l2c: Kernel panic in l2c310_enable() in non-secure mode

Marc Gonzalez marc_gonzalez at sigmadesigns.com
Wed Oct 14 07:47:59 PDT 2015

On 14/10/2015 16:17, Marc Gonzalez wrote:

> Internal error: Oops - undefined instruction: 0 [#1] PREEMPT SMP ARM
> On my platform, Linux v4.2 running in non-secure mode panics in
> l2c310_enable() with the pc pointing at this instruction:
> c03390cc: ee013f30 mcr 15, 0, r3, cr1, cr0, {1}
> which corresponds to set_auxcr IIUC.
> static inline void set_auxcr(unsigned int val)
> {
>   asm volatile("mcr p15, 0, %0, c1, c0, 1  @ set AUXCR" : : "r" (val));
>   isb();
> }
> which comes from this part in l2c310_enable:
> 	if (aux & L310_AUX_CTRL_FULL_LINE_ZERO) {
> 		set_auxcr(get_auxcr() | BIT(3) | BIT(2) | BIT(1));
> 		cpu_notifier(l2c310_cpu_enable_flz, 0);
> 	}

Just to be sure, I changed set_auxcr() to a NOP. The kernel does not
panic with that setup, and I can see the boot messages:

[    0.000000] l2x0_of_init: FOO
[    0.000000] L2C-310 enabling early BRESP for Cortex-A9
[    0.000000] L2C-310: enabling full line of zeros but not enabled in Cortex-A9
[    0.000000] reg=0x104 val=0x66460801
[    0.000000] reg=0x100 val=0x1
[    0.000000] L2C-310 I prefetch enabled, offset 1 lines
[    0.000000] L2C-310 dynamic clock gating enabled, standby mode enabled
[    0.000000] L2C-310 cache controller enabled, 8 ways, 512 kB
[    0.000000] L2C-310: CACHE_ID 0x410000c8, AUX_CTRL 0x62460801

The "reg=0x%x val=0x%lx\n" lines are from tango_l2c_write_sec()

It seems the firmware forgot to enable FLOZ.

> The L2C-310 documentation states:
>> When the L2C-310 AXI slave ports receive a write transaction with
>> AWUSERSx[10], it indicates that the write actually targets a whole
>> cache line and that all data of this cache line must be reset to
>> zero. The Cortex-A9 processor is likely to use this feature when a
>> CPU is executing a memset routine to initialise a particular memory
>> area. When the L2C-310 receives such a write transaction it ignores
>> the AXI attributes attached to the transaction, size, length, data,
>> and strobes for example, because the whole cache line must be reset.
>> This behavior is not compatible with the AXI protocol, it is disabled
>> by default. You can enable it by setting the Full Line of Zero Enable
>> bit of the Auxiliary Control Register, bit[0]. This behavior also
>> relies on an enable bit in the Cortex-A9 processor. You must take
>> care if you enable this feature because correct behavior relies on
>> consistent enabling in both the Cortex-A9 processor and the L2C-310.
> According to the Cortex A9 documentation,
> ACTLR is RO in non-secure mode if NSACR[18]=0 and RW if NSACR[18]=1
> I suppose writing to a RO register cause the exception I see?
> Commit 8abd259f657d5 ("l2c: provide generic hook to intercept
> writes to secure registers") introduced a mechanism for non-secure
> platforms to define how to write to the L2CC AUXCTRL register.
>>    When Linux is running in the non-secure world, any write to a secure
>>    L2C register will generate an abort.  Platforms normally have to call
>>    firmware to work around this.  Provide a hook for them to intercept
>>    any L2C secure register write.
> Is there a similar mechanism for asking the firmware to write
> to the CP15 ACTRL?
> I suppose a work-around might be to set NSACR[18]?


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