Cortex-A9 SCU + ARM_ERRATA_764369

[slash.tmp@free.fr (Mason)]

On 03/11/2015 18:27, Mark Rutland wrote:
> On Tue, Nov 03, 2015 at 05:51:40PM +0100, Mason wrote:
>> Hello,
>>
>> I'm using a dual Cortex-A9 MPCore r3p0 setup.
>> Thus, I have selected the following ERRATA work-arounds:
>>
>> 	select ARM_ERRATA_754322
>> 	select ARM_ERRATA_764369 if SMP
>> 	select ARM_ERRATA_775420
>>
>> (I hope I didn't miss any.)
>>
>> config ARM_ERRATA_764369
>> 	bool "ARM errata: Data cache line maintenance operation by MVA may not succeed"
>> 	depends on CPU_V7 && SMP
>> 	help
>> 	  This option enables the workaround for erratum 764369
>> 	  affecting Cortex-A9 MPCore with two or more processors (all
>> 	  current revisions). Under certain timing circumstances, a data
>> 	  cache line maintenance operation by MVA targeting an Inner
>> 	  Shareable memory region may fail to proceed up to either the
>> 	  Point of Coherency or to the Point of Unification of the
>> 	  system. This workaround adds a DSB instruction before the
>> 	  relevant cache maintenance functions and sets a specific bit
>> 	  in the diagnostic control register of the SCU.
>>
>> I'm confused by some code in scu_enable() introduced by commit f630c1bdfbf8f
>>
>> #ifdef CONFIG_ARM_ERRATA_764369
>> 	/* Cortex-A9 only */
>> 	if ((read_cpuid_id() & 0xff0ffff0) == 0x410fc090) {
>> 		scu_ctrl = readl_relaxed(scu_base + 0x30);
>> 		if (!(scu_ctrl & 1))
>> 			writel_relaxed(scu_ctrl | 0x1, scu_base + 0x30);
>> 	}
>> #endif
>>
>> <confused>
>> What is scu_base + 0x30? (SCU diagnostic control register?)
>> I don't see this register/offset documented in
>> Cortex-A9 MPCore r3p0 Technical Reference Manual (2.2 SCU registers)
> 
> It's documented (admittedly very sparsely) in the Software Developers
> Errata Notice for Cortex-A9 processors, in the section regarding erratum
> 764369.

Sigh. ARM documentation is such a treasure hunt experience.

Errata Notices are not freely available, thus probably not
indexed by search engines.

The other day, I couldn't find what I was looking for in the
A9MPC TRM, because it was in the A9 TRM. And I suspect the
architected registers are only documented in the ARM ARM.

Regards.

Related discussion for my own reference:
http://thread.gmane.org/gmane.linux.ports.arm.kernel/313051