EDAC, dmc520:: add DMC520 EDAC driver

[James Morse <james.morse@arm.com>]

Hi Rui,

On 23/01/2019 22:08, Rui Zhao wrote:
> On Wednesday, January 23, 2019 10:36 AM, James Morse wrote:
>>> If firmware enables it, they're suppose to handle the interrupt.

>> Ah, so you still have resident firmware!
>> How come your firmware trusts linux not to turn off the memory controller?!
>> These things are usually protected by trust zone so the OS can't pull the memory from under firmware's feet.

> We have firmware to config the memory controller and want to have an EDAC driver to report ECC status.

> Could you please elaborate a bit on the security concern on this approach? Like some malicious app/driver can access
> memory controller registers can cause issue? 

I'm remembering this:
https://lore.kernel.org/linux-arm-kernel/9b9c4cd5-4428-c08d-d4a3-7352c6c80583@arm.com/

Robin Murphy wrote:
| [ For anyone interested, it puts the DRAM controller into sleep mode.
| The kernel can't even panic if all the memory suddenly disappears :D ]

This would be a problem if you need your Secure-world software needs to keep
working, and depends on the memory behind this controller.

It might be that your secure-world software only uses some other memory, in
which case this wouldn't matter.
It may be linux _is_ your secure-world software, in which case it wouldn't
matter either.


> What's the recommend approach if Linux won't be able to access memory controller
> registers? Have firmware do the ECC 
> status monitoring and some sort of driver to query ECC status from firmware?

If Linux runs in the normal-world, can't you use trust-zone to prevent Linux
from accessing the memory controller?

If you did this, you'd need to handle the UE interrupts in firmware, and
wouldn't be able to use this driver in linux. Your platform hasn't gone this
way, so I guess one of the above cases applies.


Thanks,

James