Re: [v3] drm/amdgpu/mst: Stop ignoring error codes and deadlocking
From: Lyude Paul
Date: Fri Nov 18 2022 - 14:57:22 EST
On Fri, 2022-11-18 at 13:54 -0600, Limonciello, Mario wrote:
> On 11/18/2022 13:25, Lyude Paul wrote:
> > It appears that amdgpu makes the mistake of completely ignoring the return
> > values from the DP MST helpers, and instead just returns a simple
> > true/false. In this case, it seems to have come back to bite us because as
> > a result of simply returning false from
> > compute_mst_dsc_configs_for_state(), amdgpu had no way of telling when a
> > deadlock happened from these helpers. This could definitely result in some
> > kernel splats.
> >
> > V2:
> > * Address Wayne's comments (fix another bunch of spots where we weren't
> > passing down return codes)
> > V3:
> > * Fix uninitialized var in pre_compute_mst_dsc_configs_for_state()
>
> FYI v2 was just merged recently, it's in the 6.2 pull request that was
> sent out and Alex planned to add it to 6.1-fixes next week too.
>
> Can you send just the delta from v2->v3 as another patch so it can layer
> in cleanly?
Just did that :), it should be on the list now
> >
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > Fixes: 8c20a1ed9b4f ("drm/amd/display: MST DSC compute fair share")
> > Reviewed-by: Wayne Lin <Wayne.Lin@xxxxxxx>
> > Cc: Harry Wentland <harry.wentland@xxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> # v5.6+
> > ---
> > .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 18 +-
> > .../display/amdgpu_dm/amdgpu_dm_mst_types.c | 235 ++++++++++--------
> > .../display/amdgpu_dm/amdgpu_dm_mst_types.h | 12 +-
> > 3 files changed, 147 insertions(+), 118 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> > index 0db2a88cd4d7..852a2100c6b3 100644
> > --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> > +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> > @@ -6462,7 +6462,7 @@ static int dm_update_mst_vcpi_slots_for_dsc(struct drm_atomic_state *state,
> > struct drm_connector_state *new_con_state;
> > struct amdgpu_dm_connector *aconnector;
> > struct dm_connector_state *dm_conn_state;
> > - int i, j;
> > + int i, j, ret;
> > int vcpi, pbn_div, pbn, slot_num = 0;
> >
> > for_each_new_connector_in_state(state, connector, new_con_state, i) {
> > @@ -6509,8 +6509,11 @@ static int dm_update_mst_vcpi_slots_for_dsc(struct drm_atomic_state *state,
> > dm_conn_state->pbn = pbn;
> > dm_conn_state->vcpi_slots = slot_num;
> >
> > - drm_dp_mst_atomic_enable_dsc(state, aconnector->port, dm_conn_state->pbn,
> > - false);
> > + ret = drm_dp_mst_atomic_enable_dsc(state, aconnector->port,
> > + dm_conn_state->pbn, false);
> > + if (ret < 0)
> > + return ret;
> > +
> > continue;
> > }
> >
> > @@ -9523,10 +9526,9 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
> >
> > #if defined(CONFIG_DRM_AMD_DC_DCN)
> > if (dc_resource_is_dsc_encoding_supported(dc)) {
> > - if (!pre_validate_dsc(state, &dm_state, vars)) {
> > - ret = -EINVAL;
> > + ret = pre_validate_dsc(state, &dm_state, vars);
> > + if (ret != 0)
> > goto fail;
> > - }
> > }
> > #endif
> >
> > @@ -9621,9 +9623,9 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
> > }
> >
> > #if defined(CONFIG_DRM_AMD_DC_DCN)
