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

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Message-ID: <5775427B.9040907@wwwdotorg.org>
Date:	Thu, 30 Jun 2016 10:02:03 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Joseph Lo <josephl@...dia.com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	linux-tegra@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Matthew Longnecker <MLongnecker@...dia.com>,
	devicetree@...r.kernel.org, Jassi Brar <jassisinghbrar@...il.com>,
	linux-kernel@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 01/10] Documentation: dt-bindings: mailbox: tegra: Add
 binding for HSP mailbox

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@...0 (doorbell func) <-> host CPU
>>>>> remote_processor_C-/
>>>>>
>>>>> remote_processor_D -> hsp@...0 (shared mailbox) <-> CPU
>>>>>
>>>>> remote_processor_E -> hsp@...0 (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@...0 {
>      ...
>      #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.