Re: [Intel-gfx] [PATCH] drm/i915/display/debugfs: Add connector debugfs for "output_bpc"

["Modem, Bhanuprakash" <bhanuprakash.modem@intel.com>]

On Mon-04-04-2022 04:16 pm, Jani Nikula wrote:
> On Mon, 04 Apr 2022, "Modem, Bhanuprakash" <bhanuprakash.modem@intel.com> wrote:
>> On Fri-01-04-2022 06:10 pm, Jani Nikula wrote:
>>> On Tue, 29 Mar 2022, Bhanuprakash Modem <bhanuprakash.modem@intel.com> wrote:
>>>> This new debugfs will expose the connector's max supported bpc
>>>> and the bpc currently using. It is very useful for verifying
>>>> whether we enter the correct output color depth from IGT.
>>>>
>>>> Example:
>>>> cat /sys/kernel/debug/dri/0/DP-1/output_bpc
>>>> Current: 8
>>>> Maximum: 10
>>>>
>>>> V2: Add connector's max bpc to i915_display_info
>>>>
>>>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>>> Cc: Uma Shankar <uma.shankar@intel.com>
>>>> Cc: Swati Sharma <swati2.sharma@intel.com>
>>>> Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
>>>> ---
>>>>    .../drm/i915/display/intel_display_debugfs.c  | 46 +++++++++++++++++++
>>>>    1 file changed, 46 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/display/intel_display_debugfs.c b/drivers/gpu/drm/i915/display/intel_display_debugfs.c
>>>> index c1e74a13a0828..694d27f3b109c 100644
>>>> --- a/drivers/gpu/drm/i915/display/intel_display_debugfs.c
>>>> +++ b/drivers/gpu/drm/i915/display/intel_display_debugfs.c
>>>> @@ -663,6 +663,8 @@ static void intel_connector_info(struct seq_file *m,
>>>>    	seq_puts(m, "\tHDCP version: ");
>>>>    	intel_hdcp_info(m, intel_connector);
>>>>    
>>>> +	seq_printf(m, "\tmax bpc: %u\n", connector->display_info.bpc);
>>>> +
>>>>    	intel_panel_info(m, intel_connector);
>>>>    
>>>>    	seq_printf(m, "\tmodes:\n");
>>>> @@ -2275,6 +2277,47 @@ static const struct file_operations i915_dsc_bpp_fops = {
>>>>    	.write = i915_dsc_bpp_write
>>>>    };
>>>>    
>>>> +/*
>>>> + * Returns the maximum output bpc for the connector.
>>>> + * Example usage: cat /sys/kernel/debug/dri/0/DP-1/output_bpc
>>>> + */
>>>> +static int output_bpc_show(struct seq_file *m, void *data)
>>>> +{
>>>> +	struct drm_connector *connector = m->private;
>>>> +	struct drm_device *dev = connector->dev;
>>>> +	struct drm_crtc *crtc;
>>>> +	struct intel_crtc_state *crtc_state;
>>>> +	struct intel_encoder *encoder = intel_attached_encoder(to_intel_connector(connector));
>>>> +	int res;
>>>> +
>>>> +	if (!encoder)
>>>> +		return -ENODEV;
>>>> +
>>>> +	res = drm_modeset_lock_single_interruptible(&dev->mode_config.connection_mutex);
>>>> +	if (res)
>>>> +		return res;
>>>> +
>>>> +	crtc = connector->state->crtc;
>>>> +	if (connector->status != connector_status_connected || !crtc) {
>>>> +		res = -ENODEV;
>>>> +		goto unlock;
>>>> +	}
>>>> +
>>>> +	crtc_state = to_intel_crtc_state(crtc->state);
>>>> +	if (!crtc_state->hw.active)
>>>> +		goto unlock;
>>>> +
>>>> +	seq_printf(m, "Current: %u\n", crtc_state->pipe_bpp / 3);
>>>> +	seq_printf(m, "Maximum: %u\n", connector->display_info.bpc);
>>>> +	res = 0;
>>>> +
>>>> +unlock:
>>>> +	drm_modeset_unlock(&dev->mode_config.connection_mutex);
>>>> +
>>>> +	return res;
>>>> +}
>>>> +DEFINE_SHOW_ATTRIBUTE(output_bpc);
>>>
>>> Looks like an excessive amount of code for a single value.
>>
>> Yeah, but these values are very helpful in many IGT tests like
>> kms_color, kms_hdr, kms_dither, kms_dsc etc..
>>
>> Otherwise IGT needs to request the kernel to get the EDID, and parse
>> that EDID to get the "Maximum" value which is redundant (Kernel is
>> already doing the same) and not recommended in IGT.
>>
>> And there is no way to get the "Current" value except reading it from
>> i915_display_info which is again not recommended in IGT (As
>> i915_display_info is Intel specific).
> 
> Note how we have intel_connector_debugfs_add() for connector debugfs and
> intel_crtc_debugfs_add() for crtc debugfs, and how this patch just mixes
> up the two by looking up crtc and state from the connector debugfs.

I just tried to adopt from existing AMD's implementation, and it may be 
reduce the number of debugfs nodes.

"Maximum" is from connector's display_info (Needs connector debugfs)
"Current" is from crtc state (Needs crtc debugfs)

> 
>> This debugfs is already introduced & using by AMD:
>> https://patchwork.freedesktop.org/patch/308586
> 
> Wait, what?
> 
> Both amd and i915 adding the name "output_bpc" is *not* the way to
> roll. We only add i915_ prefixed debugfs files from i915.ko.
> 
> If you need this to be a standard interface across drivers, IMO it
> should be added in common drm code, not sprinkled around in drivers.

As display_info is the member of drm_connector, perhaps we can move 
"Maximum" value to drm layer and can use common name.
Example: /sys/kernel/debug/dri/0/DP-1/max_bpc

And each hardware specific driver could create a crtc debugfs node for 
"Current" value in their name space.
Example: /sys/kernel/debug/dri/0/crtc-0/i915_current_bpc

Please suggest.

- Bhanu
> 
> I see that amd is already using this in what is basically drm core
> debugfs namespace, and there's amd specific IGT built on top.
> 
> Adding a bunch of Cc's to get some clarity on drm debugfs naming and
> usage.
> 
> 
> BR,
> Jani.
> 
> 
>>
>> - Bhanu
>>
>>>
>>> BR,
>>> Jani.
>>>
>>>> +
>>>>    /**
>>>>     * intel_connector_debugfs_add - add i915 specific connector debugfs files
>>>>     * @connector: pointer to a registered drm_connector
>>>> @@ -2330,6 +2373,9 @@ void intel_connector_debugfs_add(struct intel_connector *intel_connector)
>>>>    	    connector->connector_type == DRM_MODE_CONNECTOR_HDMIB)
>>>>    		debugfs_create_file("i915_lpsp_capability", 0444, root,
>>>>    				    connector, &i915_lpsp_capability_fops);
>>>> +
>>>> +	debugfs_create_file("output_bpc", 0444, root,
>>>> +			    connector, &output_bpc_fops);
>>>>    }
>>>>    
>>>>    /**
>>>
>>
>