Re: [PATCH] drm/msm/dp: add atomic_check to bridge ops

From: Abhinav Kumar
Date: Tue Aug 23 2022 - 18:59:45 EST



On 8/23/2022 3:41 PM, Dmitry Baryshkov wrote:
On Wed, 24 Aug 2022 at 01:07, Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> 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.