linux-kernel - Re: [PATCH] drm/amd/debugfs: Expose GFXOFF state to userspace

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Message-ID: <CADnq5_NASpkVUpPa=CPad39AYvzueGx9vsNbUcSgqzQhh+0Lzg@mail.gmail.com>
Date:   Thu, 14 Jul 2022 12:42:07 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     André Almeida <andrealmeid@...lia.com>
Cc:     Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        Pan Xinhui <Xinhui.Pan@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Hawking Zhang <Hawking.Zhang@....com>,
        Tao Zhou <tao.zhou1@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jack Xiao <Jack.Xiao@....com>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom St Denis <tom.stdenis@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        kernel-dev@...lia.com
Subject: Re: [PATCH] drm/amd/debugfs: Expose GFXOFF state to userspace

On Wed, Jul 13, 2022 at 11:15 AM André Almeida <andrealmeid@...lia.com> wrote:
>
> GFXOFF has two different "state" values: one to define if the GPU is
> allowed/disallowed to enter GFXOFF, usually called state; and another
> one to define if currently GFXOFF is being used, usually called status.
> Even when GFXOFF is allowed, GPU firmware can decide to not used it
> accordingly to the GPU load.
>
> Userspace can allow/disallow GPUs to enter into GFXOFF via debugfs. The
> kernel maintains a counter of requests for GFXOFF (gfx_off_req_count)
> that should be decreased to allow GFXOFF and increased to disallow.
>
> The issue with this interface is that userspace can't be sure if GFXOFF
> is currently allowed. Even by checking amdgpu_gfxoff file, one might get
> an ambiguous 2, that means that GPU is currently out of GFXOFF, but that
> can be either because it's currently disallowed or because it's allowed
> but given the current GPU load it's enabled. Then, userspace needs to
> rely on the fact that GFXOFF is enabled by default on boot and to track
> this information.
>
> To make userspace life easier and GFXOFF more reliable, return the
> current state of GFXOFF to userspace when reading amdgpu_gfxoff with the
> same semantics of writing: 0 means not allowed, not 0 means allowed.
>

This looks good. Can you document this in the amdgpu kerneldoc?

Alex

> Expose the current status of GFXOFF through a new file,
> amdgpu_gfxoff_status.
>
> Signed-off-by: André Almeida <andrealmeid@...lia.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c | 49 ++++++++++++++++++++-
>  1 file changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
> index f3b3c688e4e7..e2eec985adb3 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
> @@ -1117,13 +1117,50 @@ static ssize_t amdgpu_debugfs_gfxoff_read(struct file *f, char __user *buf,
>         }
>
>         while (size) {
> -               uint32_t value;
> +               u32 value = adev->gfx.gfx_off_state;
> +
> +               r = put_user(value, (u32 *)buf);
> +               if (r)
> +                       goto out;
> +
> +               result += 4;
> +               buf += 4;
> +               *pos += 4;
> +               size -= 4;
> +       }
> +
> +       r = result;
> +out:
> +       pm_runtime_mark_last_busy(adev_to_drm(adev)->dev);
> +       pm_runtime_put_autosuspend(adev_to_drm(adev)->dev);
> +
> +       return r;
> +}
> +
> +static ssize_t amdgpu_debugfs_gfxoff_status_read(struct file *f, char __user *buf,
> +                                                size_t size, loff_t *pos)
> +{
> +       struct amdgpu_device *adev = file_inode(f)->i_private;
> +       ssize_t result = 0;
> +       int r;
> +
> +       if (size & 0x3 || *pos & 0x3)
> +               return -EINVAL;
> +
> +       r = pm_runtime_get_sync(adev_to_drm(adev)->dev);
> +       if (r < 0) {
> +               pm_runtime_put_autosuspend(adev_to_drm(adev)->dev);
> +               return r;
> +       }
> +
> +       while (size) {
> +               u32 value;
>
>                 r = amdgpu_get_gfx_off_status(adev, &value);
>                 if (r)
>                         goto out;
>
> -               r = put_user(value, (uint32_t *)buf);
> +               r = put_user(value, (u32 *)buf);
>                 if (r)
>                         goto out;
>
> @@ -1206,6 +1243,12 @@ static const struct file_operations amdgpu_debugfs_gfxoff_fops = {
>         .llseek = default_llseek
>  };
>
> +static const struct file_operations amdgpu_debugfs_gfxoff_status_fops = {
> +       .owner = THIS_MODULE,
> +       .read = amdgpu_debugfs_gfxoff_status_read,
> +       .llseek = default_llseek
> +};
> +
>  static const struct file_operations *debugfs_regs[] = {
>         &amdgpu_debugfs_regs_fops,
>         &amdgpu_debugfs_regs2_fops,
> @@ -1217,6 +1260,7 @@ static const struct file_operations *debugfs_regs[] = {
>         &amdgpu_debugfs_wave_fops,
>         &amdgpu_debugfs_gpr_fops,
>         &amdgpu_debugfs_gfxoff_fops,
> +       &amdgpu_debugfs_gfxoff_status_fops,
>  };
>
>  static const char *debugfs_regs_names[] = {
> @@ -1230,6 +1274,7 @@ static const char *debugfs_regs_names[] = {
>         "amdgpu_wave",
>         "amdgpu_gpr",
>         "amdgpu_gfxoff",
> +       "amdgpu_gfxoff_status",
>  };
>
>  /**
> --
> 2.37.0
>