Re: [PATCH v5] drm/i915: Fix DDC probe for passive adapters

Jani Nikula
2015-06-02T16:44:54Z

On Tue, 02 Jun 2015, Ville Syrjälä <ville.syrjala@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@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).
>>
>> v5: Take index reads into account (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>
>
> OK, I think I'm done shooting any more holes into this one:
> Reviewed-by: Ville Syrjälä <ville.syrjala@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