Re: [PATCH v5] ACPI: bus: For platform OSC negotiate capabilities

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Mar 9, 2022 at 5:46 PM Mario Limonciello
<mario.limonciello@amd.com> wrote:
>
> 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@amd.com>
> ---
> 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()?

> +               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
>