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drm/amd/display: Atomize IRQ register read/modify/write ops

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[Why]

The OTG_GLOBAL_SYNC_STATUS register controls various HW IRQ sources for
the output timing generator (OTG). VUPDATE_NO_LOCK is one of them.

To enable the IRQ, driver sets the VUPDATE_NO_LOCK_EN bit in the
GLOBAL_SYNC_STATUS register.

To ack the IRQ after it fires, the driver sets the VUPDATE_NO_LOCK_CLEAR
bit in the same GLOBAL_SYNC_STATUS register.

The bit sets are done through read/modify/write operations, which are
not atomic. Thus, the following race is possible:

    Thread A:                       IRQ handler:
                                    *HW IRQ fires*
    # IRQ disable
    val = read(GLOBAL_SYNC_STATUS)
    unset(val, VUPDATE_NO_LOCK_EN)
    write(val, GLOBAL_SYNC_STATUS)
                                    # ACK reads VUPDATE_NO_LOCK_EN unset
                                    val1 = read(GLOBAL_SYNC_STATUS)
                                    set(val1, VUPDATE_NO_LOCK_CLEAR)
    # IRQ enable
    val = read(GLOBAL_SYNC_STATUS)
    set(val, VUPDATE_NO_LOCK_EN)
    write(val, GLOBAL_SYNC_STATUS)
                                    # BAD! clears VUPDATE_NO_LOCK_EN
                                    write(val1, GLOBAL_SYNC_STATUS)

Regarding the tagged Fixes: change, it appears the change made this race
more likely to occur. Since VUPDATE_NO_LOCK is now the sole IRQ source
for vblank handling, a single race on high refresh panels can lead to a
time out.

[How]

The GLOBAL_SYNC_STATUS register is only one example, other IRQ control
registers also share the same scheme. On top of GLOBAL_SYNC_STATUS,
let's clean up those as well.

To keep things simple, Let's atomize the IRQ rmw ops via a single
driver-wide spinlock. Due to the small scope of this lock, it is
unlikely to cause noticeable overhead on top of all the existing locking
within the IRQ set/handle paths.

Since DM is responsible for locking, wrap dc_interrupt_set/ack with the
spinlock in the new amdgpu_dm_irq_set/ack functions. Migrate/drop all
references in DM to dc_interrupt_set/ack to use amdgpu_dm_irq_set/ack
instead.

Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/5616
Fixes: c87e6635d2 ("drm/amd/display: consolidate DCN vblank/flip handling onto vupdate_no_lock")
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 70de0a0216)
Cc: stable@vger.kernel.org
This commit is contained in:
Leo Li 2026-08-25 23:21:53 -04:00 • committed by Alex Deucher
commit 63e19ef3dd
9 changed files with 117 additions and 55 deletions
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