PL310 and QoS logic

[shiraz.linux.kernel@gmail.com (shiraz hashim)]

Hello Catalin,

On Wed, Oct 27, 2010 at 7:03 PM, Catalin Marinas
<catalin.marinas@arm.com> wrote:
> On Wed, 2010-10-27 at 14:09 +0100, shiraz hashim wrote:
>> We were going through ARM PL310 errata list and we came across bug
>> #738415, which says
>> "738415: ? A cacheable read with address bits [20:5] equal to 0x0000
>> can be stalled by non-cacheable read traffic targeting other address
>> region 2"
>>
>> We believe this could be one of the potential problems which we are
>> facing, and the
>> errata says the implication could be that there could be a deadlock if
>> there are dependent
>> reads.
>>
>> Now, how do we ensure that this is the root cause. What is the best
>> way in which I
>> can prevent kernel from using cache-lines at each 2MB boundary.
>> Have you come across this bug? Any information would be really helpful.
>
> I think more information or conditions for this to happen can be given
> by support at arm.com. I don't know the hardware details around this.

OK.

> In general you don't have any cacheable/non-cacheable dependent reads in
> Linux to be able to trigger the deadlock. That's a situation where one
> CPU is polling some non-cacheable memory location and a different CPU
> cannot continue. The only situation I'm aware of is during secondary CPU
> booting or if you boot Linux with CPU with a maxcpus less than the total
> number of CPUs (so some of them keep polling the pen_release even at
> run-time). But that's easily solvable with a WFE in the pen polling loop
> (if your platform has such booting protocol).
>
> On the RealView platforms, the secondary CPUs are waiting in a WFI in
> the boot monitor code and waken up one by one via an IPI, so they don't
> usually wait in the pen polling loop.
>
> If you want to prevent such memory from being used, you can hack the
> memory initialisation code in Linux to reserve a 4K physical page for
> every 2MB.

I am not very sure how to do it. Can I use reserve_bootmem_node from
bootmem_init.

-- 
regards
Shiraz Hashim