> On 17 Feb 2019, at 08:57, Mathias Kresin <d...@kresin.me> wrote:
>
> 08/02/2019 09:23, Petr Cvek:
>> Hello,
>> There is a wrong code in 0025-NET-MIPS-lantiq-adds-xrx200-net.patch [1], the
>> original code:
>> + link->rx_flow = !!(xrx200sw_read_x(XRX200_MAC_CTRL_0_FCON, port) &&
>> 0x0010);
>> + link->tx_flow = !!(xrx200sw_read_x(XRX200_MAC_CTRL_0_FCON, port) &&
>> 0x0020);
>> wants to mask the register value and is using a logical AND and not a
>> bitwise AND.
>> The fix should be:
>> + link->rx_flow = !!(xrx200sw_read_x(XRX200_MAC_CTRL_0_FCON, port) &
>> 0x0010);
>> + link->tx_flow = !!(xrx200sw_read_x(XRX200_MAC_CTRL_0_FCON, port) &
>> 0x0020);
>> References:
>> [1]
>> https://github.com/openwrt/openwrt/blob/master/target/linux/lantiq/patches-4.14/0025-NET-MIPS-lantiq-adds-xrx200-net.patch#L937
>>
>
> Hey Petr,
>
> it looks indeed wrong. And looking more closer at the lines, I don't get why
> we need the double negation.
Having seen this code structure before, it’s a non-obvious way of turning a
masked and hence value based true/false into a binary 0 or 1 result: Exhibit A
moronic demo c programme:
#include <stdio.h>
int main()
{
printf("%d\n", 5 & 4);
printf("%d\n", (5 & 4));
printf("%d\n", !(5 & 4));
printf("%d\n", !!(5 & 4));
}
Results in:
4
4
0
1
Cheers,
Kevin D-B
012C ACB2 28C6 C53E 9775 9123 B3A2 389B 9DE2 334A
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