> > - if (!compute_mst_dsc_configs_for_state(state, dm_state->context, vars)) {
> > + ret = compute_mst_dsc_configs_for_state(state, dm_state->context, vars);
> > + if (ret) {
> > DRM_DEBUG_DRIVER("compute_mst_dsc_configs_for_state() failed\n");
> > - ret = -EINVAL;
> > goto fail;
> > }
> >
> > diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
> > index 6ff96b4bdda5..2f72745660fb 100644
> > --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
> > +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
> > @@ -703,13 +703,13 @@ static int bpp_x16_from_pbn(struct dsc_mst_fairness_params param, int pbn)
> > return dsc_config.bits_per_pixel;
> > }
> >
> > -static bool increase_dsc_bpp(struct drm_atomic_state *state,
> > - struct drm_dp_mst_topology_state *mst_state,
> > - struct dc_link *dc_link,
> > - struct dsc_mst_fairness_params *params,
> > - struct dsc_mst_fairness_vars *vars,
> > - int count,
> > - int k)
> > +static int increase_dsc_bpp(struct drm_atomic_state *state,
> > + struct drm_dp_mst_topology_state *mst_state,
> > + struct dc_link *dc_link,
> > + struct dsc_mst_fairness_params *params,
> > + struct dsc_mst_fairness_vars *vars,
> > + int count,
> > + int k)
> > {
> > int i;
> > bool bpp_increased[MAX_PIPES];
> > @@ -719,6 +719,7 @@ static bool increase_dsc_bpp(struct drm_atomic_state *state,
> > int remaining_to_increase = 0;
> > int link_timeslots_used;
> > int fair_pbn_alloc;
> > + int ret = 0;
> >
> > for (i = 0; i < count; i++) {
> > if (vars[i + k].dsc_enabled) {
> > @@ -757,52 +758,60 @@ static bool increase_dsc_bpp(struct drm_atomic_state *state,
> >
> > if (initial_slack[next_index] > fair_pbn_alloc) {
> > vars[next_index].pbn += fair_pbn_alloc;
> > - if (drm_dp_atomic_find_time_slots(state,
> > - params[next_index].port->mgr,
> > - params[next_index].port,
> > - vars[next_index].pbn) < 0)
> > - return false;
> > - if (!drm_dp_mst_atomic_check(state)) {
> > + ret = drm_dp_atomic_find_time_slots(state,
> > + params[next_index].port->mgr,
> > + params[next_index].port,
> > + vars[next_index].pbn);
> > + if (ret < 0)
> > + return ret;
> > +
> > + ret = drm_dp_mst_atomic_check(state);
> > + if (ret == 0) {
> > vars[next_index].bpp_x16 = bpp_x16_from_pbn(params[next_index], vars[next_index].pbn);
> > } else {
> > vars[next_index].pbn -= fair_pbn_alloc;
> > - if (drm_dp_atomic_find_time_slots(state,
> > - params[next_index].port->mgr,
> > - params[next_index].port,
> > - vars[next_index].pbn) < 0)
> > - return false;
> > + ret = drm_dp_atomic_find_time_slots(state,
> > + params[next_index].port->mgr,
> > + params[next_index].port,
> > + vars[next_index].pbn);
> > + if (ret < 0)
> > + return ret;
> > }
> > } else {
> > vars[next_index].pbn += initial_slack[next_index];
> > - if (drm_dp_atomic_find_time_slots(state,
> > - params[next_index].port->mgr,
> > - params[next_index].port,
> > - vars[next_index].pbn) < 0)
> > - return false;
> > - if (!drm_dp_mst_atomic_check(state)) {
> > + ret = drm_dp_atomic_find_time_slots(state,
> > + params[next_index].port->mgr,
> > + params[next_index].port,
> > + vars[next_index].pbn);
> > + if (ret < 0)
> > + return ret;
> > +
> > + ret = drm_dp_mst_atomic_check(state);
> > + if (ret == 0) {
