Re: [PATCH] drm/i915/vbt: ignore extraneous child devices for a port

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

On Mon, 14 Aug 2017, Manasi Navare <manasi.d.navare@intel.com> wrote:
> On Fri, Aug 11, 2017 at 02:39:07PM +0300, Jani Nikula wrote:
>> Ever since we've parsed VBT child devices, starting from 6acab15a7b0d
>> ("drm/i915: use the HDMI DDI buffer translations from VBT"), we've
>> ignored the child device information if more than one child device
>> references the same port. The rationale for this seems lost in time.
>> 
>> Since commit 311a20949f04 ("drm/i915: don't init DP or HDMI when not
>> supported by DDI port") we started using this information more to skip
>> HDMI/DP init if the port wasn't there per VBT child devices. However, at
>> the same time it added port defaults without further explanation.
>> 
>> Thus, if the child device info was skipped due to multiple child devices
>> referencing the same port, the device info would be retrieved from the
>> somewhat arbitrary defaults.
>> 
>> Finally, when commit bb1d132935c2 ("drm/i915/vbt: split out defaults
>> that are set when there is no VBT") stopped initializing the defaults
>> whenever VBT is present, thus trusting the VBT more, we stopped
>> initializing ports which were referenced by more than one child device.
>> 
>> Apparently at least Asus UX305UA, UX305U, and UX306U laptops have VBT
>> child device blocks which cause this behaviour. Arguably they were
>> shipped with a broken VBT.
>> 
>> Relax the rules for multiple references to the same port, and use the
>> first child device info to reference a port. Retain the logic to debug
>> log about this, though.
>> 
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101745
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=196233
>> Fixes: bb1d132935c2 ("drm/i915/vbt: split out defaults that are set when there is no VBT")
>> Tested-by: Oliver Weißbarth <mail@oweissbarth.de>
>> Reported-by: Oliver Weißbarth <mail@oweissbarth.de>
>> Reported-by: Didier G <didierg-divers@orange.fr>
>> Reported-by: Giles Anderson <agander@gmail.com>
>> Cc: Manasi Navare <manasi.d.navare@intel.com>
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
>> Cc: <stable@vger.kernel.org> # v4.12+
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>> ---
>>  drivers/gpu/drm/i915/intel_bios.c | 15 +++++++++------
>>  1 file changed, 9 insertions(+), 6 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/i915/intel_bios.c b/drivers/gpu/drm/i915/intel_bios.c
>> index 82b144cdfa1d..183e87e8ea31 100644
>> --- a/drivers/gpu/drm/i915/intel_bios.c
>> +++ b/drivers/gpu/drm/i915/intel_bios.c
>> @@ -1120,8 +1120,8 @@ static void parse_ddi_port(struct drm_i915_private *dev_priv, enum port port,
>>  	bool is_dvi, is_hdmi, is_dp, is_edp, is_crt;
>>  	uint8_t aux_channel, ddc_pin;
>>  	/* Each DDI port can have more than one value on the "DVO Port" field,
>> -	 * so look for all the possible values for each port and abort if more
>> -	 * than one is found. */
>> +	 * so look for all the possible values for each port.
>> +	 */
>>  	int dvo_ports[][3] = {
>>  		{DVO_PORT_HDMIA, DVO_PORT_DPA, -1},
>>  		{DVO_PORT_HDMIB, DVO_PORT_DPB, -1},
>> @@ -1130,7 +1130,10 @@ static void parse_ddi_port(struct drm_i915_private *dev_priv, enum port port,
>>  		{DVO_PORT_CRT, DVO_PORT_HDMIE, DVO_PORT_DPE},
>>  	};
>>  
>> -	/* Find the child device to use, abort if more than one found. */
>> +	/*
>> +	 * Find the first child device to reference the port, report if more
>> +	 * than one found.
>> +	 */
>>  	for (i = 0; i < dev_priv->vbt.child_dev_num; i++) {
>>  		it = dev_priv->vbt.child_dev + i;
>>  
>> @@ -1140,11 +1143,11 @@ static void parse_ddi_port(struct drm_i915_private *dev_priv, enum port port,
>>  
>>  			if (it->common.dvo_port == dvo_ports[port][j]) {
>>  				if (child) {
>> -					DRM_DEBUG_KMS("More than one child device for port %c in VBT.\n",
>> +					DRM_DEBUG_KMS("More than one child device for port %c in VBT, using the first.\n",
>>  						      port_name(port));
>> -					return;
>
> So the bug here was that in case of port referenced by multiple child devices because it would return from the function,
> it would skip the initialization of flags like is_dp/is_hdmi?
> It almost feels like they meant to have a break; here instead of return.
> Now that this patch removes return it will still iterate through all the chile devices even
> after finding second refernce to the same port, isnt that unnecessary?
> Would adding a break there instead optimize it?

The commit message explicitly says I want to retain the behaviour of
flagging the duplicates. There is nothing particularly slow about the
loop that needs optimization; debugging machines out the there in the
wild is the slow part eating developer time.

BR,
Jani.


>
> Regards
> Manasi
>
>> +				} else {
>> +					child = it;
>>  				}
>> -				child = it;
>>  			}
>>  		}
>>  	}
>> -- 
>> 2.11.0
>> 

-- 
Jani Nikula, Intel Open Source Technology Center