Re: [PATCH v5 13/13] drm/bridge: cdns-dsi: Use pre_enable/post_disable to enable/disable

[Aradhya Bhatia <aradhya.bhatia@linux.dev>]

Hi Dmitry,

Thank you for reviewing the patches!

On 10/20/24 17:27, Dmitry Baryshkov wrote:
> On Sun, Oct 20, 2024 at 01:35:30AM +0530, Aradhya Bhatia wrote:
>> From: Aradhya Bhatia <a-bhatia1@ti.com>
>>
>> The cdns-dsi controller requires that it be turned on completely before
>> the input DPI's source has begun streaming[0]. Not having that, allows
>> for a small window before cdns-dsi enable and after cdns-dsi disable
>> where the previous entity (in this case tidss's videoport) to continue
>> streaming DPI video signals. This small window where cdns-dsi is
>> disabled but is still receiving signals causes the input FIFO of
>> cdns-dsi to get corrupted. This causes the colors to shift on the output
>> display. The colors can either shift by one color component (R->G, G->B,
>> B->R), or by two color components (R->B, G->R, B->G).
>>
>> Since tidss's videoport starts streaming via crtc enable hooks, we need
>> cdns-dsi to be up and running before that. Now that the bridges are
>> pre_enabled before crtc is enabled, and post_disabled after crtc is
>> disabled, use the pre_enable and post_disable hooks to get cdns-dsi
>> ready and running before the tidss videoport to get pass the color shift
>> issues.
>>
> 
> Not being an expert in the TI DSS driver, would it be more proper to
> handle that in the TI driver instead? I mean, sending out DPI signals
> isn't a part of the CRTC setup, it's a job of the encoder.

I haven't done a feasibility analysis of moving the CRTC bits of TIDSS
into the encoder, but even if it were possible, it wouldn't solve the
issue.

The bridge_enable() sequence gets called _after_ the encoder has been
enabled - causing the TIDSS's DPI (enabled from encoder) to still be
up and running before the DSI has had a chance to be ready.