> > vars[next_index].bpp_x16 = params[next_index].bw_range.max_target_bpp_x16;
> > } else {
> > vars[next_index].pbn -= initial_slack[next_index];
> > - if (drm_dp_atomic_find_time_slots(state,
> > - params[next_index].port->mgr,
> > - params[next_index].port,
> > - vars[next_index].pbn) < 0)
> > - return false;
> > + ret = drm_dp_atomic_find_time_slots(state,
> > + params[next_index].port->mgr,
> > + params[next_index].port,
> > + vars[next_index].pbn);
> > + if (ret < 0)
> > + return ret;
> > }
> > }
> >
> > bpp_increased[next_index] = true;
> > remaining_to_increase--;
> > }
> > - return true;
> > + return 0;
> > }
> >
> > -static bool try_disable_dsc(struct drm_atomic_state *state,
> > - struct dc_link *dc_link,
> > - struct dsc_mst_fairness_params *params,
> > - struct dsc_mst_fairness_vars *vars,
> > - int count,
> > - int k)
> > +static int try_disable_dsc(struct drm_atomic_state *state,
> > + struct dc_link *dc_link,
> > + struct dsc_mst_fairness_params *params,
> > + struct dsc_mst_fairness_vars *vars,
> > + int count,
> > + int k)
> > {
> > int i;
> > bool tried[MAX_PIPES];
> > @@ -810,6 +819,7 @@ static bool try_disable_dsc(struct drm_atomic_state *state,
> > int max_kbps_increase;
> > int next_index;
> > int remaining_to_try = 0;
> > + int ret;
> >
> > for (i = 0; i < count; i++) {
> > if (vars[i + k].dsc_enabled
> > @@ -840,49 +850,52 @@ static bool try_disable_dsc(struct drm_atomic_state *state,
> > break;
> >
> > vars[next_index].pbn = kbps_to_peak_pbn(params[next_index].bw_range.stream_kbps);
> > - if (drm_dp_atomic_find_time_slots(state,
> > - params[next_index].port->mgr,
> > - params[next_index].port,
> > - vars[next_index].pbn) < 0)
> > - return false;
> > + ret = drm_dp_atomic_find_time_slots(state,
> > + params[next_index].port->mgr,
> > + params[next_index].port,
> > + vars[next_index].pbn);
> > + if (ret < 0)
> > + return ret;
> >
> > - if (!drm_dp_mst_atomic_check(state)) {
> > + ret = drm_dp_mst_atomic_check(state);
> > + if (ret == 0) {
> > vars[next_index].dsc_enabled = false;
> > vars[next_index].bpp_x16 = 0;
> > } else {
> > vars[next_index].pbn = kbps_to_peak_pbn(params[next_index].bw_range.max_kbps);
> > - if (drm_dp_atomic_find_time_slots(state,
> > - params[next_index].port->mgr,
> > - params[next_index].port,
> > - vars[next_index].pbn) < 0)
> > - return false;
> > + ret = drm_dp_atomic_find_time_slots(state,
> > + params[next_index].port->mgr,
> > + params[next_index].port,
> > + vars[next_index].pbn);
> > + if (ret < 0)
> > + return ret;
> > }
> >
> > tried[next_index] = true;
> > remaining_to_try--;
> > }
> > - return true;
> > + return 0;
> > }
> >
> > -static bool compute_mst_dsc_configs_for_link(struct drm_atomic_state *state,
> > - struct dc_state *dc_state,
> > - struct dc_link *dc_link,
> > - struct dsc_mst_fairness_vars *vars,
> > - struct drm_dp_mst_topology_mgr *mgr,
> > - int *link_vars_start_index)
> > +static int compute_mst_dsc_configs_for_link(struct drm_atomic_state *state,
> > + struct dc_state *dc_state,
> > + struct dc_link *dc_link,
> > + struct dsc_mst_fairness_vars *vars,
> > + struct drm_dp_mst_topology_mgr *mgr,
> > + int *link_vars_start_index)
> > {
> > struct dc_stream_state *stream;
> > struct dsc_mst_fairness_params params[MAX_PIPES];
