[PATCH] drm/amdgpu/display: fix dmub invalid register read

[Alex Deucher]

On Tue, Mar 23, 2021 at 4:18 AM Thomas Lambertz <mail at thomaslambertz.de> wrote:
>
> DMCUB_SCRATCH_0 sometimes contains 0xdeadbeef during initialization.
> If this is detected, return 0 instead. This prevents wrong bit-flags
> from being read.
>
> The main impact of this bug is in the status check loop in
> dmub_srv_wait_for_auto_load. As it is waiting for the device to become
> ready, returning too early leads to a race condition. It is usually won
> on first boot, but lost when laptop resumes from sleep, breaking screen
> brightness control.
>
> This issue was always present, but previously mitigated by the fact that
> the full register was compared to the wanted value. Currently, only the
> bottom two bits are tested, which are also set in 0xdeadbeef, thus
> returning readiness to early.
>
> Fixes: 5fe6b98ae00d ("drm/amd/display: Update dmub code")

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1518

Harry, Leo, Rodrigo, any ideas?

Alex

> Signed-off-by: Thomas Lambertz <mail at thomaslambertz.de>
> ---
>  drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.c | 8 +++++++-
>  drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.h | 2 ++
>  2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.c b/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.c
> index 8e8e65fa83c0..d6fcae182f68 100644
> --- a/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.c
> +++ b/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.c
> @@ -323,8 +323,14 @@ uint32_t dmub_dcn20_get_gpint_response(struct dmub_srv *dmub)
>  union dmub_fw_boot_status dmub_dcn20_get_fw_boot_status(struct dmub_srv *dmub)
>  {
>         union dmub_fw_boot_status status;
> +       uint32_t value;
> +
> +       value = REG_READ(DMCUB_SCRATCH0);
> +       if (value == DMCUB_SCRATCH0_INVALID)
> +               status.all = 0;
> +       else
> +               status.all = value;
>
> -       status.all = REG_READ(DMCUB_SCRATCH0);
>         return status;
>  }
>
> diff --git a/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.h b/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.h
> index a62be9c0652e..9557e76cf5d4 100644
> --- a/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.h
> +++ b/drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.h
> @@ -154,6 +154,8 @@ struct dmub_srv_common_regs {
>
>  extern const struct dmub_srv_common_regs dmub_srv_dcn20_regs;
>
> +#define DMCUB_SCRATCH0_INVALID 0xdeadbeef
> +
>  /* Hardware functions. */
>
>  void dmub_dcn20_init(struct dmub_srv *dmub);
> --
> 2.31.0
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