[Intel-gfx] [PATCH v5] drm/i915: Fix DDC probe for passive adapters

[Jani Nikula]

On Tue, 02 Jun 2015, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
> On Tue, Jun 02, 2015 at 07:21:15PM +0300, Jani Nikula wrote:
>> Passive DP->DVI/HDMI dongles on DP++ ports show up to the system as HDMI
>> devices, as they do not have a sink device in them to respond to any AUX
>> traffic. When probing these dongles over the DDC, sometimes they will
>> NAK the first attempt even though the transaction is valid and they
>> support the DDC protocol. The retry loop inside of
>> drm_do_probe_ddc_edid() would normally catch this case and try the
>> transaction again, resulting in success.
>> 
>> That, however, was thwarted by the fix for [1]:
>> 
>> commit 9292f37e1f5c79400254dca46f83313488093825
>> Author: Eugeni Dodonov <eugeni.dodonov at intel.com>
>> Date:   Thu Jan 5 09:34:28 2012 -0200
>> 
>>     drm: give up on edid retries when i2c bus is not responding
>> 
>> This added code to exit immediately if the return code from the
>> i2c_transfer function was -ENXIO in order to reduce the amount of time
>> spent in waiting for unresponsive or disconnected devices. That was
>> possible because the underlying i2c bit banging algorithm had retries of
>> its own (which, of course, were part of the reason for the bug the
>> commit fixes).
>> 
>> Since its introduction in
>> 
>> commit f899fc64cda8569d0529452aafc0da31c042df2e
>> Author: Chris Wilson <chris at chris-wilson.co.uk>
>> Date:   Tue Jul 20 15:44:45 2010 -0700
>> 
>>     drm/i915: use GMBUS to manage i2c links
>> 
>> we've been flipping back and forth enabling the GMBUS transfers, but
>> we've settled since then. The GMBUS implementation does not do any
>> retries, however, bailing out of the drm_do_probe_ddc_edid() retry loop
>> on first encounter of -ENXIO. This, combined with Eugeni's commit, broke
>> the retry on -ENXIO.
>> 
>> Retry GMBUS once on -ENXIO on first message to mitigate the issues with
>> passive adapters.
>> 
>> This patch is based on the work, and commit message, by Todd Previte
>> <tprevite at gmail.com>.
>> 
>> [1] https://bugs.freedesktop.org/show_bug.cgi?id=41059
>> 
>> v2: Don't retry if using bit banging.
>> 
>> v3: Move retry within gmbux_xfer, retry only on first message.
>> 
>> v4: Initialize GMBUS0 on retry (Ville).
>> 
>> v5: Take index reads into account (Ville).
>> 
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85924
>> Cc: Todd Previte <tprevite at gmail.com>
>> Cc: stable at vger.kernel.org
>> Signed-off-by: Jani Nikula <jani.nikula at intel.com>
>
> OK, I think I'm done shooting any more holes into this one:
> Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

\o/

Thanks for all the review. I suck.

> Hopefully it works too. I don't have very good intuition into the gmbus
> hardware. I think one day I need to set up a bit-banged i2c slave and
> play around with it just for fun.

AFAICT it should be functionally equivalent to the v1 patch that got
tested in the referenced bug. I'm hoping to get a test result for this
one too.


>
>> ---
>>  drivers/gpu/drm/i915/intel_i2c.c | 20 +++++++++++++++++---
>>  1 file changed, 17 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
>> index 92072f56e418..a64f26c670af 100644
>> --- a/drivers/gpu/drm/i915/intel_i2c.c
>> +++ b/drivers/gpu/drm/i915/intel_i2c.c
>> @@ -486,7 +486,7 @@ gmbus_xfer(struct i2c_adapter *adapter,
>>  					       struct intel_gmbus,
>>  					       adapter);
>>  	struct drm_i915_private *dev_priv = bus->dev_priv;
>> -	int i, reg_offset;
>> +	int i = 0, inc, try = 0, reg_offset;
>>  	int ret = 0;
>>  
>>  	intel_aux_display_runtime_get(dev_priv);
>> @@ -499,12 +499,14 @@ gmbus_xfer(struct i2c_adapter *adapter,
>>  
>>  	reg_offset = dev_priv->gpio_mmio_base;
>>  
>> +retry:
>>  	I915_WRITE(GMBUS0 + reg_offset, bus->reg0);
>>  
>> -	for (i = 0; i < num; i++) {
>> +	for (; i < num; i += inc) {
>> +		inc = 1;
>>  		if (gmbus_is_index_read(msgs, i, num)) {
>>  			ret = gmbus_xfer_index_read(dev_priv, &msgs[i]);
>> -			i += 1;  /* set i to the index of the read xfer */
>> +			inc = 2; /* an index read is two msgs */
>>  		} else if (msgs[i].flags & I2C_M_RD) {
>>  			ret = gmbus_xfer_read(dev_priv, &msgs[i], 0);
>>  		} else {
>> @@ -576,6 +578,18 @@ clear_err:
>>  			 adapter->name, msgs[i].addr,
>>  			 (msgs[i].flags & I2C_M_RD) ? 'r' : 'w', msgs[i].len);
>>  
>> +	/*
>> +	 * Passive adapters sometimes NAK the first probe. Retry the first
>> +	 * message once on -ENXIO for GMBUS transfers; the bit banging algorithm
>> +	 * has retries internally. See also the retry loop in
>> +	 * drm_do_probe_ddc_edid, which bails out on the first -ENXIO.
>> +	 */
>> +	if (ret == -ENXIO && i == 0 && try++ == 0) {
>> +		DRM_DEBUG_KMS("GMBUS [%s] NAK on first message, retry\n",
>> +			      adapter->name);
>> +		goto retry;
>> +	}
>> +
>>  	goto out;
>>  
>>  timeout:
>> -- 
>> 2.1.4
>
> -- 
> Ville Syrjälä
> Intel OTC

-- 
Jani Nikula, Intel Open Source Technology Center