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From: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
To: Stefan Wahren <stefan.wahren@i2se.com>
Cc: linux-rpi-kernel@lists.infradead.org, eric@anholt.net,
	gregkh@linuxfoundation.org, linux-arm-kernel@lists.infradead.org,
	devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org,
	dave.stevenson@raspberrypi.org
Subject: Re: [PATCH RFC 09/18] staging: vchiq_core: do not initialize semaphores twice
Date: Tue, 06 Nov 2018 19:28:14 +0100	[thread overview]
Message-ID: <a790978e8bcb869708833c73da1a05923f66b2d2.camel@suse.de> (raw)
In-Reply-To: <9366d835-a291-2770-4409-b88ea1a155e2@i2se.com>

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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@suse.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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WARNING: multiple messages have this Message-ID
From: nsaenzjulienne@suse.de (Nicolas Saenz Julienne)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH RFC 09/18] staging: vchiq_core: do not initialize semaphores twice
Date: Tue, 06 Nov 2018 19:28:14 +0100	[thread overview]
Message-ID: <a790978e8bcb869708833c73da1a05923f66b2d2.camel@suse.de> (raw)
In-Reply-To: <9366d835-a291-2770-4409-b88ea1a155e2@i2se.com>

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@suse.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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  reply	other threads:[~2018-11-06 18:28 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-26 13:47 [PATCH RFC 00/18] staging: vchiq: remove dead code & misc fixes Nicolas Saenz Julienne
2018-10-26 13:47 ` Nicolas Saenz Julienne
2018-10-26 13:47 ` [PATCH RFC 01/18] staging: vchiq_core: rework vchiq_get_config Nicolas Saenz Julienne
2018-10-26 13:47   ` Nicolas Saenz Julienne
2018-10-26 13:47 ` [PATCH RFC 02/18] staging: vchiq_arm: rework close/remove_service IOCTLS Nicolas Saenz Julienne
2018-10-26 13:47   ` Nicolas Saenz Julienne
2018-10-26 13:47 ` [PATCH RFC 03/18] staging: vchiq_shim: delete vchi_service_create Nicolas Saenz Julienne
2018-10-26 13:47   ` Nicolas Saenz Julienne
2018-10-26 13:47 ` [PATCH RFC 04/18] stagning: vchiq_arm: use list_for_each_entry when accessing bulk_waiter_list Nicolas Saenz Julienne
2018-10-26 13:47   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 05/18] staging: vchiq_arm: get rid of vchi_mh.h Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 06/18] staging: vchiq_arm: rework vchiq_ioc_copy_element_data Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 07/18] staging: vchiq-core: get rid of is_master distinction Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 08/18] staging: vchiq_core: remove unnecessary safety checks in vchiq_init_state Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 09/18] staging: vchiq_core: do not initialize semaphores twice Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-28 20:45   ` Stefan Wahren
2018-10-28 20:45     ` Stefan Wahren
2018-11-06 15:41     ` Nicolas Saenz Julienne
2018-11-06 15:41       ` Nicolas Saenz Julienne
2018-11-06 16:06       ` Stefan Wahren
2018-11-06 16:06         ` Stefan Wahren
2018-11-06 18:28         ` Nicolas Saenz Julienne [this message]
2018-11-06 18:28           ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 10/18] staging: vchiq_core: don't add a wmb() before remote_event_signal() Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 11/18] staging: vchiq_arm: use completions instead of semaphores Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-28 21:00   ` Stefan Wahren
2018-10-28 21:00     ` Stefan Wahren
2018-10-26 13:48 ` [PATCH RFC 12/18] staging: vchiq_util: " Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 13/18] staging: vchiq_core: " Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 14/18] staging: vchiq_util: get rid of unneeded memory barriers Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 15/18] stagning: vchiq_core: fix logic redundancy in parse_open Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-28 20:58   ` Stefan Wahren
2018-10-28 20:58     ` Stefan Wahren
2018-10-26 13:48 ` [PATCH RFC 16/18] staging: vchiq_arm: rework probe and init functions Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-28 21:02   ` Stefan Wahren
2018-10-28 21:02     ` Stefan Wahren
2018-10-26 13:48 ` [PATCH RFC 17/18] staging: vchiq_arm: fix open/release cdev functions Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-26 13:48 ` [PATCH RFC 18/18] staging: vchiq: add more tasks to the TODO list Nicolas Saenz Julienne
2018-10-26 13:48   ` Nicolas Saenz Julienne
2018-10-28 21:11   ` Stefan Wahren
2018-10-28 21:11     ` Stefan Wahren

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