linux-kernel - Re: [PATCH RFC 09/18] staging: vchiq_core: do not initialize semaphores twice

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Message-ID: <a790978e8bcb869708833c73da1a05923f66b2d2.camel@suse.de>
Date:   Tue, 06 Nov 2018 19:28:14 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     linux-rpi-kernel@...ts.infradead.org, eric@...olt.net,
        gregkh@...uxfoundation.org, linux-arm-kernel@...ts.infradead.org,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        dave.stevenson@...pberrypi.org
Subject: Re: [PATCH RFC 09/18] staging: vchiq_core: do not initialize
 semaphores twice

On Tue, 2018-11-06 at 17:06 +0100, Stefan Wahren wrote:
> Am 06.11.18 um 16:41 schrieb Nicolas Saenz Julienne:
> > Hi Stefan,
> > thanks for spending the time reviewing the code. I took note of the
> > rest of comments.
> > 
> > On Sun, 2018-10-28 at 21:45 +0100, Stefan Wahren wrote:
> > > Hi Nicolas,
> > > 
> > > > Nicolas Saenz Julienne <nsaenzjulienne@...e.de> hat am 26.
> > > > Oktober
> > > > 2018 um 15:48 geschrieben:
> > > > 
> > > > 
> > > > vchiq_init_state() initialises a series of semaphores to then
> > > > call
> > > > remote_event_create() on the same semaphores, which initializes
> > > > them
> > > > again.
> > > i would prefer to have all init stuff at one place in
> > > vchiq_init_state() and drop this ugliness from
> > > remote_event_create()
> > > instead. Is this possible?
> > As I'm sure you're aware of, REMOTE_EVENT_T is shared between the
> > CPU
> > and VC4, which can't be expanded. And since storing a pointer is
> > out of
> > question because of arm64, I can only think of storing an index to
> > an
> > array of completions in the shared structure instead of the pointer
> > magic implemented right now. It would be a little more explicit.
> > Then
> > we could completely decouple both initializations. I'm not sure if
> > it's
> > similar to what you had in mind. 
> 
> I don't think so, this was my intention:
> 
>  static inline void
>  remote_event_create(VCHIQ_STATE_T *state, REMOTE_EVENT_T *event)
>  {
>     event->armed = 0;
>     /* Don't clear the 'fired' flag because it may already have been
> set
>     ** by the other side. */
> -    sema_init((struct semaphore *)((char *)state + event->event),
> 0);
>  }

Fair enough, even simpler.

> 
> 
> > On a semi-related topic, I'm curious to know why these shared
> > structures aren't set with the "__packed" preprocessor macro. Any
> > ideas? As fas as I've been told, in general, the compiler may
> > reorder
> > or add unexpected padding to any structure. Which would be very bad
> > in
> > this case.
> 
> This would be better, but i assume the firmware side uses the same
> source code. So using __packed only on ARM side could also break :-(

True. Yet in that case, aren't we still relying on the fact that both
compilers are going to behave the same way?
 
> 
> > Regards,
> > Nicolas


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