu, May 23, 2019 at 8:24 PM Andrew Lunn <andrew@...n.ch> wrote:
>> >
>> > > +static int fu540_macb_tx_set_rate(struct clk_hw *hw, unsigned long rate,
>> > > +                               unsigned long parent_rate)
>> > > +{
>> > > +     rate = fu540_macb_tx_round_rate(hw, rate, &parent_rate);
>> > > +     iowrite32(rate != 125000000, mgmt->reg);
>> >
>> > That looks odd. Writing the result of a comparison to a register?
>>
>> The idea was to write "1" to the register if the value of rate is
>> anything else than 125000000.
>
> I'm not a language lawyer. Is it guaranteed that an expression like
> this returns 1? Any value !0 is true, so maybe it actually returns 42?

>From Stack Overflow: https://stackoverflow.com/questions/18097922/return-value-of-operator-in-c

"C11(ISO/IEC 9899:201x) §6.5.8 Relational operators

Each of the operators < (less than), > (greater than), <= (less than or equal
to), and >= (greater than or equal to) shall yield 1 if the specified relation
is true and 0 if it is false. The result has type int."

>> To make it easier to read, I will change this to below:
>>     - iowrite32(rate != 125000000, mgmt->reg);
>>     + if (rate != 125000000)
>>     +     iowrite32(1, mgmt->reg);
>>     + else
>>     +     iowrite32(0, mgmt->reg);
>>
>> Hope that's fine. Thanks for your comment
>
> Yes, that is good.
>
>      Andrew