Re: [PATCH v3 2/2] x86: detect PIT aliasing on ports other than 0x4[0-3]

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.05.2024 15:57, Jason Andryuk wrote:
> On 2024-05-22 08:59, Jan Beulich wrote:
>> --- a/xen/arch/x86/time.c
>> +++ b/xen/arch/x86/time.c
>> @@ -427,6 +427,74 @@ static struct platform_timesource __init
>>       .resume = resume_pit,
>>   };
>>   
>> +unsigned int __initdata pit_alias_mask;
>> +
>> +static void __init probe_pit_alias(void)
>> +{
>> +    unsigned int mask = 0x1c;
>> +    uint8_t val = 0;
>> +
>> +    if ( !opt_probe_port_aliases )
>> +        return;
>> +
>> +    /*
>> +     * Use channel 2 in mode 0 for probing.  In this mode even a non-initial
>> +     * count is loaded independent of counting being / becoming enabled.  
>> Thus
>> +     * we have a 16-bit value fully under our control, to write and then 
>> check
>> +     * whether we can also read it back unaltered.
>> +     */
>> +
>> +    /* Turn off speaker output and disable channel 2 counting. */
>> +    outb(inb(0x61) & 0x0c, 0x61);
>> +
>> +    outb(PIT_LTCH_CH(2) | PIT_RW_LSB_MSB | PIT_MODE_EOC | PIT_BINARY,
>> +         PIT_MODE);
>> +
>> +    do {
>> +        uint8_t val2;
>> +        unsigned int offs;
>> +
>> +        outb(val, PIT_CH2);
>> +        outb(val ^ 0xff, PIT_CH2);
>> +
>> +        /* Wait for the Null Count bit to clear. */
>> +        do {
>> +            /* Latch status. */
>> +            outb(PIT_RDB | PIT_RDB_NO_COUNT | PIT_RDB_CH2, PIT_MODE);
>> +
>> +            /* Try to make sure we're actually having a PIT here. */
>> +            val2 = inb(PIT_CH2);
>> +            if ( (val2 & ~(PIT_STATUS_OUT_PIN | PIT_STATUS_NULL_COUNT)) !=
>> +                 (PIT_RW_LSB_MSB | PIT_MODE_EOC | PIT_BINARY) )
>> +                return;
>> +        } while ( val2 & (1 << 6) );
> 
> You can use PIT_STATUS_NULL_COUNT here.

Indeed, and I meant to but then forgot. Thanks for noticing.

> With that:
> Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>

Thanks.

Jan