linux-kernel - Re: [PATCH v6 10/11] ARM: dts: stm32: add ETZPC as a system bus for STM32MP15x boards

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Date:   Mon, 16 Oct 2023 14:02:39 +0200
From:   Gatien CHEVALLIER <gatien.chevallier@...s.st.com>
To:     Rob Herring <robh@...nel.org>
CC:     <Oleksii_Moisieiev@...m.com>, <gregkh@...uxfoundation.org>,
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Subject: Re: [PATCH v6 10/11] ARM: dts: stm32: add ETZPC as a system bus for
 STM32MP15x boards

Hi Rob,

On 10/12/23 17:30, Rob Herring wrote:
> On Wed, Oct 11, 2023 at 10:49:58AM +0200, Gatien CHEVALLIER wrote:
>> Hi Rob,
>>
>> On 10/10/23 20:42, Rob Herring wrote:
>>> On Tue, Oct 10, 2023 at 02:57:18PM +0200, Gatien Chevallier wrote:
>>>> ETZPC is a firewall controller. Put all peripherals filtered by the
>>>> ETZPC as ETZPC subnodes and reference ETZPC as an
>>>> access-control-provider.
>>>>
>>>> For more information on which peripheral is securable or supports MCU
>>>> isolation, please read the STM32MP15 reference manual.
>>>>
>>>> Signed-off-by: Gatien Chevallier <gatien.chevallier@...s.st.com>
>>>> ---
>>>>
>>>> Changes in V6:
>>>>       	- Renamed access-controller to access-controllers
>>>>       	- Removal of access-control-provider property
>>>>
>>>> Changes in V5:
>>>>       	- Renamed feature-domain* to access-control*
>>>>
>>>>    arch/arm/boot/dts/st/stm32mp151.dtsi  | 2756 +++++++++++++------------
>>>>    arch/arm/boot/dts/st/stm32mp153.dtsi  |   52 +-
>>>>    arch/arm/boot/dts/st/stm32mp15xc.dtsi |   19 +-
>>>>    3 files changed, 1450 insertions(+), 1377 deletions(-)
>>>
>>> This is not reviewable. Change the indentation and any non-functional
>>> change in one patch and then actual changes in another.
>>
>> Ok, I'll make it easier to read.
>>
>>>
>>> This is also an ABI break. Though I'm not sure it's avoidable. All the
>>> devices below the ETZPC node won't probe on existing kernel. A
>>> simple-bus fallback for ETZPC node should solve that.
>>>
>>
>> I had one issue when trying with a simple-bus fallback that was the
>> drivers were probing even though the access rights aren't correct.
>> Hence the removal of the simple-bus compatible in the STM32MP25 patch.
> 
> But it worked before, right? So the difference is you have either added
> new devices which need setup or your firmware changed how devices are
> setup (or not setup). Certainly can't fix the latter case. You just need
> to be explicit about what you are doing to users.
> 

I should've specified it was during a test where I deliberately set
incorrect rights on a peripheral and enabled its node to see if the
firewall would allow the creation of the device.

> 
>> Even though a node is tagged with the OF_POPULATED flag when checking
>> the access rights with the firewall controller, it seems that when
>> simple-bus is probing, there's no check of this flag.
> 
> It shouldn't. Those flags are for creating the devices (or not) and
> removing only devices of_platform_populate() created.
> 

About the "simple-bus" being a fallback, I think I understood why I saw
that the devices were created.

All devices under a node whose compatible is "simple-bus" are created
in of_platform_device_create_pdata(), called by
of_platform_default_populate_init() at arch_initcall level. This
before the firewall-controller has a chance to populate it's bus.

Therefore, when I flag nodes when populating the firewall-bus, the
devices are already created. The "simple-bus" mechanism is not a
fallback here as it precedes the driver probe.

Is there a safe way to safely remove/disable a device created this way?
Devices that are under the firewall controller (simple-bus) node
should not be probed before it as they're child of it.

Best regards,
Gatien

>> of_platform_populate() checks and sets the OF_POPULATED_BUS flag.
>> Maybe that is my error and the firewall bus populate should set
>> OF_POPULATED_BUS instead of OF_POPULATED. Is that correct?
> 
> Shrug. Off hand, I'd say probably not, but am not certain.
> 
> Rob