Re: [PATCH v4] drm/i915: Fix DDC probe for passive adapters

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Jun 02, 2015 at 01:42:52PM +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@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@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@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).
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85924
> Cc: Todd Previte <tprevite@gmail.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_i2c.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
> index 92072f56e418..5f2e88d39baf 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, try = 0, reg_offset;
>  	int ret = 0;
>  
>  	intel_aux_display_runtime_get(dev_priv);
> @@ -499,9 +499,10 @@ 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++) {
>  		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 */

This increment means we'll never retry if we decice to use
gmbus_xfer_index_read() for the first two messages. AFAICS that
should always be the case for EDID reads where we don't send the
segment address.

> @@ -576,6 +577,18 @@ 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 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
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