[Bug 92740] Unclaimed register detected before writing to register 0x20a8

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92740

--- Comment #1 from andrew at lunn.ch ---
I tested again, this time with linux-next, 4.5.0-rc4-next-20160216:

[ 2557.065919] ------------[ cut here ]------------
[ 2557.065950] WARNING: CPU: 2 PID: 0 at
drivers/gpu/drm/i915/intel_uncore.c:649 __unclaimed_reg_debug+0x77/0x80
[i915]()
[ 2557.065952] Unclaimed register detected before reading register 0x23a0
[ 2557.065953] Modules linked in: intel_rapl x86_pkg_temp_thermal
intel_powerclamp coretemp kvm_intel kvm irqbypass crct10dif_pclmul crc32_pclmul
ghash_clmulni_intel sha256_ssse3 sha256_generic joydev hmac drbg ansi_cprng
aesni_intel arc4 aes_x86_64 iwlmvm ir_lirc_codec lrw btusb btrtl btbcm btintel
iTCO_wdt bluetooth iTCO_vendor_support mac80211 lirc_dev gf128mul
snd_hda_codec_realtek snd_hda_codec_hdmi glue_helper psmouse serio_raw
snd_hda_codec_generic ablk_helper pcspkr evdev rc_rc6_mce sg iwlwifi cfg80211
rfkill snd_hda_intel snd_hda_codec cryptd nuvoton_cir snd_hda_core rc_core i915
snd_hwdep battery snd_pcm snd_soc_sst_acpi dw_dmac snd_soc_sst_match
dw_dmac_core snd_timer shpchp i2c_designware_platform i2c_designware_core
lpc_ich mfd_core video drm_kms_helper drm i2c_i801 i2c_algo_bit snd button
[ 2557.065989]  mei_me mei soundcore tpm_tis acpi_pad tpm tun autofs4 ext4
crc16 mbcache jbd2 hid_logitech_hidpp hid_logitech_dj usbhid sd_mod
crc32c_intel ahci libahci libata xhci_pci ehci_pci ehci_hcd xhci_hcd scsi_mod
e1000e usbcore ptp pps_core usb_common fan thermal sdhci_acpi sdhci mmc_core
i2c_hid hid
[ 2557.066009] CPU: 2 PID: 0 Comm: swapper/2 Not tainted
4.5.0-rc4-next-20160216 #5
[ 2557.066010] Hardware name:                  /NUC5i3RYB, BIOS
RYBDWi35.86A.0350.2015.0812.1722 08/12/2015
[ 2557.066012]  0000000000000000 ffffffff812dda15 ffff880256d03d60
ffffffffa052e858
[ 2557.066014]  ffffffff810723ed 00000000000023a0 ffff880256d03db0
0000000000000001
[ 2557.066016]  ffff88024c0120f8 0000000000000001 ffffffff8107246c
ffffffffa052e8f0
[ 2557.066019] Call Trace:
[ 2557.066020]  <IRQ>  [<ffffffff812dda15>] ? dump_stack+0x5c/0x77
[ 2557.066028]  [<ffffffff810723ed>] ? warn_slowpath_common+0x7d/0xb0
[ 2557.066030]  [<ffffffff8107246c>] ? warn_slowpath_fmt+0x4c/0x50
[ 2557.066049]  [<ffffffffa04a3867>] ? __unclaimed_reg_debug+0x77/0x80 [i915]
[ 2557.066066]  [<ffffffffa04a65c2>] ? gen6_read32+0x1b2/0x1f0 [i915]
[ 2557.066086]  [<ffffffffa04998ea>] ? intel_lrc_irq_handler+0x3a/0x230 [i915]
[ 2557.066100]  [<ffffffffa0457936>] ? gen8_gt_irq_handler+0x226/0x240 [i915]
[ 2557.066115]  [<ffffffffa04579b6>] ? gen8_irq_handler+0x66/0x690 [i915]
[ 2557.066119]  [<ffffffff810c2128>] ? handle_irq_event_percpu+0x78/0x180
[ 2557.066122]  [<ffffffff810c2269>] ? handle_irq_event+0x39/0x60
[ 2557.066124]  [<ffffffff810c53bb>] ? handle_edge_irq+0x7b/0x140
[ 2557.066127]  [<ffffffff8101cb49>] ? handle_irq+0x19/0x30
[ 2557.066130]  [<ffffffff81582746>] ? do_IRQ+0x46/0xd0
[ 2557.066132]  [<ffffffff81580882>] ? common_interrupt+0x82/0x82
[ 2557.066133]  <EOI>  [<ffffffff814535fe>] ? cpuidle_enter_state+0x13e/0x280
[ 2557.066139]  [<ffffffff810aeb75>] ? cpu_startup_entry+0x245/0x300
[ 2557.066142]  [<ffffffff8104992a>] ? start_secondary+0x14a/0x170
[ 2557.066143] ---[ end trace d496279b545ad521 ]---

I took a closer look at the code:

For intel_lrc_irq_handler+0x3a/0x230 [i915] we have:

0000000000000d90 <intel_lrc_irq_handler>:
 *
 * Check the unread Context Status Buffers and manage the submission of new
 * contexts to the ELSP accordingly.
 */
void intel_lrc_irq_handler(struct intel_engine_cs *ring)
{
     d90:       e8 00 00 00 00          callq  d95 <intel_lrc_irq_handler+0x5>
                        d91: R_X86_64_PC32      __fentry__-0x4
     d95:       41 57                   push   %r15
     d97:       41 56                   push   %r14
        u8 write_pointer;
        u32 status = 0;
        u32 status_id;
        u32 submit_contexts = 0;
...
...
     dbf:       48 89 44 24 08          mov    %rax,0x8(%rsp)
        u8 write_pointer;
        u32 status = 0;
        u32 status_id;
        u32 submit_contexts = 0;

        status_pointer = I915_READ(RING_CONTEXT_STATUS_PTR(ring));
     dc4:       ff 90 a8 00 00 00       callq  *0xa8(%rax)

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