[Public]
> >
> > According to the ACPI 6.4 spec:
> > It is strongly recommended that the OS evaluate _OSC with the Query
> > Support Flag set until _OSC returns the Capabilities Masked bit clear,
> > to negotiate the set of features to be granted to the OS for native
> > support; a platform may require a specific combination of features
> > to be supported natively by an OS before granting native control
> > of a given feature. After negotiation with the query flag set,
> > the OS should evaluate without it so that any negotiated values
> > can be made effective to hardware.
> >
> > Currently the code sends the exact same values in both executions of the
> > _OSC and this leads to some problems on some AMD platforms in certain
> > configurations.
> >
> > The following notable capabilities are set by OSPM when query is enabled:
> > * OSC_SB_PR3_SUPPORT
> > * OSC_SB_PCLPI_SUPPORT
> > * OSC_SB_NATIVE_USB4_SUPPORT
> >
> > The first call to the platform OSC returns back a masked capabilities
> > error because the firmware did not acknowledge OSC_SB_PCLPI_SUPPORT
> but
> > it acknolwedged the others.
> >
> > The second call to the platform _OSC without the query flag set then
> > fails because the OSPM still sent the exact same values. This leads
> > to not acknowledging OSC_SB_NATIVE_USB4_SUPPORT and later USB4
> PCIe
> > tunnels can't be authorized.
> >
> > This problem was first introduced by commit 159d8c274fd9 ("ACPI: Pass the
> > same capabilities to the _OSC regardless of the query flag") which subtly
> > adjusted the behavior from 719e1f5 ("ACPI: Execute platform _OSC also
> > with query bit clear").
> >
> > The _OSC was called exactly 2 times:
> > * Once to query and request from firmware
> > * Once to commit to firmware without query
> >
> > To fix this problem, continue to call the _OSC until the firmware has
> > indicated that capabilities are no longer masked or after an arbitrary
> > number of negotiation attempts.
> >
> > Furthermore, to avoid the problem that commit 159d8c274fd9 ("ACPI: Pass
> > the same capabilities to the _OSC regardless of the query flag")
> > introduced, explicitly mark support for CPC and CPPCv2 even if they
> > were masked by the series of query calls due to table loading order on
> > some systems.
> >
> > Fixes: 159d8c274fd9 ("ACPI: Pass the same capabilities to the _OSC
> regardless of the query flag")
> > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > ---
> > This series was accepted but showed a regression in another use of
> acpi_run_osc
> > so the series was dropped.
> >
> > Changes from v4->v5:
> > * Move negotiation entirely into
> acpi_bus_osc_negotiate_platform_control
> > drivers/acpi/bus.c | 31 ++++++++++++++++++++++++++++---
> > 1 file changed, 28 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
> > index b96c54813886..86d88bd72c07 100644
> > --- a/drivers/acpi/bus.c
> > +++ b/drivers/acpi/bus.c
> > @@ -294,6 +294,7 @@ static void
> acpi_bus_osc_negotiate_platform_control(void)
> > .cap.pointer = capbuf,
> > };
> > acpi_handle handle;
> > + int i;
> >
> > capbuf[OSC_QUERY_DWORD] = OSC_QUERY_ENABLE;
> > capbuf[OSC_SUPPORT_DWORD] = OSC_SB_PR3_SUPPORT; /* _PR3 is
> in use */
> > @@ -329,10 +330,34 @@ static void
> acpi_bus_osc_negotiate_platform_control(void)
> > if (ACPI_FAILURE(acpi_get_handle(NULL, "\\_SB", &handle)))
> > return;
> >
> > - if (ACPI_FAILURE(acpi_run_osc(handle, &context)))
> > - return;
> > + /*
> > + * Check if bits were masked, we need to negotiate
> > + * prevent potential endless loop by limited number of
> > + * negotiation cycles.
> > + */
> > + for (i = 0; i < 5; i++) {
>
> Why 5 iterations?
>
> Why cannot it work in analogy with the loop in acpi_pci_osc_control_set()?
5 was an arbitrary number selected just to guarantee that bad firmware couldn't
deadlock the negotiation. It's admittedly unlikely, and if you would prefer I'll swap
over to an endless loop design like acpi_pci_osc_control_set.
>
> > + bool retry = false;
> > +
> > + if (ACPI_FAILURE(acpi_run_osc(handle, &context)))
> > + return;
> > + capbuf_ret = context.ret.pointer;
> > + retry = capbuf_ret[OSC_SUPPORT_DWORD] !=
> capbuf[OSC_SUPPORT_DWORD];
> > + capbuf[OSC_SUPPORT_DWORD] =
> capbuf_ret[OSC_SUPPORT_DWORD];
> > + kfree(context.ret.pointer);
> > + if (!retry)
> > + break;
> > + }
> >
> > - kfree(context.ret.pointer);
> > + /*
> > + * Avoid problems with BIOS dynamically loading tables by indicating
> > + * support for CPPC even if it was masked.
> > + */
> > +#ifdef CONFIG_X86
> > + if (boot_cpu_has(X86_FEATURE_HWP)) {
> > + capbuf[OSC_SUPPORT_DWORD] |= OSC_SB_CPC_SUPPORT;
> > + capbuf[OSC_SUPPORT_DWORD] |= OSC_SB_CPCV2_SUPPORT;
> > + }
> > +#endif
> >
> > /* Now run _OSC again with query flag clear */
> > capbuf[OSC_QUERY_DWORD] = 0;
> > --
> > 2.34.1
> >
