Re: [PATCH v4 6/7] ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class cpus

[Vladimir Murzin]

On 24/05/17 09:36, Arnd Bergmann wrote:
> On Wed, May 24, 2017 at 10:31 AM, Vladimir Murzin
> <vladimir.murzin@xxxxxxx> wrote:
>> On 23/05/17 21:33, Arnd Bergmann wrote:
>>> On Tue, May 23, 2017 at 10:01 PM, Russell King - ARM Linux
>>> <linux@xxxxxxxxxxxxxxx> wrote:
>>>> On Mon, Apr 24, 2017 at 11:16:56AM +0100, Vladimir Murzin wrote:
>>>>> Now, we have dedicated non-cacheable region for consistent DMA
>>>>> operations. However, that region can still be marked as bufferable by
>>>>> MPU, so it'd be safer to have barriers by default.
>>>>
>>>> What do you actually want here?  Your patch doesn't quite make sense,
>>>> the commit description seems to indicate that you require this option
>>>> to be set for V7M, but the patch says otherwise.
>>>>
>>>>>  config ARM_DMA_MEM_BUFFERABLE
>>>>> -     bool "Use non-cacheable memory for DMA" if (CPU_V6 || CPU_V6K) && !CPU_V7
>>>>> -     default y if CPU_V6 || CPU_V6K || CPU_V7
>>>>> +     bool "Use non-cacheable memory for DMA" if (CPU_V6 || CPU_V6K || CPU_V7M) && !CPU_V7
>>>>
>>>> This "if" conditional conditionalises the visibility of the option,
>>>> it doesn't conditionalise the value.
>>>>
>>>>> +     default y if CPU_V6 || CPU_V6K || CPU_V7 || CPU_V7M
>>>>
>>>> Taking both of these changes together what you end up with is an option
>>>> presented to the user for "Use non-cacheable memory for DMA" which
>>>> they can choose to disable.
>>>>
>>>> If you require this option to be set, that's incorrect - your modification
>>>> to the default line is correct, but the first line is not.  To achieve
>>>> that, you want the if condition to evaluate false for V7M, thereby hiding
>>>> the option from the user.  In that case, the default value will always be
>>>> assigned to the option.
>>>
>>> I had the opposite comment in the previous version ;-)
>>> https://lkml.org/lkml/2017/4/19/185
>>>
>>> I think the current patch is correct, but the description could still be
>>> clarified: On some of the beefier ARMv7-M machines (with DMA
>>> and write buffers) we want this enabled, while those that didn't
>>> need it until now also won't need it in the future.
>>
>> Ok. Do you want it go into commit message or option description or maybe both?
> 
> I'd say both. It would also be helpful to identify specifically which platforms
> require this, and then add a 'select ARM_DMA_MEM_BUFFERABLE' from
> the platform, as we do from Moxart.
> 

I'm a bit confused here. In case we want to control it on platform level via
'select ARM_DMA_MEM_BUFFERABLE' wouldn't we need to 'default n' for CPU_V7M?
IIUC, Moxart needs to select this option because it is neither CPU_V6(K) or
CPU_V7.

Thanks
Vladimir

>       Arnd
>