[PATCH] drm/amdgpu: add rebar parameter

[Alex Deucher]

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On Fri, Feb 28, 2025 at 3:39 PM Alex Deucher <alexander.deucher at amd.com> wrote:
>
> Add a new parameter to disable BAR resizing.  Note that this
> only disables the driver from attempting to resize the BAR,
> The BIOS may have resized the BAR at boot.
>
> Some teams have found this useful in debugging P2P DMA
> issues on systems where the available MMIO space did not allow
> for all of the GPUs present to resize their BARs.
>
> Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu.h        |  1 +
>  drivers/gpu/drm/amd/amdgpu/amdgpu_device.c |  3 +++
>  drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c    | 11 +++++++++++
>  3 files changed, 15 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu.h b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
> index 87062c1adcdf7..948f832f469ef 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
> @@ -268,6 +268,7 @@ extern int amdgpu_umsch_mm_fwlog;
>
>  extern int amdgpu_user_partt_mode;
>  extern int amdgpu_agp;
> +extern int amdgpu_rebar;
>
>  extern int amdgpu_wbrf;
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> index 0a1a1f3ee5fc0..cc1a991ad4719 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
> @@ -1662,6 +1662,9 @@ int amdgpu_device_resize_fb_bar(struct amdgpu_device *adev)
>         if (amdgpu_sriov_vf(adev))
>                 return 0;
>
> +       if (!amdgpu_rebar)
> +               return 0;
> +
>         /* resizing on Dell G5 SE platforms causes problems with runtime pm */
>         if ((amdgpu_runtime_pm != 0) &&
>             adev->pdev->vendor == PCI_VENDOR_ID_ATI &&
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
> index b161daa900198..333c78ac000e9 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
> @@ -237,6 +237,7 @@ int amdgpu_agp = -1; /* auto */
>  int amdgpu_wbrf = -1;
>  int amdgpu_damage_clips = -1; /* auto */
>  int amdgpu_umsch_mm_fwlog;
> +int amdgpu_rebar = -1; /* auto */
>
>  DECLARE_DYNDBG_CLASSMAP(drm_debug_classes, DD_CLASS_TYPE_DISJOINT_BITS, 0,
>                         "DRM_UT_CORE",
> @@ -1083,6 +1084,16 @@ MODULE_PARM_DESC(wbrf,
>         "Enable Wifi RFI interference mitigation (0 = disabled, 1 = enabled, -1 = auto(default)");
>  module_param_named(wbrf, amdgpu_wbrf, int, 0444);
>
> +/**
> + * DOC: rebar (int)
> + * Allow BAR resizing.  Disable this to prevent the driver from attempting
> + * to resize the BAR if the GPU supports it and there is available MMIO space.
> + * Note that this just prevents the driver from resizing the BAR.  The BIOS
> + * may have already resized the BAR at boot time.
> + */
> +MODULE_PARM_DESC(rebar, "Resizable BAR (-1 = auto (default), 0 = disable, 1 = enable)");
> +module_param_named(rebar, amdgpu_rebar, int, 0444);
> +
>  /* These devices are not supported by amdgpu.
>   * They are supported by the mach64, r128, radeon drivers
>   */
> --
> 2.48.1
>