> > struct amdgpu_dm_connector *aconnector;
> > struct drm_dp_mst_topology_state *mst_state = drm_atomic_get_mst_topology_state(state, mgr);
> > int count = 0;
> > - int i, k;
> > + int i, k, ret;
> > bool debugfs_overwrite = false;
> >
> > memset(params, 0, sizeof(params));
> >
> > if (IS_ERR(mst_state))
> > - return false;
> > + return PTR_ERR(mst_state);
> >
> > mst_state->pbn_div = dm_mst_get_pbn_divider(dc_link);
> > #if defined(CONFIG_DRM_AMD_DC_DCN)
> > @@ -933,7 +946,7 @@ static bool compute_mst_dsc_configs_for_link(struct drm_atomic_state *state,
> >
> > if (count == 0) {
> > ASSERT(0);
> > - return true;
> > + return 0;
> > }
> >
> > /* k is start index of vars for current phy link used by mst hub */
> > @@ -947,13 +960,17 @@ static bool compute_mst_dsc_configs_for_link(struct drm_atomic_state *state,
> > vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.stream_kbps);
> > vars[i + k].dsc_enabled = false;
> > vars[i + k].bpp_x16 = 0;
> > - if (drm_dp_atomic_find_time_slots(state, params[i].port->mgr, params[i].port,
> > - vars[i + k].pbn) < 0)
> > - return false;
> > + ret = drm_dp_atomic_find_time_slots(state, params[i].port->mgr, params[i].port,
> > + vars[i + k].pbn);
> > + if (ret < 0)
> > + return ret;
> > }
> > - if (!drm_dp_mst_atomic_check(state) && !debugfs_overwrite) {
> > + ret = drm_dp_mst_atomic_check(state);
> > + if (ret == 0 && !debugfs_overwrite) {
> > set_dsc_configs_from_fairness_vars(params, vars, count, k);
> > - return true;
> > + return 0;
> > + } else if (ret != -ENOSPC) {
> > + return ret;
> > }
> >
> > /* Try max compression */
> > @@ -962,31 +979,36 @@ static bool compute_mst_dsc_configs_for_link(struct drm_atomic_state *state,
> > vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.min_kbps);
> > vars[i + k].dsc_enabled = true;
> > vars[i + k].bpp_x16 = params[i].bw_range.min_target_bpp_x16;
> > - if (drm_dp_atomic_find_time_slots(state, params[i].port->mgr,
> > - params[i].port, vars[i + k].pbn) < 0)
> > - return false;
> > + ret = drm_dp_atomic_find_time_slots(state, params[i].port->mgr,
> > + params[i].port, vars[i + k].pbn);
> > + if (ret < 0)
> > + return ret;
> > } else {
> > vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.stream_kbps);
> > vars[i + k].dsc_enabled = false;
> > vars[i + k].bpp_x16 = 0;
> > - if (drm_dp_atomic_find_time_slots(state, params[i].port->mgr,
> > - params[i].port, vars[i + k].pbn) < 0)
> > - return false;
> > + ret = drm_dp_atomic_find_time_slots(state, params[i].port->mgr,
> > + params[i].port, vars[i + k].pbn);
> > + if (ret < 0)
> > + return ret;
> > }
> > }
> > - if (drm_dp_mst_atomic_check(state))
> > - return false;
> > + ret = drm_dp_mst_atomic_check(state);
> > + if (ret != 0)
> > + return ret;
> >
> > /* Optimize degree of compression */
> > - if (!increase_dsc_bpp(state, mst_state, dc_link, params, vars, count, k))
> > - return false;
> > + ret = increase_dsc_bpp(state, mst_state, dc_link, params, vars, count, k);
> > + if (ret < 0)
> > + return ret;
> >
> > - if (!try_disable_dsc(state, dc_link, params, vars, count, k))
> > - return false;
> > + ret = try_disable_dsc(state, dc_link, params, vars, count, k);
> > + if (ret < 0)
> > + return ret;
> >
