Date:   Tue, 23 Aug 2022 15:59:25 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
CC:     Kuogee Hsieh <quic_khsieh@...cinc.com>, <robdclark@...il.com>,
        <sean@...rly.run>, <swboyd@...omium.org>, <dianders@...omium.org>,
        <vkoul@...nel.org>, <daniel@...ll.ch>, <airlied@...ux.ie>,
        <agross@...nel.org>, <bjorn.andersson@...aro.org>,
        <quic_sbillaka@...cinc.com>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <quic_aravindh@...cinc.com>, <freedreno@...ts.freedesktop.org>
Subject: Re: [PATCH] drm/msm/dp: add atomic_check to bridge ops



On 8/23/2022 3:41 PM, Dmitry Baryshkov wrote:
> On Wed, 24 Aug 2022 at 01:07, Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
>> On 8/22/2022 11:33 AM, Dmitry Baryshkov wrote:
>>> On 22/08/2022 20:32, Abhinav Kumar wrote:
>>>>
>>>>
>>>> On 8/22/2022 9:49 AM, Dmitry Baryshkov wrote:
>>>>> On 22/08/2022 19:38, Abhinav Kumar wrote:
>>>>>> Hi Dmitry
>>>>>>
>>>>>> On 8/22/2022 9:18 AM, Dmitry Baryshkov wrote:
>>>>>>> On 17/08/2022 21:01, Kuogee Hsieh wrote:
>>>>>>>> DRM commit_tails() will disable downstream crtc/encoder/bridge if
>>>>>>>> both disable crtc is required and crtc->active is set before pushing
>>>>>>>> a new frame downstream.
>>>>>>>>
>>>>>>>> There is a rare case that user space display manager issue an extra
>>>>>>>> screen update immediately followed by close DRM device while down
>>>>>>>> stream display interface is disabled. This extra screen update will
>>>>>>>> timeout due to the downstream interface is disabled but will cause
>>>>>>>> crtc->active be set. Hence the followed commit_tails() called by
>>>>>>>> drm_release() will pass the disable downstream crtc/encoder/bridge
>>>>>>>> conditions checking even downstream interface is disabled.
>>>>>>>> This cause the crash to happen at dp_bridge_disable() due to it
>>>>>>>> trying
>>>>>>>> to access the main link register to push the idle pattern out
>>>>>>>> while main
>>>>>>>> link clocks is disabled.
>>>>>>>>
>>>>>>>> This patch adds atomic_check to prevent the extra frame will not
>>>>>>>> be pushed down if display interface is down so that crtc->active
>>>>>>>> will not be set neither. This will fail the conditions checking
>>>>>>>> of disabling down stream crtc/encoder/bridge which prevent
>>>>>>>> drm_release() from calling dp_bridge_disable() so that crash
>>>>>>>> at dp_bridge_disable() prevented.
>>>>>>>
>>>>>>> I must admit I had troubles parsing this description. However if I
>>>>>>> got you right, I think the check that the main link clock is
>>>>>>> running in the dp_bridge_disable() or dp_ctrl_push_idle() would be
>>>>>>> a better fix.
>>>>>>
>>>>>> Originally, thats what was posted
>>>>>> https://patchwork.freedesktop.org/patch/496984/.
>>>>>
>>>>> This patch is also not so correct from my POV. It checks for the hpd
>>>>> status, while in reality it should check for main link clocks being
>>>>> enabled.
>>>>>
>>>>
>>>> We can push another fix to check for the clk state instead of the hpd
>>>> status. But I must say we are again just masking something which the
>>>> fwk should have avoided isnt it?
>>>>
>>>> As per the doc in the include/drm/drm_bridge.h it says,
>>>>
>>>> "*
>>>>    * The bridge can assume that the display pipe (i.e. clocks and timing
>>>>    * signals) feeding it is still running when this callback is called.
>>>>    *"
>>>
>>> Yes, that's what I meant about this chunk begging to go to the core. In
>>> my opinion, if we are talking about the disconnected sinks, it is the
>>> framework who should disallow submitting the frames to the disconnected
>>> sinks.
>>>
>>>>
>>>> By adding an extra layers of protection in the driver, we are just
>>>> avoiding another issue but the commit should not have been issued in
>>>> the first place.
>>>>
>>>> So shouldnt we do both then? That is add protection to check if clock
>>>> is ON and also, reject commits when display is disconnected.
>>>>
>>>>>>
>>>>>> Then it seemed like we were just protecting against an issue in the
>>>>>> framework which was allowing the frames to be pushed even after the
>>>>>> display was disconnected. The DP driver did send out the disconnect
>>>>>> event correctly and as per the logs, this frame came down after that
>>>>>> and the DRM fwk did allow it.
>>>>>>
>>>>>> So after discussing on IRC with Rob, we came up with this approach that
>>>>>> if the display is not connected, then atomic_check should fail. That
>>>>>> way the commit will not happen.
>>>>>>
>>>>>> Just seemed a bit cleaner instead of adding all our protections.
>>>>>
>>>>> The check to fail atomic_check if display is not connected seems out
>>>>> of place. In its current way it begs go to the upper layer,
>>>>> forbidding using disconnected sinks for all the drivers. There is
>>>>> nothing special in the MSM DP driver with respect to the HPD events
>>>>> processing and failing atomic_check() based on that.
>>>>>
>>>>
>>>> Why all the drivers? This is only for MSM DP bridge.
>>>
>>> Yes, we change the MSM DRM driver. But the check is generic enough. I'm
>>> not actually insisting on pushing the check to the core, just trying to
>>> understand the real cause here.
>>>
>>>>
>>
>> I actually wanted to push this to the core and thats what I had
>> originally asked on IRC because it does seem to be generic enough that
>> it should belong to the core but after discussion with Rob on freedreno,
>> he felt this was a better approach because for some of the legacy
>> connectors like VGA, this need not belong to the DRM core, hence we went
>> with this approach.
> 
> It might be better to whitelist such connectors (S-VIDEO/composite
> comes to my mind rather than VGA).

