Re: [PATCH v5 2/7] mailbox: arm_mhu: add driver for ARM MHU controller

Arnd Bergmann
2015-02-04T16:42:01Z

On Wednesday 04 February 2015 21:03:50 Jassi Brar wrote:
> On 4 February 2015 at 20:18, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> > On Wednesday 04 February 2015 20:04:21 Jassi Brar wrote:
> >>
> >> > Using the bits of the pointer as the message instead of pointing
> >> > to the message feels like an abuse of the API.
> >> >
> >> I can see your POV.
> >> Now consider a client, like mine, that sends a u32 value as the data.
> >> But unlike me, the client uses the mailbox api in 'async' mode i.e,
> >> register a callback function, submit a 32bit message and move on. It
> >> is perfectly doable, but doesn't kalloc'ing a u32 for each submission,
> >> seem overkill?
> >
> > That could easily be done by dereferencing the message data in the
> > function that queues the asynchronous message: instead of queuing
> > the pointer, you queue the data in the driver.
> >
> The 'void *data' is not queued in the driver, but in the API code. Or
> do I not get your point?

My mistake. If the API keeps that pointer after returning to the caller
here, this is indeed a problem for callers that have the message on the
stack.

The solution of storing the message in a circular buffer (in the API)
would in turn require knowing the length of the data.

Another option would be to require that the asynchronous version of the
interface cannot be used with on-stack data and requires the object
lifetime to be managed by the caller, which seems reasonable to me
since you already need the completion callback.

Arnd