Re: [RFC 2/8] ARM:global_timer: Add ARM global timer support.

[Stuart MENEFY <stuart.menefy@st.com>]

On 08/05/13 15:49, Will Deacon wrote:
> On Wed, May 08, 2013 at 03:38:22PM +0100, Arnd Bergmann wrote:
>> On Wednesday 08 May 2013, Srinivas KANDAGATLA wrote:
>>> From: Stuart Menefy <stuart.menefy@st.com>
>>>
>>> This is a simple driver for the global timer module found in the Cortex
>>> A9-MP cores from revision r1p0 onwards. This should be able to perform
>>> the functions of the system timer and the local timer in an SMP system.
>>>
>>> The global timer has the following features:
>>>     The global timer is a 64-bit incrementing counter with an
>>> auto-incrementing feature. It continues incrementing after sending
>>> interrupts. The global timer is memory mapped in the private memory
>>> region.
>>>     The global timer is accessible to all Cortex-A9 processors in the
>>> cluster. Each Cortex-A9 processor has a private 64-bit comparator that
>>> is used to assert a private interrupt when the global timer has reached
>>> the comparator value. All the Cortex-A9 processors in a design use the
>>> banked ID, ID27, for this interrupt. ID27 is sent to the Interrupt
>>> Controller as a Private Peripheral Interrupt. The global timer is
>>> clocked by PERIPHCLK.
>>>
>>> Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
>>> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
>>>
>>
>> Are you sure we don't already have a driver for this? It sounds unlikely
>> that you are the first one to do this when the hardware is so common.
> 
> An additional reason why people don't make use of this timer (on top of the
> power domain concerns raised by Rob) is due to the following from the A9
> TRM:
> 
>   `From r2p0 the comparators for each processor with the global timer fire
>    when the timer value is greater than or equal to. In previous revisions
>    the comparators fired when the timer value was equal to.'
> 
> So you'll get into all sorts of horrible races on those older revisions if
> you're not careful.

Fortunately the earliest version of these SoCs uses r2p2, so we avoided that
particular issue. But as the code doesn't attempt to work around these
problems a check on the version number might be a good idea to prevent other
people accidentally hitting it.

Stuart