From: Jean Delvare <khali@linux-fr.org>
To: Andy Walls <awalls@md.metrocast.net>
Cc: linux-media@vger.kernel.org, Mike Isely <isely@isely.net>,
Kenney Phillisjr <kphillisjr@gmail.com>,
Hans Verkuil <hverkuil@xs4all.nl>
Subject: Re: [PATCH 04/17] cx25840: Make cx25840 i2c register read transactions atomic
Date: Tue, 20 Jul 2010 14:46:13 +0200 [thread overview]
Message-ID: <20100720144613.49b5ef35@hyperion.delvare> (raw)
In-Reply-To: <1279629326.2714.31.camel@morgan.silverblock.net>
On Tue, 20 Jul 2010 08:35:26 -0400, Andy Walls wrote:
> On Tue, 2010-07-20 at 08:42 +0200, Jean Delvare wrote:
> > Hi Andy,
> >
> > On Mon, 19 Jul 2010 21:11:46 -0400, Andy Walls wrote:
> > > There was a small window between writing the cx25840 register
> > > address over the i2c bus and reading the register contents back from the
> > > cx25840 device that the i2c adapter lock was released. This change ensures the
> > > adapter lock is not released until the register read is done.
> > >
> > > Signed-off-by: Andy Walls <awalls@md.metrocast.net>
> >
> > Good catch.
>
> Thanks.
>
> > Acked-by: Jean Delvare <khali@linux-fr.org>
> >
> > Note that cx25840_and_or() still has a (smaller and less dangerous)
> > race window. If several calls to cx25840_and_or() happen in parallel on
> > the same register, some of the changes may be lost. I don't know if
> > this can be a problem in practice though. If it is, then additional
> > locking around cx25840_and_or() would be needed.
>
> Ah, thank you for pointing that out.
>
> So, please bear with me while I think out loud:
>
> 1. There are many explicit cases of read-modify-write on a register in
> the cx25840 module, this is not the only one.
>
> 2. The bridge driver historically has always serialized access to the
> cx25840 module so races have never previously been an issue.
>
> 3. I have added a work handler in the cx23885 module that calls the
> cx25840 module's interrupt handler. Calls by the work handler are
> serialized with respect to themselves, but not with respect to
> serialized calls in #2.
>
> 4. IIRC, registers written to by the cx25840 interrupt handler are never
> written to by the other cx25840 module functions; and vice-versa. I
> will have to audit the cx25840 module code to be sure.
>
> 5. There is always a race on r-m-w on *some* registers in the
> 0x800-0x9ff range with the audio microcontroller that is built into the
> chip. The only way to provide locking for those is to halt the
> microcontroller. I've just looked at Patch 12/17 again. The interrupt
> handler only reads the CX23885_AUD_MC_INT_MASK_REG, which is used by the
> audio micrcontroller. The interrupt handler does do a r-m-w on the
> CX25840_AUD_INT_STAT_REG, but that register is not used by the
> microcontroller.
>
>
> So, I think I'm OK with just not dropping the i2c adapter lock in a
> cx25840 register read transaction until it is complete.
>
> Thanks for making me think that one through. :)
Looks like you're OK then, but you may want to document this somewhere
for future contributors to these drivers.
--
Jean Delvare
next prev parent reply other threads:[~2010-07-20 12:46 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1279586511.git.awalls@md.metrocast.net>
2010-07-20 1:08 ` [PATCH 01/17] cx23885: Return -ENXIO on slave nack Andy Walls
2010-07-20 1:09 ` [PATCH 02/17] cx23885: Check for slave nack on all transactions Andy Walls
2010-07-20 1:10 ` [PATCH 03/17] cx23885: i2c_wait_done returns 0 or 1, don't check for < 0 return value Andy Walls
2010-07-20 1:11 ` [PATCH 04/17] cx25840: Make cx25840 i2c register read transactions atomic Andy Walls
2010-07-20 6:42 ` Jean Delvare
2010-07-20 12:35 ` Andy Walls
2010-07-20 12:46 ` Jean Delvare [this message]
2010-07-20 1:12 ` [PATCH 05/17] cx23885: Add correct detection of the HVR-1250 model 79501 Andy Walls
2010-07-20 1:12 ` [PATCH 06/17] cx23885: Add a VIDIOC_LOG_STATUS ioctl function for analog video devices Andy Walls
2010-07-20 1:16 ` [PATCH 07/17] v4l2_subdev: Add s_io_pin_config to v4l2_subdev_core_ops Andy Walls
2010-07-20 1:18 ` [PATCH 08/17] cx25840: Add s_io_pin_config core subdev ops for the CX2388[578] Andy Walls
2010-07-20 1:18 ` [PATCH 09/17] v4l2_subdev, cx23885: Differentiate IR carrier sense and I/O pin inversion Andy Walls
2010-07-20 1:19 ` [PATCH 10/17] cx23885: For CX23888 IR, configure the IO pin mux IR pins explcitly Andy Walls
2010-07-20 1:20 ` [PATCH 11/17] v4l2_subdev: Move interrupt_service_routine ptr to v4l2_subdev_core_ops Andy Walls
2010-07-20 1:22 ` [PATCH 12/17] cx25840: Add support for CX2388[57] A/V core integrated IR controllers Andy Walls
2010-07-20 1:22 ` [PATCH 13/17] cx23885: Add a v4l2_subdev group id for the CX2388[578] integrated AV core Andy Walls
2010-07-20 1:23 ` [PATCH 14/17] cx23885: Add preliminary IR Rx support for the HVR-1250 and TeVii S470 Andy Walls
2010-07-20 1:25 ` [PATCH 15/17] cx23885: Protect PCI interrupt mask manipulations with a spinlock Andy Walls
2010-07-20 1:25 ` [PATCH 16/17] cx23885: Move AV Core irq handling to a work handler Andy Walls
2010-07-20 1:26 ` [PATCH 17/17] cx23885: Require user to explicitly enable CX2388[57] IR via module param Andy Walls
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