Regards
Aradhya


> 
>> [0]: See section 12.6.5.7.3 "Start-up Procedure" in J721E SoC TRM
>>      TRM Link: http://www.ti.com/lit/pdf/spruil1
>>
>> Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> Signed-off-by: Aradhya Bhatia <a-bhatia1@ti.com>
>> Signed-off-by: Aradhya Bhatia <aradhya.bhatia@linux.dev>
>> ---
>>  .../gpu/drm/bridge/cadence/cdns-dsi-core.c    | 62 ++++++++++---------
>>  1 file changed, 34 insertions(+), 28 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
>> index 79d8c2264c14..dfeb53841ebc 100644
>> --- a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
>> +++ b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
>> @@ -658,13 +658,28 @@ cdns_dsi_bridge_mode_valid(struct drm_bridge *bridge,
>>  	return MODE_OK;
>>  }
>>  
>> -static void cdns_dsi_bridge_atomic_disable(struct drm_bridge *bridge,
>> -					   struct drm_bridge_state *old_bridge_state)
>> +static void cdns_dsi_bridge_atomic_post_disable(struct drm_bridge *bridge,
>> +						struct drm_bridge_state *old_bridge_state)
>>  {
>>  	struct cdns_dsi_input *input = bridge_to_cdns_dsi_input(bridge);
>>  	struct cdns_dsi *dsi = input_to_dsi(input);
>>  	u32 val;
>>  
>> +	/*
>> +	 * The cdns-dsi controller needs to be disabled after it's DPI source
>> +	 * has stopped streaming. If this is not followed, there is a brief
>> +	 * window before DPI source is disabled and after cdns-dsi controller
>> +	 * has been disabled where the DPI stream is still on, but the cdns-dsi
>> +	 * controller is not ready anymore to accept the incoming signals. This
>> +	 * is one of the reasons why a shift in pixel colors is observed on
>> +	 * displays that have cdns-dsi as one of the bridges.
>> +	 *
>> +	 * To mitigate this, disable this bridge from the bridge post_disable()
>> +	 * hook, instead of the bridge _disable() hook. The bridge post_disable()
>> +	 * hook gets called after the CRTC disable, where often many DPI sources
>> +	 * disable their streams.
>> +	 */
>> +
>>  	val = readl(dsi->regs + MCTL_MAIN_DATA_CTL);
>>  	val &= ~(IF_VID_SELECT_MASK | IF_VID_MODE | VID_EN | HOST_EOT_GEN |
>>  		 DISP_EOT_GEN);
>> @@ -683,15 +698,6 @@ static void cdns_dsi_bridge_atomic_disable(struct drm_bridge *bridge,
>>  	pm_runtime_put(dsi->base.dev);
>>  }
>>  
>> -static void cdns_dsi_bridge_atomic_post_disable(struct drm_bridge *bridge,
>> -						struct drm_bridge_state *old_bridge_state)
>> -{
>> -	struct cdns_dsi_input *input = bridge_to_cdns_dsi_input(bridge);
>> -	struct cdns_dsi *dsi = input_to_dsi(input);
>> -
>> -	pm_runtime_put(dsi->base.dev);
>> -}
>> -
>>  static void cdns_dsi_hs_init(struct cdns_dsi *dsi)
>>  {
>>  	struct cdns_dsi_output *output = &dsi->output;
>> @@ -760,8 +766,8 @@ static void cdns_dsi_init_link(struct cdns_dsi *dsi)
>>  	dsi->link_initialized = true;
>>  }
>>  
>> -static void cdns_dsi_bridge_atomic_enable(struct drm_bridge *bridge,
>> -					  struct drm_bridge_state *old_bridge_state)
>> +static void cdns_dsi_bridge_atomic_pre_enable(struct drm_bridge *bridge,
>> +					      struct drm_bridge_state *old_bridge_state)
>>  {
>>  	struct cdns_dsi_input *input = bridge_to_cdns_dsi_input(bridge);
>>  	struct cdns_dsi *dsi = input_to_dsi(input);
>> @@ -776,6 +782,21 @@ static void cdns_dsi_bridge_atomic_enable(struct drm_bridge *bridge,
>>  	if (WARN_ON(pm_runtime_get_sync(dsi->base.dev) < 0))
>>  		return;
>>  
>> +	/*
>> +	 * The cdns-dsi controller needs to be enabled before it's DPI source
>> +	 * has begun streaming. If this is not followed, there is a brief window
>> +	 * after DPI source enable and before cdns-dsi controller enable where
>> +	 * the DPI stream is on, but the cdns-dsi controller is not ready to
>> +	 * accept the incoming signals. This is one of the reasons why a shift
>> +	 * in pixel colors is observed on displays that have cdns-dsi as one of
>> +	 * the bridges.
>> +	 *
>> +	 * To mitigate this, enable this bridge from the bridge pre_enable()
>> +	 * hook, instead of the bridge _enable() hook. The bridge pre_enable()
>> +	 * hook gets called before the CRTC enable, where often many DPI sources
>> +	 * enable their streams.
>> +	 */
>> +
>>  	if (dsi->platform_ops && dsi->platform_ops->enable)
>>  		dsi->platform_ops->enable(dsi);
>>  
>> @@ -912,19 +933,6 @@ static void cdns_dsi_bridge_atomic_enable(struct drm_bridge *bridge,
>>  	writel(tmp, dsi->regs + MCTL_MAIN_EN);
>>  }
>>  
>> -static void cdns_dsi_bridge_atomic_pre_enable(struct drm_bridge *bridge,
>> -					      struct drm_bridge_state *old_bridge_state)
>> -{
>> -	struct cdns_dsi_input *input = bridge_to_cdns_dsi_input(bridge);
>> -	struct cdns_dsi *dsi = input_to_dsi(input);
>> -
>> -	if (WARN_ON(pm_runtime_get_sync(dsi->base.dev) < 0))
>> -		return;
>> -
>> -	cdns_dsi_init_link(dsi);
>> -	cdns_dsi_hs_init(dsi);
>> -}
>> -
>>  static u32 *cdns_dsi_bridge_get_input_bus_fmts(struct drm_bridge *bridge,
>>  					       struct drm_bridge_state *bridge_state,
>>  					       struct drm_crtc_state *crtc_state,
>> @@ -968,9 +976,7 @@ static int cdns_dsi_bridge_atomic_check(struct drm_bridge *bridge,
>>  static const struct drm_bridge_funcs cdns_dsi_bridge_funcs = {
>>  	.attach = cdns_dsi_bridge_attach,
>>  	.mode_valid = cdns_dsi_bridge_mode_valid,
>> -	.atomic_disable = cdns_dsi_bridge_atomic_disable,
>>  	.atomic_pre_enable = cdns_dsi_bridge_atomic_pre_enable,
>> -	.atomic_enable = cdns_dsi_bridge_atomic_enable,
>>  	.atomic_post_disable = cdns_dsi_bridge_atomic_post_disable,
>>  	.atomic_check = cdns_dsi_bridge_atomic_check,
>>  	.atomic_duplicate_state = drm_atomic_helper_bridge_duplicate_state,
>> -- 
>> 2.34.1
>>
>