Message-ID: <929ce1f6-818f-4bb9-abf9-24d511bd2bf3@foss.st.com>
Date: Fri, 10 Oct 2025 12:04:40 +0200
From: Arnaud POULIQUEN <arnaud.pouliquen@...s.st.com>
To: Sumit Garg <sumit.garg@...nel.org>
CC: <devicetree@...r.kernel.org>, Conor Dooley <conor+dt@...nel.org>,
        "Mathieu
 Poirier" <mathieu.poirier@...aro.org>,
        <op-tee@...ts.trustedfirmware.org>,
        Bjorn Andersson <andersson@...nel.org>,
        <linux-remoteproc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Linux-stm32] [PATCH v19 4/6] dt-bindings: remoteproc: Add
 compatibility for TEE support

Hi Sumit,

On 10/10/25 07:44, Sumit Garg wrote:
> Hi Arnaud,
> 
> It's been some holidays followed by business travel leading to delay in
> my response here.
> 
> On Tue, Oct 07, 2025 at 03:50:32PM +0200, Arnaud POULIQUEN wrote:
>> Hello Bjorn, Mathieu, Sumit,
>>
>> On 9/22/25 10:57, Arnaud POULIQUEN wrote:
>>>
>>>
>>> On 9/19/25 08:46, Sumit Garg wrote:
>>>> On Wed, Sep 17, 2025 at 03:47:40PM +0200, Arnaud POULIQUEN wrote:
>>>>>
>>>>> On 9/17/25 12:08, Sumit Garg wrote:
>>>>>> On Tue, Sep 16, 2025 at 03:26:47PM +0200, Arnaud POULIQUEN wrote:
>>>>>>> Hello Sumit,
>>>>>>>
>>>>>>> On 9/16/25 11:14, Sumit Garg wrote:
>>>>>>>> Hi Arnaud,
>>>>>>>>
>>>>>>>> First of all apologies for such a late review comment as previously I
>>>>>>>> wasn't CCed or involved in the review of this
>>>>>>>> patch-set. In case any of
>>>>>>>> my following comments have been discussed in the
>>>>>>>> past then feel free to
>>>>>>>> point me at relevant discussions.
>>>>>>> No worries, there are too many versions of this series
>>>>>>> to follow all the
>>>>>>> past discussions. I sometimes have difficulty remembering all the
>>>>>>> discussions myself :)
>>>>>>>
>>>>>>>> On Wed, Jun 25, 2025 at 11:40:26AM +0200, Arnaud Pouliquen wrote:
>>>>>>>>> The "st,stm32mp1-m4-tee" compatible is utilized
>>>>>>>>> in a system configuration
>>>>>>>>> where the Cortex-M4 firmware is loaded by the
>>>>>>>>> Trusted Execution Environment
>>>>>>>>> (TEE).
>>>>>>>> Having a DT based compatible for a TEE service to me
>>>>>>>> just feels like it
>>>>>>>> is redundant here. I can see you have also used a
>>>>>>>> TEE bus based device
>>>>>>>> too but that is not being properly used. I know subsystems like
>>>>>>>> remoteproc, SCMI and others heavily rely on DT to
>>>>>>>> hardcode properties of
>>>>>>>> system firmware which are rather better to be discovered dynamically.
>>>>>>>>
>>>>>>>> So I have an open question for you and the remoteproc subsystem
>>>>>>>> maintainers being:
>>>>>>>>
>>>>>>>> Is it feasible to rather leverage the benefits of a
>>>>>>>> fully discoverable
>>>>>>>> TEE bus rather than relying on platform bus/ DT to hardcode firmware
>>>>>>>> properties?
>>>>>>> The discoverable TEE bus does not works if the remoteproc is probed
>>>>>>> before the OP-TEE bus, in such case  no possibility to know if the TEE
>>>>>>> TA is not yet available or not available at all.
>>>>>>> This point is mentioned in a comment in rproc_tee_register().
>>>>> For the discussion, it’s probably better if I provide more
>>>>> details on the
>>>>> current OP-TEE implementation and the stm32mp processors.
>>>>>
>>>>> 1) STM32MP topology:
>>>>>     - STM32MP1: only a Cortex-M4 remote processor
>>>>>     -  STM32MP2x: a Cortex-M33 and a Cortex-M0 remote processors
>>>>>     At this stage, only the STM32MP15 is upstreamed; the
>>>>> STM32MP25 is waiting
>>>>>     for this series to be merged.
>>>>>
>>>>> 2) OP-TEE architecture:
>>>>> - A platform-agnostic Trusted Application (TA) handles the bus
>>>>> service.[1]
>>>>>     This TA supports managing multiple remote processors. It can
>>>>> be embedded
>>>>>     regardless of the number of remote processors managed in OP-TEE.
>>>>>     The decision to embed this service is made at build time based on the
>>>>>     presence of the remoteproc driver, so it is not device tree
>>>>> dependent.
>>>>>     - STM32MP15: TA activated only if the remoteproc OP-TEE
>>>>> driver is probed
>>>>>     - STM32MP2x: TA always activated as the OP-TEE remoteproc
