Re: [Linaro-acpi] [PATCH v8 11/21] ARM64 / ACPI: Get PSCI flags in FADT for PSCI init

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Fri, Feb 06, 2015 at 07:56:07AM +0000, Hanjun Guo wrote:
> Hi Lorenzo, Al,
> 
> On 2015年02月06日 03:03, Al Stone wrote:
> > On 02/05/2015 10:49 AM, Lorenzo Pieralisi wrote:
> >> Hi Al,
> >
> > Howdy, Lorenzo.
> >
> >> On Thu, Feb 05, 2015 at 05:11:31PM +0000, Al Stone wrote:
> >>> On 02/04/2015 09:43 AM, Lorenzo Pieralisi wrote:
> >>>> On Mon, Feb 02, 2015 at 12:45:39PM +0000, Hanjun Guo wrote:
> >>>>> From: Graeme Gregory <graeme.gregory@linaro.org>
> >>>>>
> >>>>> There are two flags: PSCI_COMPLIANT and PSCI_USE_HVC. When set,
> >>>>> the former signals to the OS that the firmware is PSCI compliant.
> >>>>> The latter selects the appropriate conduit for PSCI calls by
> >>>>> toggling between Hypervisor Calls (HVC) and Secure Monitor Calls
> >>>>> (SMC).
> >>>>>
> >>>>> FADT table contains such information in ACPI 5.1, FADT table was
> >>>>> parsed in ACPI table init and copy to struct acpi_gbl_FADT, so
> >>>>> use the flags in struct acpi_gbl_FADT for PSCI init.
> >>>>
> >>>> So you do rely on a global FADT being available, if you use it for PSCI
> >>>> detection you can use it for ACPI revision detection too, right ?
> >>>>
> >>>> Point is, either we should not use the global FADT table, or we use
> >>>> it consistently, or there is something I am unaware of that prevents
> >>>> you from using in some code paths and I would like to understand
> >>>> why.
> >>>
> >>> The FADT is a required table for arm64, as noted in the documentation
> >>> and the SBBR.  While unfortunately the spec does not say it is mandatory,
> >>> even x86 systems are pretty useless without it.  So yes, we rely on it
> >>> being available, not only for the PSCI info, but other flags such as
> >>> HW_REDUCED_ACPI.
> >>>
> >>> I suppose it does not have to be globally scoped.  However, the FADT is
> >>> frequently used, especially on x86, so it makes sense to me from an
> >>> efficiency standpoint to have a global reference to it.
> >>>
> >>> I'm not sure I understand what is meant by using FADT for ACPI revision
> >>> detection; there are fields in the FADT that provide a major and minor
> >>> number for the FADT itself, but I don't believe there's any guarantee
> >>> those will be the same as the level of the specification that is being
> >>> supported by the kernel (chances are they will, but it's not mandatory).
> >>>
> >>> I've probably just missed a part of a thread somewhere; could you point
> >>> me to where the inconsistency lies?  I'm just not understanding right this
> >>> second....
> >>
> >> Yes, it is my fault, I was referring to another thread/patch (9), where you
> >> need to check the FADT revision to "validate it" (ie >= 5.1) for the arm64
> >> kernel. What I am saying is: if the global FADT is there to parse PSCI
> >> info, it is there to check the FADT revision too, I do not necessarily
> >> see the need for calling acpi_table_parse() again to do it, the FADT
> >> revision checking can be carried out as for PSCI, that's all I wanted
> >> to say.
> >
> > Aha.  I understand now.  Another colleague was also trying to explain this
> > to me and I think I just hadn't had enough coffee yet.  The underlying ACPI
> > code maps tables into the kernel in two phases; it may be that when the code
> > in patch 9 is run, the global table is not yet available, while here it is;
> > I don't recall off-hand.
> >
> > I'll take a look at this and talk it over with Hanjun.  If the global table
> > is available, it would indeed make sense to be consistent.
> 
> I had dig into the code and found out that struct acpi_gbl_FADT will be
> available with correct value only if FADT is presented by firmware.
> 
> acpi_table_init() will be called before parsing FADT for PSCI flag in
> this patch set.
> 
> In acpi_table_init()
>     acpi_initialize_tables()
>        acpi_tb_parse_root_table()
> 
> In acpi_tb_parse_root_table()
> 
> if (ACPI_SUCCESS(status) &&
>     ACPI_COMPARE_NAME(&acpi_gbl_root_table_list.
>       tables[table_index].signature,
>       ACPI_SIG_FADT)) {
> 	acpi_tb_parse_fadt(table_index);
> }
> 
> And acpi_tb_parse_fadt(table_index) will copy the
> fadt table to global struct acpi_gbl_FADT.
> 
> so it seems that we can use global struct acpi_gbl_FADT directly to
> check the FADT revision, but it is only available with firmware
> presented the FADT table, so check for the FADT table is still needed
> for some bad firmware without FADT.
> 
> Why PSCI flag can be used without any check for the availability
> of FADT? because we already disable ACPI if 
> (acpi_table_parse(ACPI_SIG_FADT, acpi_parse_fadt))
> failed (no FADT tabled found), and PSCI flag will not be used
> later.
> 
> So I think we can keep the code as it is for now, and I think
> it is the safest way to do it, does it make sense?

Understood. Basically, given current ACPI code, you have to call
acpi_table_parse() to make sure FADT is there, even if the handler
to parse it can be left to a void empty function, and while at it
within the handler passed to acpi_table_parse() you check the
revision; it makes sense but we end up having disable_acpi() scattered
all over the place.

You can leave your code as it is, or we check with Rafael if
acpi_table_parse() can be made to propagate the handler return value,
my fear is that the acpi_table_parse() is not expected to return
failure if the handler fails, only if table is not found, I will have
a look into this.

Thanks,
Lorenzo
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