> > set_dsc_configs_from_fairness_vars(params, vars, count, k);
> >
> > - return true;
> > + return 0;
> > }
> >
> > static bool is_dsc_need_re_compute(
> > @@ -1087,15 +1109,16 @@ static bool is_dsc_need_re_compute(
> > return is_dsc_need_re_compute;
> > }
> >
> > -bool compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> > - struct dc_state *dc_state,
> > - struct dsc_mst_fairness_vars *vars)
> > +int compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> > + struct dc_state *dc_state,
> > + struct dsc_mst_fairness_vars *vars)
> > {
> > int i, j;
> > struct dc_stream_state *stream;
> > bool computed_streams[MAX_PIPES];
> > struct amdgpu_dm_connector *aconnector;
> > int link_vars_start_index = 0;
> > + int ret = 0;
> >
> > for (i = 0; i < dc_state->stream_count; i++)
> > computed_streams[i] = false;
> > @@ -1118,17 +1141,19 @@ bool compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> > continue;
> >
> > if (dcn20_remove_stream_from_ctx(stream->ctx->dc, dc_state, stream) != DC_OK)
> > - return false;
> > + return -EINVAL;
> >
> > if (!is_dsc_need_re_compute(state, dc_state, stream->link))
> > continue;
> >
> > mutex_lock(&aconnector->mst_mgr.lock);
> > - if (!compute_mst_dsc_configs_for_link(state, dc_state, stream->link, vars,
> > - &aconnector->mst_mgr,
> > - &link_vars_start_index)) {
> > +
> > + ret = compute_mst_dsc_configs_for_link(state, dc_state, stream->link, vars,
> > + &aconnector->mst_mgr,
> > + &link_vars_start_index);
> > + if (ret != 0) {
> > mutex_unlock(&aconnector->mst_mgr.lock);
> > - return false;
> > + return ret;
> > }
> > mutex_unlock(&aconnector->mst_mgr.lock);
> >
> > @@ -1143,22 +1168,22 @@ bool compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> >
> > if (stream->timing.flags.DSC == 1)
> > if (dc_stream_add_dsc_to_resource(stream->ctx->dc, dc_state, stream) != DC_OK)
> > - return false;
> > + return -EINVAL;
> > }
> >
> > - return true;
> > + return ret;
> > }
> >
> > -static bool
> > - pre_compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> > - struct dc_state *dc_state,
> > - struct dsc_mst_fairness_vars *vars)
> > +static int pre_compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> > + struct dc_state *dc_state,
> > + struct dsc_mst_fairness_vars *vars)
> > {
> > int i, j;
> > struct dc_stream_state *stream;
> > bool computed_streams[MAX_PIPES];
> > struct amdgpu_dm_connector *aconnector;
> > int link_vars_start_index = 0;
> > + int ret = 0;
> >
> > for (i = 0; i < dc_state->stream_count; i++)
> > computed_streams[i] = false;
> > @@ -1184,11 +1209,12 @@ static bool
> > continue;
> >
> > mutex_lock(&aconnector->mst_mgr.lock);
> > - if (!compute_mst_dsc_configs_for_link(state, dc_state, stream->link, vars,
> > - &aconnector->mst_mgr,
> > - &link_vars_start_index)) {
> > + ret = compute_mst_dsc_configs_for_link(state, dc_state, stream->link, vars,
> > + &aconnector->mst_mgr,
> > + &link_vars_start_index);
> > + if (ret != 0) {
> > mutex_unlock(&aconnector->mst_mgr.lock);
> > - return false;
> > + return ret;
> > }
> > mutex_unlock(&aconnector->mst_mgr.lock);
> >
> > @@ -1198,7 +1224,7 @@ static bool
> > }
> > }
> >
> > - return true;
> > + return ret;
> > }
> >
> > static int find_crtc_index_in_state_by_stream(struct drm_atomic_state *state,
