[U-Boot] [PATCH] Add a mailbox driver framework/uclass

[Stephen Warren <swarren@wwwdotorg.org>]

On 05/13/2016 02:05 PM, Simon Glass wrote:
> Hi Stephen,
>
> On 12 May 2016 at 16:27, Stephen Warren <swarren@wwwdotorg.org> wrote:
>> From: Stephen Warren <swarren@nvidia.com>
>>
>> A mailbox is a hardware mechanism for transferring small message and/or
>> notifications between the CPU on which U-Boot runs and some other device
>> such as an auxilliary CPU running firmware or a hardware module.
>>
>> This patch defines a standard API that connects mailbox clients to mailbox
>> providers (drivers). Initially, DT is the only supported method for
>> connecting the two.
>>
>> The DT binding specification (mailbox.txt) was taken from Linux kernel
>> v4.5's Documentation/devicetree/bindings/mailbox/mailbox.txt.
>>
>> Signed-off-by: Stephen Warren <swarren@nvidia.com>

> Also see the remoteproc uclass. I'd just like to make sure that these
> should not be combined.

I think that's quite a different API; more to do with loading 
firmware(?)/booting/resetting remote processors than communicating with 
them once they're booted. FWIW, the Linux kernel has separate mailbox 
and remoteproc APIs too.

>> diff --git a/drivers/mailbox/mailbox-uclass.c b/drivers/mailbox/mailbox-uclass.c

>> +int mbox_send(struct mbox_chan *chan, const void *data)
>
> Is there no length on the data?

The length is implicit; each mailbox implementation defines (the HW) 
defines the exact message size it transfers. Clients would always use 
this exact size.

>> +int mbox_recv(struct mbox_chan *chan, void *data, ulong timeout_us)

>> +       start_time = get_ticks();
>> +       /*
>> +        * Account for partial us ticks, but if timeout_us is 0, ensure we
>> +        * still don't wait at all.
>> +        */
>> +       if (timeout_us)
>> +               timeout_us++;
>
> This seems a bit picky - it is only a microsecond!

Is there anything wrong with being correct? Equally, "only a 
microsecond" admittedly isn't relevant if timeout_us is 1000 * 1000, but 
is a massive percentage of the delay if timeout_us is something tiny 
like 1 or 2.