Re: Patches for arch/arm/mm/cache-l2x0.c

[Mason <slash.tmp@free.fr>]

On 01/12/2015 13:59, Russell King - ARM Linux wrote:
> On Tue, Dec 01, 2015 at 09:56:23AM +0100, Mason wrote:
>> [ Back-porting v4.2 patches to v4.1 LTS ]
>>
>> On 26/11/2015 09:54, Mason wrote:
>>
>>> Over the past few months, I wrote board-support code based on kernel v4.2
>>> and things worked as expected.
>>>
>>> Then I had to rebase to a LTS kernel (I used v4.1.13) and something broke
>>> in the L2 cache setup. Apparently, Russell King did a lot of fixing and
>>> cleaning up between v4.1 and v4.2
>>>
>>> I cherry-picked the following 5 patches, and my 4.1 kernel works again:
>>>
>>> 346248a2d1e3a815297125c1347d90dafcc51990 ARM: l2c: avoid passing auxiliary control register through enable method
>>>  arch/arm/mm/cache-l2x0.c | 32 +++++++++++++++++---------------
>>>  1 file changed, 17 insertions(+), 15 deletions(-)
>>> dc63c0733050996143a82f2b095fc378a04274f0 ARM: l2c: only unlock caches if NS_LOCKDOWN bit is set
>>>  arch/arm/mm/cache-l2x0.c | 26 +++++++++++++++++++++++++-
>>>  1 file changed, 25 insertions(+), 1 deletion(-)
>>> 7787be2a74dc618bf32348a0f588eebf7ebe0a06 ARM: l2c: clean up l2c_configure()
>>>  arch/arm/mm/cache-l2x0.c | 23 ++++++++++++++---------
>>>  1 file changed, 14 insertions(+), 9 deletions(-)
>>> 8f80afa16002e9b4784dc1d51c48f95f52838cfb ARM: l2c: write auxiliary control register first
>>>  arch/arm/mm/cache-l2x0.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>> 9749167eec6a057122b7a1ab2193abd079645aba ARM: l2c: restore the behaviour documented above l2c_enable()
>>>  arch/arm/mm/cache-l2x0.c | 10 +++++-----
>>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>>
>>>
>>> The problem is that my system runs Linux in NS (non-secure) mode, and there
>>> are a bunch of instructions that Linux can't use in that mode.
>>>
>>> I suspect the crash comes from trying to unconditionally unlock the caches,
>>> thus patch dc63c0733050.
>>>
>>> But 9749167eec6a and 8f80afa16002 also look like good candidates for
>>> back-porting to v4.1
>>>
>>> What do you think?
>>
>> Any opinion on this topic?
> 
> You can find the answer to this question yourself if you read
> Documentation/stable_kernel_rules.txt, which gives the rules for what's
> suitable to be backported to stable trees.  Let's go through them one
> by one:

I get the feeling you've also brought tar and feathers :-)

>  - It must be obviously correct and tested.
> Tested yes. Obviously correct - maybe (it's been some time since I wrote
> the patches.)
> 
>  - It cannot be bigger than 100 lines, with context.
> Doubtful, though for individual patches this may be true.

FWIW, I provided the diffstat in my original message. (32, 26, 23, 4, 10)

>  - It must fix only one thing.
> True of each individual patch.
> 
>  - It must fix a real bug that bothers people (not a, "This could be a
>    problem..." type thing).
> Maybe for your situation, but you're an out of tree user, and the
> problem doesn't exist with mainline kernels.

<confused>
Problem would hit anyone running Linux in non-secure mode. The kernel
will crash when it executes l2c_unlock() which unconditionally writes
to a secure register.

>  - It must fix a problem that causes a build error (but not for things
>    marked CONFIG_BROKEN), an oops, a hang, data corruption, a real
>    security issue, or some "oh, that's not good" issue.  In short, something
>    critical.
> Maybe causes an oops for you, but not for any already merged platform,
> so it isn't a problem that is seen with mainline kernels.

<confused>
Why would this problem not affect e.g. arch/arm/mach-highbank ?
(CCing Rob)

(BTW, thanks for the wait_event_interruptible_timeout code outline.)

Regards.