[PATCH v3] ARM: l2c: add options to overwrite prefetching behavior

[Hauke Mehrtens]

On 05/29/2015 08:35 PM, Florian Fainelli wrote:
> On 29/05/15 11:30, Florian Fainelli wrote:
>> On 29/05/15 11:11, Hauke Mehrtens wrote:
>>>
>>>
>>> On 05/29/2015 07:52 PM, Florian Fainelli wrote:
>>>> On 15/05/15 14:52, Hauke Mehrtens wrote:
>>>>> These options make it possible to overwrites the data and instruction
>>>>> prefetching behavior of the arm pl310 cache controller.
>>>>>
>>>>> Signed-off-by: Hauke Mehrtens <hauke at hauke-m.de>
>>>>> ---
>>>>> v2: only set prefetch
>>>>> v1: set prefetch and aux
>>>>>
>>>>>  Documentation/devicetree/bindings/arm/l2cc.txt |  4 ++++
>>>>>  arch/arm/mm/cache-l2x0.c                       | 20 ++++++++++++++++++++
>>>>>  2 files changed, 24 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/arm/l2cc.txt b/Documentation/devicetree/bindings/arm/l2cc.txt
>>>>> index 0dbabe9..528821a 100644
>>>>> --- a/Documentation/devicetree/bindings/arm/l2cc.txt
>>>>> +++ b/Documentation/devicetree/bindings/arm/l2cc.txt
>>>>> @@ -67,6 +67,10 @@ Optional properties:
>>>>>    disable if zero.
>>>>>  - arm,prefetch-offset : Override prefetch offset value. Valid values are
>>>>>    0-7, 15, 23, and 31.
>>>>> +- arm,prefetch-data : Enable data prefetch. Enabling prefetching
>>>>> +  can improve performance.
>>>>
>>>> I do not think the "can improve performance" has a place in a binding,
>>>> this is either not technical enough about what this does, or marketing
>>>> enough it does not buy us much.
>>>>
>>>> data/instruction pre-fetching are commonly found on cache controller
>>>> these days, so I would be tempted to remove the "arm," prefixing here
>>>> since this can be generalized to other kinds of cache controllers.
>>>> Documenting that this can be either a boolean, or accept a value (see
>>>> below) could help.
>>>
>>> So you think I should only add prefetch-data and prefetch-instr without
>>> the arm prefix.
>>
>> That's what I think yes, others may disagree.

Are there any other opinions on this issue?

>>
>>>>
>>>>> +- arm,prefetch-instr : Enable instruction prefetch. Enabling prefetching
>>>>> +  can improve performance.
>>>>>  
>>>>>  Example:
>>>>>  
>>>>> diff --git a/arch/arm/mm/cache-l2x0.c b/arch/arm/mm/cache-l2x0.c
>>>>> index e309c8f..1aa970a 100644
>>>>> --- a/arch/arm/mm/cache-l2x0.c
>>>>> +++ b/arch/arm/mm/cache-l2x0.c
>>>>> @@ -1199,6 +1199,26 @@ static void __init l2c310_of_parse(const struct device_node *np,
>>>>>  		pr_err("L2C-310 OF arm,prefetch-offset property value is missing\n");
>>>>>  	}
>>>>>  
>>>>> +	ret = of_property_read_u32(np, "arm,prefetch-data", &val);
>>>>> +	if (ret == 0) {
>>>>> +		if (val)
>>>>> +			prefetch |= L310_PREFETCH_CTRL_DATA_PREFETCH;
>>>>> +		else
>>>>> +			prefetch &= ~L310_PREFETCH_CTRL_DATA_PREFETCH;
>>>>> +	} else if (ret != -EINVAL) {
>>>>> +		pr_err("L2C-310 OF arm,prefetch-data property value is missing\n");
>>>>> +	}
>>>>
>>>> If we want to generalize the use of this property, there could indeed be
>>>> a value associated with it, if the cache controller supports different
>>>> pre-fetching strides, however this is not the cache for these cache
>>>> controllers it seems, are not we going to show error messages more often
>>>> than desired?
>>> I did this so it is possible to deactivate the prefech mode. I do not
>>> know if somebody wants to do that. I haven't understood how you suggest
>>> I should change.
>>> When you do not associate a value with an entry in device tree it is
>>> there or not there, so we could only activate it when it was not
>>> automatically detected, but we could not deactivate it, because the case
>>> when this value is not specified in device tree would be, use to auto
>>> detected the value.
>>
>> My point is that you use of_read_property_u32, but your example does not
>> state what should be the value associated with this property in the
>> binding, so it is unclear what are the results without looking at the
>> code between these examples:
>>
>> /* Enable data pre-fetching */
>> #1 arm,data-prefetch;
>> #2 arm,data-prefetch = <1>;
>>
>> /* Disable data pre-fetching */
>> #3 arm,data-prefetch = <0>;
>>
>> /* do nothing, empty aka retain existing settings set by firmware */
>> #4
>>
>> Based on the code, only #2 and #3 are intended, which raises the
>> question, do not we want of_read_property_bool() instead then? The
>> binding does seem to suggest that only #1 and #4 are valid though.
> 
> I re-read the code, and I think if you just clarify the values in the
> binding to be: 0 (forcibly disable), 1 (forcibly enable), property
> absent (retain settings), this would be crystal clear.

Ok I will do that.

Hauke