[RFC] ARM: Make CPU_DCACHE_DISABLE depend on !SMP

[Gregory Fong]

On Thu, Jan 8, 2015 at 1:54 PM, Gregory Fong <gregory.0xf0 at gmail.com> wrote:
> On Thu, Jan 8, 2015 at 7:53 AM, Florian Fainelli <f.fainelli at gmail.com> wrote:
>> Le 08/01/2015 03:52, Russell King - ARM Linux a écrit :
>>> On Thu, Jan 08, 2015 at 09:33:05AM +0100, Arnd Bergmann wrote:
>>>> On Wednesday 07 January 2015 18:11:28 Florian Fainelli wrote:
>>>>> Enabling CPU_DCACHE_DISABLE on a SMP capable system will prevent the
>>>>> kernel from booting because of the following ldrex instruction in
>>>>> arch_spin_lock:
>>>>>
>>>>> (gdb) x/10i $pc
>>>>> => 0xc053cfa8 <_raw_spin_lock+4>:       ldrex   r3, [r0]
>>>>>    0xc053cfac <_raw_spin_lock+8>:       add     r2, r3, #65536  ; 0x10000
>>>>>
>>>>> which is taken by the very first printk call:
>>>>>
>>>>>     at /home/fainelli/work/linux/arch/arm/include/asm/spinlock.h:65
>>>>>     fmt=0xc0637650 "\001\066Booting Linux on physical CPU 0x%x\n", args=<incomplete type>)
>>>>>     at kernel/printk/printk.c:1525
>>>>>     fmt=0xc05370f4 <printk+52> "\024\320\215\342\004\340\235\344\020\320\215\342\036\377/\341\017") at kernel/printk/printk.c:1688
>>>>>
>>>>> ldrex requires exclusive monitor(s) (local or global) which are no longer
>>>>> working when the Data cache is disabled in CP15 and will just hang the CPU
>>>>> there.
>>>>>
>>>>> Signed-off-by: Florian Fainelli <f.fainelli at gmail.com>
>>>>
>>>> Acked-by: Arnd Bergmann <arnd at arndb.de>
>>>>
>>>> We stumbled on this a while back when Kaixu was trying to fix allmodconfig
>>>> builds to run on real hardware, but never submitted it in the end when
>>>> Russell didn't like some of the other parts required for that to work.
>>>>
>>>> This one clearly makes sense independently.
>>>
>>> What about platforms where exclusives to strongly ordered memory do work?
>>
>> Do we have a comprehensive list of these platforms?
>
> I haven't had a chance to go through all of the TRMs, but section
> A3.4.5 of the ARMv7-A ARM says:
>
> "It is IMPLEMENTATION DEFINED whether LDREX and STREX operations can
> be performed to a memory region
> with the Device or Strongly-ordered memory attribute. Unless the
> implementation documentation explicitly
> states that LDREX and STREX operations to a memory region with the
> Device or Strongly-ordered attribute are
> permitted, the effect of such operations is UNPREDICTABLE ."
>
> From 6.4.5 "Synchronization primitives" in the Cortex-A15 TRM:
>
> "Use of synchronization primitives on addresses in regions marked as
> Strongly-ordered or
> Device is UNPREDICTABLE in the ARMv7-A Architecture. Code that makes
> such accesses
> is not portable."
>
> It looks like this is not generally permissible for ARMv7, maybe
> someone else can check the other versions.  Perhaps it would make
> sense to disallow disabling the D-cache for !SMP in general and
> special-case those architectures which do support exclusives to
> strongly ordered memory.

Er, I meant "disallow disabling the D-cache for SMP", sorry.  Too many
negatives.