I am fine with that approach, if Rob is onboard with that.

> 
>>>>>>>> SError Interrupt on CPU7, code 0x00000000be000411 -- SError
>>>>>>>> CPU: 7 PID: 3878 Comm: Xorg Not tainted 5.19.0-stb-cbq #19
>>>>>>>> Hardware name: Google Lazor (rev3 - 8) (DT)
>>>>>>>> pstate: a04000c9 (NzCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>>>>>>> pc : __cmpxchg_case_acq_32+0x14/0x2c
>>>>>>>> lr : do_raw_spin_lock+0xa4/0xdc
>>>>>>>> sp : ffffffc01092b6a0
>>>>>>>> x29: ffffffc01092b6a0 x28: 0000000000000028 x27: 0000000000000038
>>>>>>>> x26: 0000000000000004 x25: ffffffd2973dce48 x24: 0000000000000000
>>>>>>>> x23: 00000000ffffffff x22: 00000000ffffffff x21: ffffffd2978d0008
>>>>>>>> x20: ffffffd2978d0008 x19: ffffff80ff759fc0 x18: 0000000000000000
>>>>>>>> x17: 004800a501260460 x16: 0441043b04600438 x15: 04380000089807d0
>>>>>>>> x14: 07b0089807800780 x13: 0000000000000000 x12: 0000000000000000
>>>>>>>> x11: 0000000000000438 x10: 00000000000007d0 x9 : ffffffd2973e09e4
>>>>>>>> x8 : ffffff8092d53300 x7 : ffffff808902e8b8 x6 : 0000000000000001
>>>>>>>> x5 : ffffff808902e880 x4 : 0000000000000000 x3 : ffffff80ff759fc0
>>>>>>>> x2 : 0000000000000001 x1 : 0000000000000000 x0 : ffffff80ff759fc0
>>>>>>>> Kernel panic - not syncing: Asynchronous SError Interrupt
>>>>>>>> CPU: 7 PID: 3878 Comm: Xorg Not tainted 5.19.0-stb-cbq #19
>>>>>>>> Hardware name: Google Lazor (rev3 - 8) (DT)
>>>>>>>> Call trace:
>>>>>>>>    dump_backtrace.part.0+0xbc/0xe4
>>>>>>>>    show_stack+0x24/0x70
>>>>>>>>    dump_stack_lvl+0x68/0x84
>>>>>>>>    dump_stack+0x18/0x34
>>>>>>>>    panic+0x14c/0x32c
>>>>>>>>    nmi_panic+0x58/0x7c
>>>>>>>>    arm64_serror_panic+0x78/0x84
>>>>>>>>    do_serror+0x40/0x64
>>>>>>>>    el1h_64_error_handler+0x30/0x48
>>>>>>>>    el1h_64_error+0x68/0x6c
>>>>>>>>    __cmpxchg_case_acq_32+0x14/0x2c
>>>>>>>>    _raw_spin_lock_irqsave+0x38/0x4c
>>>
>>> You know, after re-reading the trace, I could not help but notice that
>>> the issue seems to be related to completion/timer/spinlock memory
>>> becoming unavailable rather than disabling the main link clock.
>>> See, the SError comes in the spin_lock path, not during register read.
>>>
>>> Thus I think the commit message is a bit misleading.
>>>
>>
>> No, this issue is due to unclocked access. Please check this part of the
>> stack:
> 
> Well, if it were for the unlocked access, we would see SError on the
> register access, wouldn't we? However in this case the SError comes
> from the raw spinlock code.

This is not uncommon. With unclocked access, we have seen in the past 
that sometimes the stack is off by one line. The fact that this issue 
got resolved even with the older version of the patch 
https://patchwork.freedesktop.org/patch/496984/ is pointing towards an 
unclocked access and not the dp/dp->ctrl memory pointers.

> 
>>   >>>>>>   wait_for_completion_timeout+0x2c/0x54
>>   >>>>>>   dp_ctrl_push_idle+0x40/0x88
>>   >>>>>>   dp_bridge_disable+0x24/0x30
>>   >>>>>>   drm_atomic_bridge_chain_disable+0x90/0xbc
>>   >>>>>>   drm_atomic_helper_commit_modeset_disables+0x198/0x444
>>   >>>>>>   msm_atomic_commit_tail+0x1d0/0x374
>>   >>>>>>   commit_tail+0x80/0x108
>>   >>>>>>   drm_atomic_helper_commit+0x118/0x11c
>>   >>>>>>   drm_atomic_commit+0xb4/0xe0
>>   >>>>>>   drm_client_modeset_commit_atomic+0x184/0x224
>>   >>>>>>   drm_client_modeset_commit_locked+0x58/0x160
>>   >>>>>>   drm_client_modeset_commit+0x3c/0x64
>>
>>> Can we please get a trace checking which calls were actually made for
>>> the dp bridge and if the dp/dp->ctrl memory pointers are correct?
>>>
>>> I do not see the dp_display_disable() being called. Maybe I just missed
>>> the call.
>>>
>>
>> Yes it is called, please refer to the above part of the stack that I
>> have pasted.
> 
> The stacktrace mentions dp_bridge_disable(), not dp_display_disable()
> (which I asked for).
> 

So whats happening here is the crash is happening in dp_bridge_disable().

dp_display_disable() is called from post_disable() thats why it doesnt 
show up in the stack.

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