Re: [PATCH 1/1] x86/acpi: Use FADT flags to determine the PMTMR width

[Jan Beulich <jbeulich@suse.com>]

On 16.06.2020 12:32, Roger Pau Monné wrote:
> On Tue, Jun 16, 2020 at 10:07:05AM +0200, Jan Beulich wrote:
>> On 14.06.2020 16:36, Grzegorz Uriasz wrote:
>>> --- a/xen/arch/x86/acpi/boot.c
>>> +++ b/xen/arch/x86/acpi/boot.c
>>> @@ -480,7 +480,10 @@ static int __init acpi_parse_fadt(struct acpi_table_header *table)
>>>  		if (fadt->xpm_timer_block.space_id ==
>>>  		    ACPI_ADR_SPACE_SYSTEM_IO) {
>>>  			pmtmr_ioport = fadt->xpm_timer_block.address;
>>> -			pmtmr_width = fadt->xpm_timer_block.bit_width;
>>> +			if (fadt->flags & ACPI_FADT_32BIT_TIMER)
>>> +				pmtmr_width = 32;
>>> +			else
>>> +				pmtmr_width = 24;
>>
>> I think disagreement of the two wants logging, and you want to
>> default to using the smaller of the two (or even to ignoring the
>> timer altogether). Then there wants to be a way to override
>> (unless we already have one) our defaulting, in case it's wrong.
> 
> TBH, I presume timer_block will always return 32bits, because that's
> the size of the register. Then the timer can implement less bits than
> the full size of the register, and that's what gets signaled using the
> ACPI flags. What we care about here is the number of bits used by the
> timer, not the size of the register.

The first random system I checked this on reports 24 bits as bit_width
(and the flag clear, i.e. both are consistent). The flag, aiui, is
really important only in the ACPI v1 case, where the size of the
register was a byte-granular value. The spec isn't helpful in
clarifying applicability of the flag though, i.e. one can interpret it
either way imo.

Jan

> I think we should only ignore the timer if pm_timer_block.bit_width <
> pmtmr_width.
> 
> Printing a (debug) message when those values disagree is fine, but I
> bet it's going to trigger always when the implemented timer is only
> using 24bits.
> 
> Roger.
>