[lvc-project] [PATCH net-next] net: phy: mdio-bcm-unimac: Cast denominator to unsigned long to avoid overflow

[Doug Berger]

On 3/12/2024 10:23 AM, Florian Fainelli wrote:
> On 3/12/24 10:18, Russell King (Oracle) wrote:
>> On Tue, Mar 12, 2024 at 07:53:58PM +0300, Daniil Dulov wrote:
>>> The expression priv->clk_freq * 2 can lead to overflow that will cause
>>> a division by zero. So, let's cast it to unsigned long to avoid it.
>>
>> How does casting this help? "unsigned long" can still be 32-bit.
>> Maybe unimac_mdio_probe() should be validating the value it read from
>> DT won't overflow? I suspect that a value of 2.1GHz is way too large
>> for this property in any case.
>>
>> https://en.wikipedia.org/wiki/Management_Data_Input/Output#Electrical_specification
>>
>> (note, this driver is clause-22 only.)
>>
> 
> Had commented on the previous version (not sure why this was not 
> prefixed with v2) that the maximum clock frequency for this clock is 
> 250MHz, the driver could check that to prevent for an overflow, most 
> certainly.

Could also use:
-	div = (rate / (2 * priv->clk_freq)) - 1;
+	div = ((rate / priv->clk_freq) >> 1) - 1;
which is mathematically equivalent without the risk of overflow.

-Doug