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

[Jani Nikula <jani.nikula@linux.intel.com>]

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.

> 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.

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);
>>>   }
>>>   
>>>   /**
>> 
>

-- 
Jani Nikula, Intel Open Source Graphics Center