Re: [PATCH] hw/i386/acpi-build: Explain bits in OSC method

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 23, 2020 at 09:55:14AM +0200, Julia Suvorova wrote:
> Provide a better explanation for the enabled bits in the _OSC control
> field. Base it on the latest specification due to new bits 5 and 6.
> 
> Signed-off-by: Julia Suvorova <jusual@redhat.com>
> ---
>  hw/i386/acpi-build.h | 11 +++++++++++
>  hw/i386/acpi-build.c |  7 ++++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/acpi-build.h b/hw/i386/acpi-build.h
> index 74df5fc612..bb5269d162 100644
> --- a/hw/i386/acpi-build.h
> +++ b/hw/i386/acpi-build.h
> @@ -5,6 +5,17 @@
>  
>  extern const struct AcpiGenericAddress x86_nvdimm_acpi_dsmio;
>  
> +/* PCI Firmware Specification 3.2, Table 4-5 */
> +typedef enum {
> +    ACPI_OSC_NATIVE_HP_EN = 0,
> +    ACPI_OSC_SHPC_EN = 1,
> +    ACPI_OSC_PME_EN = 2,
> +    ACPI_OSC_AER_EN = 3,
> +    ACPI_OSC_PCIE_CAP_EN = 4,
> +    ACPI_OSC_LTR_EN = 5,
> +    ACPI_OSC_ALLONES_INVALID = 6,
> +} AcpiOSCField;
> +
>  void acpi_setup(void);
>  
>  #endif
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index 0e0535d2e3..b08f005a35 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -1417,6 +1417,7 @@ static Aml *build_q35_osc_method(void)
>      Aml *method;
>      Aml *a_cwd1 = aml_name("CDW1");
>      Aml *a_ctrl = aml_local(0);
> +    unsigned osc_ctrl;
>  
>      method = aml_method("_OSC", 4, AML_NOTSERIALIZED);
>      aml_append(method, aml_create_dword_field(aml_arg(3), aml_int(0), "CDW1"));
> @@ -1432,7 +1433,11 @@ static Aml *build_q35_osc_method(void)
>       * Always allow native PME, AER (no dependencies)
>       * Allow SHPC (PCI bridges can have SHPC controller)
>       */
> -    aml_append(if_ctx, aml_and(a_ctrl, aml_int(0x1F), a_ctrl));
> +    osc_ctrl = BIT(ACPI_OSC_PME_EN) | BIT(ACPI_OSC_AER_EN) |
> +               BIT(ACPI_OSC_PCIE_CAP_EN) | BIT(ACPI_OSC_NATIVE_HP_EN) |
> +               BIT(ACPI_OSC_SHPC_EN);
> +
> +    aml_append(if_ctx, aml_and(a_ctrl, aml_int(osc_ctrl), a_ctrl));

Now the comment above somewhat duplicates the enums.  And decoding what
does each value mean is a head scratcher.  As long as there's a single
use, we prefer comments to an enum, they can match spec exactly:

BIT(0) /* PCI Express Native Hotplug control */ |
BIT(4) /* SHPC Native Hotplug control */ |

etc ...

>      if_ctx2 = aml_if(aml_lnot(aml_equal(aml_arg(1), aml_int(1))));
>      /* Unknown revision */
> -- 
> 2.25.4