>>>>> driver is always
>>>>> probed
>>>>>
>>>>> - A pseudo Trusted Application implements the platform porting[2],
>>>>>     relying on registered remoteproc platform drivers.
>>>>>
>>>>> - Platform driver(s) manage the remote processors.[3][4]
>>>>>     - If remoteproc is managed by OP-TEE: manages the remoteproc
>>>>> lifecycle
>>>>>     - If remoteproc is managed by Linux: provides access rights
>>>>> to Linux to
>>>>> manage
>>>>>       the remoteproc
>>>>>
>>>>>     - STM32MP15: driver probed only if the remoteproc is managed
>>>>> in OP-TEE
>>>>>     - STM32MP2x: driver probed in both cases for the Cortex-M33
>>>>>       For the STM32MP25, the TA is always present and queries the
>>>>> driver to
>>>>> check
>>>>>       if it supports secure loading.
>>>>>
>>>>>
>>>>> [1] https://elixir.bootlin.com/op-tee/4.7.0/source/ta/remoteproc
>>>>> [2]
>>>>> https://elixir.bootlin.com/op-tee/4.7.0/source/core/pta/stm32mp/
>>>>> remoteproc_pta.c
>>>>> [3]https://elixir.bootlin.com/op-tee/4.7.0/source/core/drivers/
>>>>> remoteproc/stm32_remoteproc.c
>>>>> [4]https://github.com/STMicroelectronics/optee_os/blob/4.0.0-stm32mp/
>>>>> core/drivers/remoteproc/stm32_remoteproc.c
>>>> Thanks for the background here.
>>>>
>>>>>> The reason here is that you are mixing platform and TEE bus
>>>>>> for remoteproc
>>>>>> driver. For probe, you rely on platform bus and then try to migrate to
>>>>>> TEE bus via rproc_tee_register() is the problem here.
>>>>>> Instead you should
>>>>>> rather probe remoteproc device on TEE bus from the beginning.
>>>>> The approach is interesting, but how can we rely on Device Tree (DT) for
>>>>> hardware configuration in this case?
>>>>> At a minimum, I need to define memory regions and mailboxes.
>>>> The hardware configuration in DT should be consumed by OP-TEE and the
>>>> kernel probes remoteproc properties from OP-TEE since it's an OP-TEE
>>>> mediated remoteproc service you are adding here.
>>>>>   From my perspective, I would still need a driver probed by DT
>>>>> that registers
>>>>> a driver on the TEE bus. Therefore, I still need a mechanism to decide
>>>>> whether the remote firmware is managed by the secure or
>>>>> non-secure context.
>>>> As I mentioned below, this should be achievable using the secure-status
>>>> property without introducing the new compatible:
>>>>
>>>> Kernel managed remoteproc:
>>>>      status = "okay"; secure-status = "disabled";     /* NS-only */
>>>>
>>>> OP-TEE managed remoteproc:
>>>>      status = "disabled"; secure-status = "okay";     /* S-only */
>>>>
>>>>> Another issue would be to be able to share the remoteproc TEE service
>>>>> between
>>>>> several platform remoteproc drivers, in case of multi remote processor
>>>>> support.
>>>> Making the TEE based remoteproc service independent of DT will surely
>>>> make it more scalable to other platforms too. Have a look at how OP-TEE
>>>> based HWRNG service scales across platforms.
>>>
>>> Another important service is SCMI, which drivers use to manage clocks
>>> and resets.
>>> These clocks and resets are declared in the Device Tree (DT). It seems
>>> to me that
>>> in this case, we are closer to SCMI than to the RNG service.
>>>
>>> I propose we discuss this based on a concrete example with the STM32MP25.
>>> Although not yet upstreamed, our plan is to manage signed firmware for the
>>> Cortex-M33 and Cortex-M0.
>>>
>>> Please find below my view of the DT resources to address.
>>>
>>> STM32MP25  Cortex-M33 and Cortex-M0 nodes:
>>>
>>> m33_rproc {
>>>     /* M33 watchdog interrupt */
>>>     interrupt-parent = <&intc>;
>>>     interrupts = <GIC_SPI 4 IRQ_TYPE_LEVEL_HIGH>;
>>>     /* power domain management */
>>>     power-domains = <&cluster_pd>, <&ret_pd>;
>>>     power-domain-names = "default", "sleep";
>>>     /* RPMsg mailboxes + M33 graceful shutdown request */
>>>     mboxes = <&ipcc1 0x0>, <&ipcc1 0x1>, <&ipcc1 2>;
>>>     mbox-names = "vq0", "vq1", "shutdown";
>>>     memory-region = <&vdev0vring0>, <&vdev0vring1>, <&vdev0buffer>;
