ARM64 SMMU Setup

[Ard Biesheuvel]

> On 31 Jan 2018, at 23:31, Thor Thayer <thor.thayer at linux.intel.com> wrote:
> 
> Hi Ard,
> 
>> On 01/31/2018 04:21 PM, Ard Biesheuvel wrote:
>>> On 31 January 2018 at 22:09, Thor Thayer <thor.thayer at linux.intel.com> wrote:
>>> Hi,
>>> 
>>> I'm enabling the ARM SMMU-500 on an ARM64 A53. I'm hitting a data abort in
>>> the probe functions because I'm accessing the registers in EL1 mode.
>>> 
>> Why do you think the fact that you are running at EL1 is causing the data abort?
> Yes, I may not be diagnosing this correctly. I narrowed it down to failing on the first read in arm_smmu_device_cfg_probe().
> 
> When I read the same register with a debugger (I'm in EL1), it fails. I just realized the read also fails in EL2.
> 
> If I elevate the read to EL3, I can read the default reset value from the register but I mistakenly thought this was EL2.
> 

This confirms my suspicion. It is up to the secure firmware running in EL3 (or secure EL1) to configure the SMMU itself and the interconnect in a way that allows code running in EL2 or non-secure EL1 to access it (note that EL3 is always secure and EL2 is always non-secure)

I don’t know the details of the MMU-500 or the way it is integrated into your platform, so I can’t be of any more help, unfortunately.


> 
>>> Linux starts in EL2 mode but drops down to EL1 mode by the time it reaches
>>> the arm-smmu probe function.
>>> 
>>> Is there something else I need to add to either move the arm-smmu probe
>>> earlier or remain in EL2 until after the arm-smmu probe?
>>> 
>> No. I am pretty sure EL2 vs EL1 is not causing your issue. Could you
>> elaborate on the nature of the exception you are getting?
> The error I'm getting is:
> 
> Unhandled fault: synchronous external abort (0x96000010) at 0xffff000008e40020
> Internal error: : 96000010 [#1] PREEMPT SMP

> To be honest I got lost trying to decode 0x96000010 and I may have used the wrong ARM document. I ended up interpreting this as Data Abort taken without an Exception level from the ARMv8 ARM.
> 
> Additionally, the SMMU documentation seemed to indicate the Hypervisor would need to run at EL2 so I ran with that.
> 
> Thank you for the reply and I'll go back and examine my assumptions again.
> 
> Thor
> 
>>> A Google search didn't turn up anything so I hoped posting to the list would
>>> help.
>>> 
>>> Thanks in advance,
>>> 
>>> Thor
>>> 
>>> 
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>