There are frequent hangs on Baytrail CPUs according to
https://bugzilla.kernel.org/show_bug.cgi?id=109051.
This patch works around the errata by disabling C6.

According to the discussion in the bug and private mail it might be
useful to include the workaround in mainline.

Signed-off-by: Jochen Hein <redacted>
Cc: stable@vger.kernel.org

--- linux-4.7.5/drivers/idle/intel_idle.c.orig  2016-09-24 10:10:18.000000000 +0200
+++ linux-4.7.5/drivers/idle/intel_idle.c       2016-10-16 07:36:51.142862573 +0200

@@ -1210,6 +1210,34 @@

 }
 /*
+ * byt_idle_state_table_update(void)
+ *
+ * On BYT, we have errata VLP52 and disable C6.
+ * https://bugzilla.kernel.org/show_bug.cgi?id=109051A
+ * http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/pentium-n3520-j2850-celeron-n2920-n2820-n2815-n2806-j1850-j1750-spec-update.pdf
+ * VLP52 EOI Transactions May Not be Sent if Software Enters Core C6 During an Interrupt Service Routine.
+
+Problem:
+If core C6 is entered after the start of an interrupt service routine but before a write
+to the APIC EOI (End of Interrupt) register, and the core is woken up by an event
+other than a fixed interrupt source the core may drop the EOI transaction the next
+time APIC EOI register is written and further interrupts from the same or lower
+priority level will be blocked.
+
+Implication:
+EOI transactions may be lost and interrupts may be blocked when core C6 is used
+during interrupt service routines.
+
+Workaround:
+It is possible for the firmware to contain a workaround for this erratum.
+ */
+static void byt_idle_state_table_update(void)
+{
+       printk(PREFIX "byt_idle_state_table_update reached\n");
+       byt_cstates[1].disabled = 1;    /* C6N-BYT */
+       byt_cstates[2].disabled = 1;    /* C6S-BYT */
+}
+/*
  * sklh_idle_state_table_update(void)
  *
  * On SKL-H (model 0x5e) disable C8 and C9 if:

@@ -1264,6 +1292,10 @@
        case 0x3e: /* IVT */
                ivt_idle_state_table_update();
                break;
+       case 0x37: /* BYT */
+               printk(PREFIX "intel_idle_state_table_update BYT 0x37 reached\n");
+               byt_idle_state_table_update();
+               break;
        case 0x5c: /* BXT */
                bxt_idle_state_table_update();
                break;

--

The only problem with troubleshooting is that the trouble shoots back.

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