From: Wolfram Sang <wsa@the-dreams.de>
To: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Thor Thayer <thor.thayer@linux.intel.com>,
Andy Shevchenko <andy.shevchenko@gmail.com>,
David Laight <David.Laight@ACULAB.COM>,
linux-i2c@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] i2c: altera: Fix potential integer overflow
Date: Thu, 13 Feb 2020 10:09:59 +0100 [thread overview]
Message-ID: <20200213090959.GA2123@ninjato> (raw)
In-Reply-To: <20200211144704.GA6461@embeddedor>
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On Tue, Feb 11, 2020 at 08:47:04AM -0600, Gustavo A. R. Silva wrote:
> Factor out 100 from the equation and do 32-bit arithmetic (3 * clk_mhz / 10)
> instead of 64-bit.
>
> Notice that clk_mhz is MHz, so the multiplication will never wrap 32 bits
> and there is no need for div_u64().
Was there ever? With
u32 clk_mhz = clk_get_rate(idev->i2c_clk) / 1000000;
a later multiplication with 300 should not wrap u32?
> /* SDA Hold Time, 300ns */
> - writel(div_u64(300 * clk_mhz, 1000), idev->base + ALTR_I2C_SDA_HOLD);
> + writel(3 * clk_mhz / 10, idev->base + ALTR_I2C_SDA_HOLD);
The change itself is OK, yet I wonder about the comment above:
'clk_mhz * 0.3' will not give a constant 300ns, or?
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next prev parent reply other threads:[~2020-02-13 9:10 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-11 14:47 [PATCH v2] i2c: altera: Fix potential integer overflow Gustavo A. R. Silva
2020-02-11 19:54 ` Thor Thayer
2020-02-13 9:09 ` Wolfram Sang [this message]
2020-02-13 9:58 ` David Laight
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