[PATCH v1] drm/dp/mst: fix missing modeset unlock for MST port detect

[Jani Nikula]

On Fri, 29 Sep 2023, Ramya SR <rsr at qti.qualcomm.com> wrote:
> Hi All ,
>
> Please review the reply comment.

Please read the responses you do get [1]. Please stop
top-posting. Please fix your mail client.

BR,
Jani.


[1] https://lore.kernel.org/r/877coak8o3.fsf@intel.com

>
> Regards,
> Ramya SR
>
> -----Original Message-----
> From: Ramya SR 
> Sent: Thursday, September 28, 2023 7:55 AM
> To: 'Alex Deucher' <alexdeucher at gmail.com>; 'imre.deak at intel.com' <imre.deak at intel.com>
> Cc: 'Lyude Paul' <lyude at redhat.com>; 'Jani Nikula' <jani.nikula at intel.com>; 'Jeff Layton' <jlayton at kernel.org>; 'linux-kernel at vger.kernel.org' <linux-kernel at vger.kernel.org>; 'dri-devel at lists.freedesktop.org' <dri-devel at lists.freedesktop.org>; 'Wayne Lin' <Wayne.Lin at amd.com>; 'Alex Deucher' <alexander.deucher at amd.com>; Ramya SR (QUIC) <quic_rsr at quicinc.com>
> Subject: RE: [PATCH v1] drm/dp/mst: fix missing modeset unlock for MST port detect
>
> Reminder. Please review the reply comment.
>
> Thanks and Regards,
> Ramya SR
>
> -----Original Message-----
> From: Ramya SR 
> Sent: Tuesday, September 26, 2023 4:12 PM
> To: Alex Deucher <alexdeucher at gmail.com>; imre.deak at intel.com
> Cc: Lyude Paul <lyude at redhat.com>; Jani Nikula <jani.nikula at intel.com>; Jeff Layton <jlayton at kernel.org>; linux-kernel at vger.kernel.org; dri-devel at lists.freedesktop.org; Wayne Lin <Wayne.Lin at amd.com>; Alex Deucher <alexander.deucher at amd.com>; Ramya SR (QUIC) <quic_rsr at quicinc.com>
> Subject: RE: [PATCH v1] drm/dp/mst: fix missing modeset unlock for MST port detect
>
>
>
> -----Original Message-----
> From: Alex Deucher <alexdeucher at gmail.com>
> Sent: Monday, September 25, 2023 8:27 PM
> To: imre.deak at intel.com
> Cc: Lyude Paul <lyude at redhat.com>; Jani Nikula <jani.nikula at intel.com>; Jeff Layton <jlayton at kernel.org>; linux-kernel at vger.kernel.org; dri-devel at lists.freedesktop.org; Wayne Lin <Wayne.Lin at amd.com>; Alex Deucher <alexander.deucher at amd.com>; Ramya SR (QUIC) <quic_rsr at quicinc.com>
> Subject: Re: [PATCH v1] drm/dp/mst: fix missing modeset unlock for MST port detect
>
> WARNING: This email originated from outside of Qualcomm. Please be wary of any links or attachments, and do not enable macros.
>
> On Fri, Sep 22, 2023 at 3:22 PM Imre Deak <imre.deak at intel.com> wrote:
>>
>> On Fri, Sep 22, 2023 at 03:02:23PM -0400, Lyude Paul wrote:
>> >
>> > Oh! wow thank you for catching this:
>> >
>> > Reviewed-by: Lyude Paul <lyude at redhat.com>
>> >
>> > I will go and push this to drm-misc-next in just a moment
>> >
>> > On Fri, 2023-09-15 at 10:24 +0530, Ramya SR wrote:
>> > > Modeset mutex unlock is missing in drm_dp_mst_detect_port function.
>> > > This will lead to deadlock if calling the function multiple times 
>> > > in an atomic operation, for example, getting imultiple MST ports 
>> > > status for a DP MST bonding scenario.
>> > >
>> > > Signed-off-by: Ramya SR <quic_rsr at quicinc.com>
>> > > ---
>> > >  drivers/gpu/drm/display/drm_dp_mst_topology.c | 4 +++-
>> > >  1 file changed, 3 insertions(+), 1 deletion(-)
>> > >
>> > > diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > b/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > index ed96cfc..d6512c4 100644
>> > > --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > @@ -4154,7 +4154,7 @@ drm_dp_mst_detect_port(struct drm_connector 
>> > > *connector,
>> > >
>> > >     ret = drm_modeset_lock(&mgr->base.lock, ctx);
>> > >     if (ret)
>> > > -           goto out;
>> > > +           goto fail;
>> > >
>> > >     ret = connector_status_disconnected;
>> > >
>> > > @@ -4181,6 +4181,8 @@ drm_dp_mst_detect_port(struct drm_connector *connector,
>> > >             break;
>> > >     }
>> > >  out:
>> > > +   drm_modeset_unlock(&mgr->base.lock);
>>
>> Isn't this supposed to be unlocked only by
>> drm_helper_probe_detect_ctx() / drm_helper_probe_detect() ?
>
> Maybe adding a comment to that effect would be helpful for the future.
>
> Alex
>
>>this is different lock, above function mentioned is locking/unlocking the global connection_mutex, that is required always locked during the atomic >check/commit. Here we are talking about MST topology manager mutex "mgr->base.lock".
>
>>For normal non-bond MST, it's ok, the calling sequence for detecting 
>>MST connector status is  dp_mst_connector_detect 
>>->mst->mst_fw_cbs->detect_port_ctx = dp_mst_detect_port 
>>->drm_dp_mst_detect_port (where mgr->base.lock is locked)
>
>> Then for the bond MST case, to figure out if the sibling connectors 
>> are also connected, so that the bonding is possible, we need detect 
>> two or more connectors >in single dp_mst_connector_detect call
>
>>dp_mst_connector_detect ->mst->mst_fw_cbs->detect_port_ctx = 
>>dp_mst_detect_port ->drm_dp_mst_detect_port (where mgr->base.lock is 
>>locked) dp_mst_find_sibling_connector 
>>->mst->mst_fw_cbs->detect_port_ctx = dp_mst_detect_port 
>>->drm_dp_mst_detect_port (blocked by mgr->base.lock)
>
>>
>> > > +fail:
>> > >     drm_dp_mst_topology_put_port(port);
>> > >     return ret;
>> > >  }
>> >
>> > --
>> > Cheers,
>> >  Lyude Paul (she/her)
>> >  Software Engineer at Red Hat
>> >

-- 
Jani Nikula, Intel