linux-kernel - Re: [PATCH] media: cedrus: Remove global IRQ spin lock from the driver

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Message-ID: <9aa0953ea2d6c3f5363c811efe7586b705a2ee28.camel@bootlin.com>
Date:   Thu, 29 Nov 2018 10:02:15 +0100
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
Cc:     linux-media@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-sunxi@...glegroups.com, Hans Verkuil <hverkuil@...all.nl>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: [PATCH] media: cedrus: Remove global IRQ spin lock from the
 driver

Hi,

On Fri, 2018-11-16 at 11:47 +0100, Maxime Ripard wrote:
> On Thu, Nov 15, 2018 at 03:39:55PM +0100, Paul Kocialkowski wrote:
> > We initially introduced a spin lock to ensure that the VPU registers
> > are not accessed concurrently between our setup function and IRQ
> > handler. Because the V4L2 M2M API only allows one job to run at a time
> > and our jobs are completed following the IRQ handler, there is actually
> > no chance of an interrupt happening while programming the VPU registers.
> 
> That's not entirely true. There's no chance that the interrupt
> signaling the end of the frame decoding can happen.
> 
> However, spurious interrupts can happen at any point in time as soon
> as you have the interrupts enabled.
> 
> > In addition, holding a spin lock is problematic when doing more than
> > simply configuring the registers in the setup operation. H.265 support
> > currently involves a CMA allocation in that step, which is not
> > compatible with an atomic context.
> 
> That's not really true either. Any allocation can be done with
> GFP_ATOMIC or GFP_NOWAIT, and be compatible with an atomic
> context. Whether it's something we want is a different story :)
> 
> And since the h265 code isn't upstream, I'm not really sure it's
> relevant to mention it here.

Thanks for your comments, I have just made associated changes and sent
out v2.

Cheers!

 * Paul

> > As a result, remove the global IRQ spin lock.
> > 
> > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@...tlin.com>
> 
> Acked-by: Maxime Ripard <maxime.ripard@...tlin.com>
> 
> Maxime
> 
-- 
Paul Kocialkowski, Bootlin (formerly Free Electrons)
Embedded Linux and kernel engineering
https://bootlin.com

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