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

[Russell King - ARM Linux <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>]

On Wed, Feb 04, 2015 at 11:29:55AM +0100, Arnd Bergmann wrote:
> On Wednesday 04 February 2015 08:57:43 Jassi Brar wrote:
> > On 3 February 2015 at 20:55, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> > > On Tuesday 03 February 2015 14:46:11 Russell King - ARM Linux wrote:
> > >> On Tue, Feb 03, 2015 at 08:09:34PM +0530, Jassi Brar wrote:
> 
> > > I had expected to see here something like:
> > >
> > > static int mhu_send_data(struct mbox_chan *chan, void *data)
> > > {
> > >         struct mhu_link *mlink = chan->con_priv;
> > >         u32 *arg = data;
> > >
> > >         writel_relaxed(*arg, mlink->tx_reg + INTR_SET_OFS);
> > > }
> > >
> > > i.e. dereferencing the pointer instead of using the actual value.
> > >
> > OK, just curious how is this (dereferencing to the u32 variable on
> > stack of the client driver) better?
> 
> The API as I understand is defined to use the pointer to point to
> a chunk of data of fixed size, with the size being known to both
> the client driver and the mailbox driver. This is the reason for
> having a pointer in the first place.
> 
> Using the bits of the pointer as the message instead of pointing
> to the message feels like an abuse of the API.

I agree on those two points.  However, passing the address of something
on the stack to mbox_send_message() is also not particularly on - it
may save the pointer to use later on if its operating on non-blocking
mode.

A possible alternative would be if the user of mbox_send_message()
stored the message in an array, operated as a circular buffer, and
passed the address of the word to send.  That would avoid the need
to repeatedly allocate and free memory (which would be expensive for
the sake of a u32.)

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