Re: [PATCH 01/10] Documentation: dt-bindings: mailbox: tegra: Add binding for HSP mailbox

[Stephen Warren]

On 06/30/2016 03:25 AM, Joseph Lo wrote:
> On 06/29/2016 11:28 PM, Stephen Warren wrote:
> > On 06/28/2016 11:56 PM, Joseph Lo wrote:
> > > On 06/29/2016 03:08 AM, Stephen Warren wrote:
> > > > On 06/28/2016 03:15 AM, Joseph Lo wrote:
> > > > > On 06/27/2016 11:55 PM, Stephen Warren wrote:
> > > > > > On 06/27/2016 03:02 AM, Joseph Lo wrote:
> > > snip.
> > > > > Currently the usage of HSP HW in the downstream kernel is something
> > > > > like
> > > > > the model below.
> > > > >
> > > > > remote_processor_A-\
> > > > > remote_processor_B--->hsp@1000 (doorbell func) <-> host CPU
> > > > > remote_processor_C-/
> > > > >
> > > > > remote_processor_D -> hsp@2000 (shared mailbox) <-> CPU
> > > > >
> > > > > remote_processor_E -> hsp@3000 (shared mailbox) <-> CPU
> > > > >
> > > > > I am thinking if we can just add the appropriate compatible strings
> > > > > for
> > > > > it to replace "nvidia,tegra186-hsp". e.g.
> > > > > "nvidia,tegra186-hsp-doorbell"
> > > > > and "nvidia,tegra186-hsp-sharedmailbox". So the driver can probe and
> > > > > initialize correctly depend on the compatible property. How do you
> > > > > think
> > > > > about it? Is this the same as the (b) you mentioned above?
> > > >
> > > > Yes, that would be (b) above.
> > > >
> > > > However, please do note (a): I expect that splitting things up will
> > > > turn
> > > > out to be a mistake, as it has for other HW modules in the past. I
> > > > would
> > > > far rather see a single hsp node in DT, since there is a single HSP
> > > > block in HW. Sure that block has multiple sub-functions. However, there
> > > > is common logic that affects all of those sub-functions and binds
> > > > everything into a single HW module. If you represent the HW module
> > > > using
> > > > multiple different DT nodes, it will be hard to correctly represent
> > > > that
> > > > common logic. Conversely, I see no real advantage to splitting up
> > > > the DT
> > > > node. I strongly believe we should have a single "hsp" node in DT.
> > >
> > > We have 6 HSP block in HW. FYI.
> >
> > Yes, we have 6 /instances/ of the overall HSP block. Those should each
> > have their own node, since they're entirely separate modules, all
> > instances of the same configurable IP block.
> >
> > Above, I was talking about the sub-blocks within each HSP instance,
> > which should all be represented into a single node per instance, for a
> > total of 6 DT nodes overall.
> Yes.
>
> So, one thing still concerns me is that the binding and driver still
> can't work with multiple HSP sub-modules per HSP block. It only supports
> one HSP module per HSP block right how.

The driver can be enhanced without affecting the DT binding, providing 
the binding is reasonably designed, as I believe it is.

I believe the existing binding can work fine for multiple HSP 
sub-modules, or at least be extended in a backwards-compatible way. 
Aside from the mailbox cells issue you mention below, is there any other 
reason you believe the binding can't be extended in a 
backwards-compatible way? Interrupts are already accessed solely by 
name, so we can add more later without issue. The node can become a 
provider for any other resource type besides mailboxes in a 
backwards-compatible way without issue.

> Although, I said it matches the
> model that we are using in the downstream kernel. But I still concern if
> we need to enable and work with multiple HSP modules per HSP block at
> sometime in future, then the binding and driver need lots of change to
> achieve that. And the binding is not back-ward compatible obviously.
>
> So I want to revise it again.
>
> #mbox-cells: should be 2.
>
> The mobxes property in the client node should contain the phandle of the
> HSP block, HSP sub-module ID and the specifier of the module.
>
> Ex.
> hsp_top0: hsp@1000 {
>      ...
>      #mbox-cells = <2>;
> };
>
> clientA {
>      ....
>      mboxes = <&hsp_top0 HSP_DOORBELL DB_MASTER_XXX>;
> };
>
> clientB {
>      ...
>      mboxes = <&hsp_top0 HSP_SHARED_MAILBOX SM_MASTER_XXX>;
> };
>
> Stephen, How do you think of this change?

Well, we could do that. Or, since we won't have 2^32 instances of 
doorbells, we could also have #mbox-cells=<1> as we do now, and encode 
mailbox IDs as "(type << 16) | id" where TEGRA186_HSP_MAILBOX_TYPE_DB is 
0. That would be backwards-compatible with no change to the binding. I 
think either way is fine. I have a slight preference for keeping 
#mbox-cells=<1> to avoid revising the U-Boot driver code I wrote, but I 
can deal with changing it if I have to.