>>>     status = "okay";
>>> };
>>>
>>> m0_rproc {
>>>     /* mailbox for graceful shutdown */
>>>     mboxes = <&ipcc2 2>;
>>>     mbox-names = "shutdown";
>>>     /* M0 watchdog */
>>>    interrupt-parent = <&intc>;
>>>    interrupts = <GIC_SPI 7 IRQ_TYPE_LEVEL_HIGH>;
>>>    /* M0 peripheral clocking (not accessible by the M0) */
>>>    clocks = <&scmi_clk CK_SCMI_GPIOZ_AM>,
>>>    <&scmi_clk CK_SCMI_GPIOZ>,
>>>    <&scmi_clk CK_SCMI_IPCC2>,
>>>    <&scmi_clk CK_SCMI_IPCC2_AM>,
>>>    <&rcc CK_LPTIM3_AM>,
>>>    <&rcc CK_LPUART1_AM>,
>>>    <&rcc CK_CPU3_AM>,
>>>    <&rcc CK_CPU3>,
>>>    <&rcc CK_LPUART1_C3>,
>>>    <&rcc CK_GPIOZ_C3>,
>>>    <&rcc CK_LPTIM3_C3>,
>>>    <&rcc CK_KER_LPUART1>,
>>>    <&rcc CK_KER_LPTIM3>,
>>>    <&scmi_clk CK_SCMI_GPIOZ>,
>>>    <&scmi_clk CK_SCMI_IPCC2>;
>>>    status = "okay";
>>> };
>>>
>>> If we want to remove the DT, we need to consider:
>>>
>>> - Mechanism to differentiate Cortex-M33 and Cortex-M0:
>>>     Similar to SCMI, the remoteproc OP-TEE service should support
>>>    multiprocessor setups without instantiating multiple services.
>>>
>>> - Mailboxes:
>>>
>>>     A phandle is needed because the mailbox driver is managed by the
>>>     Linux mailbox driver. STM32MP2 has two mailboxes.
>>>     Moving towards your proposal would imply creating a mailbox service
>>>     in TEE to manage non-secure mailboxes for non-secure IPC. This might
>>>     not be efficient for inter-processor communication. Hardware-wise, it
>>>     would require the IRQ to be handled by the secure context.
>>>
>>> - Memory regions:
>>>    - Hardware limitation: OP-TEE is limited in the number of memory regions
>>>      it can declare due to Firewall configuration. Moving IPC memory regions
>>>      reaches this limit. Currently, OP-TEE defines a single region with
>>> shareable
>>>      access rights, which Linux splits into at least three memory regions
>>> for RPMsg.
>>>    - Memory mapping: Memory regions still need to be declared in Linux to
>>> prevent
>>>      Linux from using them.
>>>    - Virtio/RPMsg: Memory region names are fixed (e.g., dev<X>vring<Y>),
>>> so OP-TEE
>>>     must declare memory regions in its DT according to Linux naming
>>> conventions.
>>>
>>> - Clock and reset:
>>>      Some clocks and resets are managed via SCMI, others are not. This
>>> would require
>>>     managing all clocks and resets through SCMI, with possible side
>>> effect on the
>>>     "unused" clock mechanism in Linux ( to be confirmed)
>>>
>>> - Power domain:
>>>     Information is needed at the Linux level to determine the low power
>>> mode.
>>>
>>> - Watchdog interrupt:
>>>     Should be managed by OP-TEE, which requires the hardware to have an
>>> associated
>>>     secure IRQ.
>>>
>>> - Miscellaneous vendor DT properties:
>>>      How to be sure that these can be addressed through TEE services?
>>>
>>> Regarding the existing DT needs, it seems to me that removing the DT
>>> would require
>>> moving all node resource management into TEE ( if really possible). This
>>> would
>>> increase TEE complexity and footprint, reduce system efficiency, and
>>> make supporting
>>> other platforms less scalable.
> 
> I can see your arguments regarding some DT properties which are hard to
> discover via OP-TEE service and I have been trying to think of how to
> handle it in a proper manner for a device on discoverable TEE bus. One
> of my fellow kernel maintainers pointed out that other discoverable
> buses in the kernel like PCI etc. already solved this DT dependencies
> via having device specific bindings. You can have a look at a one
> particular example of PCI device binding here [1]. In case of OP-TEE,
> you can have a similar device binding with UUID under the OP-TEE
> firmware DT node.
> 
> The current approach you are taking via probing device on platform bus
> and then trying to move onto TEE bus isn't at all the compatible with
> the driver model, it simply sets a bad example for the driver model.
> 
> [1] Documentation/devicetree/bindings/net/wireless/qcom,ath11k-pci.yaml
> 

