[PATCH] misc: atmel-secumod: Driver for Atmel "security module".

[Boris Brezillon]

On Mon, 23 May 2016 14:04:24 +0200
Boris Brezillon <boris.brezillon at free-electrons.com> wrote:

> Hi David,
> 
> Sorry for the late review (I know you've posted new versions but I want
> to comment on this one).
> 
> On Sun, 31 Jan 2016 12:34:09 +0100
> Alexandre Belloni <alexandre.belloni at free-electrons.com> wrote:
> 
> > On 29/01/2016 at 11:13:05 +1100, Finn Thain wrote :  
> > > 
> > > On Mon, 25 Jan 2016, David Mosberger wrote:
> > >     
> > > > On Mon, Jan 25, 2016 at 4:09 AM, Alexandre Belloni 
> > > > <alexandre.belloni at free-electrons.com> wrote:
> > > >     
> > > > > I know this does more than that but I think those thre sections should 
> > > > > be registered using the nvmem framework. The sysfs file creation and 
> > > > > accesses then comes for free.    
> > > > 
> > > > I think Finn's patches would have to go in for that first, since the 
> > > > existing nvram code is a mess. Even with Finn's patches in, I think it 
> > > > could go either way.    
> > > 
> > > I think Alexandre is speaking of the nvmem subsystem (not nvram).
> > > Documentation/devicetree/bindings/nvmem
> > > Documentation/nvmem
> > > drivers/nvmem
> > >     
> > 
> > absolutely.
> >   
> > > > I'm not exactly sure how some of the features of the security module 
> > > > would be used: key management, auto erasing, there is a strange "backup 
> > > > mode" vs "normal mode" which is not well documented, etc.  So I think it 
> > > > may well end up being sufficiently different to warrant a separate 
> > > > driver.    
> > > 
> > > nvmem is not a subsystem I am familiar with, so it's not immediately clear 
> > > to me what your driver would look like if re-written that way.
> > > 
> > > Maybe it would become simpler. But if you did end up needing a separate 
> > > misc driver as well, maybe use of the nvmem framework would actually 
> > > increase complexity.
> > > 
> > > It would depend on your requirements. But I would focus on the actual 
> > > requirement rather than uncertain future possibilities.
> > >     
> > > >     
> > > > > Another idea is also to expose it using a genpool so it can be 
> > > > > accessed as sram from inside the kernel.    
> > > > 
> > > > That may be a fine idea, but as far as our application is concerned, we 
> > > > need user-level access to the battery-backed RAM.    
> > > 
> > > Right. I don't see how adding a memory allocator would help either.
> > >     
> > 
> > While the immediate need is to use that sram from userspace, I think
> > this is valuable to already think that at some point we will need to be
> > able to partition and access that sram from the kernel.
> > 
> > 
> >   
> 
> Well, I think we're reaching this point right now: I have to implement
> "freeze" mode (entering a deep sleep mode by cutting all power domains
> except VDDBU), and in order to do that I need to access BUREGs which
> are part of the secu-sram you're trying to expose here.
> 
> Two comments on the nvmem approach:
> 1/ first of all it's not really a non-volative memory: if you loose
> VDDBU you also loose the whole SRAM content.
> 2/ I need to be able to reserve the BUREG region (at least part of it)
> for in kernel usage (need to store the SDRAM address I should jump to
> when exiting freeze mode).

Forget this aspect. As Alexandre pointed out, the nvmem framework
provides an in-kernel API, so reserving space for the "freeze" mode
implementation is doable. But need to use the securam for advanced
stuff (like executing code from there) then the SRAM driver approach is
more future-proof IMO.


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com