> > @@ -1253,9 +1279,9 @@ static bool is_dsc_precompute_needed(struct drm_atomic_state *state)
> > return ret;
> > }
> >
> > -bool pre_validate_dsc(struct drm_atomic_state *state,
> > - struct dm_atomic_state **dm_state_ptr,
> > - struct dsc_mst_fairness_vars *vars)
> > +int pre_validate_dsc(struct drm_atomic_state *state,
> > + struct dm_atomic_state **dm_state_ptr,
> > + struct dsc_mst_fairness_vars *vars)
> > {
> > int i;
> > struct dm_atomic_state *dm_state;
> > @@ -1264,11 +1290,12 @@ bool pre_validate_dsc(struct drm_atomic_state *state,
> >
> > if (!is_dsc_precompute_needed(state)) {
> > DRM_INFO_ONCE("DSC precompute is not needed.\n");
> > - return true;
> > + return 0;
> > }
> > - if (dm_atomic_get_state(state, dm_state_ptr)) {
> > + ret = dm_atomic_get_state(state, dm_state_ptr);
> > + if (ret != 0) {
> > DRM_INFO_ONCE("dm_atomic_get_state() failed\n");
> > - return false;
> > + return ret;
> > }
> > dm_state = *dm_state_ptr;
> >
> > @@ -1280,7 +1307,7 @@ bool pre_validate_dsc(struct drm_atomic_state *state,
> >
> > local_dc_state = kmemdup(dm_state->context, sizeof(struct dc_state), GFP_KERNEL);
> > if (!local_dc_state)
> > - return false;
> > + return -ENOMEM;
> >
> > for (i = 0; i < local_dc_state->stream_count; i++) {
> > struct dc_stream_state *stream = dm_state->context->streams[i];
> > @@ -1316,9 +1343,9 @@ bool pre_validate_dsc(struct drm_atomic_state *state,
> > if (ret != 0)
> > goto clean_exit;
> >
> > - if (!pre_compute_mst_dsc_configs_for_state(state, local_dc_state, vars)) {
> > + ret = pre_compute_mst_dsc_configs_for_state(state, local_dc_state, vars);
> > + if (ret != 0) {
> > DRM_INFO_ONCE("pre_compute_mst_dsc_configs_for_state() failed\n");
> > - ret = -EINVAL;
> > goto clean_exit;
> > }
> >
> > @@ -1349,7 +1376,7 @@ bool pre_validate_dsc(struct drm_atomic_state *state,
> >
> > kfree(local_dc_state);
> >
> > - return (ret == 0);
> > + return ret;
> > }
> >
> > static unsigned int kbps_from_pbn(unsigned int pbn)
> > diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.h b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.h
> > index b92a7c5671aa..97fd70df531b 100644
> > --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.h
> > +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.h
> > @@ -53,15 +53,15 @@ struct dsc_mst_fairness_vars {
> > struct amdgpu_dm_connector *aconnector;
> > };
> >
> > -bool compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> > - struct dc_state *dc_state,
> > - struct dsc_mst_fairness_vars *vars);
> > +int compute_mst_dsc_configs_for_state(struct drm_atomic_state *state,
> > + struct dc_state *dc_state,
> > + struct dsc_mst_fairness_vars *vars);
> >
> > bool needs_dsc_aux_workaround(struct dc_link *link);
> >
> > -bool pre_validate_dsc(struct drm_atomic_state *state,
> > - struct dm_atomic_state **dm_state_ptr,
> > - struct dsc_mst_fairness_vars *vars);
> > +int pre_validate_dsc(struct drm_atomic_state *state,
> > + struct dm_atomic_state **dm_state_ptr,
> > + struct dsc_mst_fairness_vars *vars);
> >
> > enum dc_status dm_dp_mst_is_port_support_mode(
> > struct amdgpu_dm_connector *aconnector,
>
--
Cheers,
Lyude Paul (she/her)
Software Engineer at Red Hat