Thank you for your feedback. I am not very familiar with PCIe, so I need 
to take a deeper look at the implementation.

To help me better understand your expectations, could you please confirm 
if your proposal is close to what I have described below?


1) Device tree:
Notice that an intermediate ahb bus is used on ST platform for address 
conversion between remote processor address and local physical address
I will probably need to find a solution for that.


firmware {
    remoteproc_tee {
	compatible = "remoteproc-tee";
	mlahb: ahb {
	    compatible = "st,mlahb", "simple-bus";
	    #address-cells = <1>;
	    #size-cells = <1>;
	    ranges;
	    dma-ranges = <>;
	    m33_rproc@0 {
       		compatible = "st,stm32mp1-m33";
      		/* M33 watchdog interrupt */
      		interrupt-parent = <&intc>;
		interrupts = <GIC_SPI 4 IRQ_TYPE_LEVEL_HIGH>;
		/* power domain management */
		power-domains = <&cluster_pd>, <&ret_pd>;
		power-domain-names = "default", "sleep";
		/* RPMsg mailboxes + M33 graceful shutdown request */
		mboxes = <&ipcc1 0x0>, <&ipcc1 0x1>, <&ipcc1 2>;
		mbox-names = "vq0", "vq1", "shutdown";
		memory-region = <&vdev0vring0>, <&vdev0vring1>,
				<&vdev0buffer>;
		status = "okay";
	    };
         };

	ahbsr: ahb@2 {
	   compatible = "st,mlahb", "simple-bus";
	   #address-cells = <1>;
	   #size-cells = <1>;
	   ranges = <>;
	   dma-ranges = <>;
	   m0_rproc@1 {
       		compatible = "st,stm32mp1-m0";
		mboxes = <&ipcc2 2>;
		mbox-names = "shutdown";
		interrupt-parent = <&intc>;
		interrupts = <GIC_SPI 7 IRQ_TYPE_LEVEL_HIGH>;
		/* M0 peripheral clocking (not accessible by the M0) */
		clocks = <>, <>;
     		status = "okay";
	   };
        };
     };
};

2) remoteproc_driver:
   - platform driver that registers a TEE driver with specified UUID
   - on TEE driver probing it parses sub device (reg property replacing
     st, proc_id property)
   - tries to open associated TEE session and in case of success
     probes associated rproc driver.

3) st,remoteproc drivers
   -  unique DT compatible, no secure-status use (legacy compatibility)
   -  need to add a mechanism to detect probing
      by the TEE bus, or directly probed by the ahb bus.


Thanks,
Arnaud

>>>
>>> That said, it probably also depends on the TEE implementation.
>>> And  we should support both. This could be done by introducing a second
>>> UUID.
>>> but in this case should it be the same driver?
>>
>> I am unsure how to move forward here. It seems to me that addressing Sumit's
>> request for a TEE without a device tree is not compatible with the current
>> OP-TEE implementation, at least for the STM32MP platforms.
>>
>> Perhaps the simplest approach is to abandon the effort to make this generic
>> and instead rename tee_remoteproc.c to stm32_tee_remoteproc.c, making it
>> platform-dependent. Then, if another platform wants to reuse it with OP-TEE
>> FFA or another TEE, the file can be renamed.
>>
>> Does this proposal would make sense to you?
> 
> No, we should try to avoid a vendor specific TEE driver especially if it's based
> on similar OP-TEE based service which should be able to abstract out the vendor
> implementation for the kernel.
> 
> -Sumit

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