From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jani Nikula <jani.nikula@linux.intel.com>
Subject: Re: CDF meeting @FOSDEM report
Date: Wed, 06 Feb 2013 14:11:12 +0200
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References: <1990856.qS9uisuiVF@avalon> <51123A5F.9050604@ti.com>
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To: Tomi Valkeinen <tomi.valkeinen@ti.com>, Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: linux-fbdev@vger.kernel.org, Sebastien Guiriec <s-guiriec@ti.com>, dri-devel@lists.freedesktop.org, Jesse Barnes <jesse.barnes@intel.com>, Sumit Semwal <sumit.semwal@linaro.org>, Tom Gall <tom.gall@linaro.org>, Kyungmin Park <kyungmin.park@samsung.com>, linux-media@vger.kernel.org, Stephen Warren <swarren@wwwdotorg.org>, Mark Zhang <markz@nvidia.com>, Thierry Reding <thierry.reding@avionic-desi.gn.de>, linaro-mm-sig@lists.linaro.org, =?utf-8?Q?St=C3=A9phane?= Marchesin <stephane.marchesin@gmail.com>, Alexandre Courbot <acourbot@nvidia.com>, Ragesh Radhakrishnan <Ragesh.R@linaro.org>, Thomas Petazzoni <thomas.petazzoni@free-electrons.com>, Sunil Joshi <joshi@samsung.com>, Maxime Ripard <maxime.ripard@free-electrons.com>, Vikas Sajjan <vikas.sajjan@linaro.org>
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On Wed, 06 Feb 2013, Tomi Valkeinen <tomi.valkeinen@ti.com> wrote:
>> 6. Miscellaneous
>> ----------------
>> 
>> - If the OMAP3 DSS driver is used as a model for the DSI support 
>> implementation, Daniel Vetter requested the DSI bus lock semaphore to be 
>> killed as it prevents lockdep from working correctly (reference needed ;-)).

[...]

> As for the semaphore, sure, it can be removed, although I'm not aware of
> this lockdep problem. If there's a problem it should be fixed in any case.

The problem is that lockdep does not support semaphores out of the
box. I'm not sure how hard it would be to manually lockdep annotate the
bus lock, and whether it would really work. In any case, as I think we
learned in the past, getting locking right in a DSI command mode panel
driver with an asynchronous update callback, DSI bus lock, and a driver
data specific mutex can be a PITA. Lockdep would be extremely useful
there.

AFAICS simply replacing the semaphore with a mutex would work for all
other cases except DSI command mode display update, unless you're
prepared to wait in the call until the next tearing effect interrupt
plus framedone. Which would suck. I think you and I have talked about
this part in the past...


BR,
Jani.