linux-kernel - Re: [PATCH v3 1/2] media: imx: csi: Disable SMFC before disabling IDMA channel

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Date:   Tue, 22 Jan 2019 10:58:16 +0100
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Steve Longerbeam <slongerbeam@...il.com>,
        linux-media@...r.kernel.org
Cc:     Gael PORTAY <gael.portay@...labora.com>,
        Peter Seiderer <ps.report@....net>, stable@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "open list:STAGING SUBSYSTEM" <devel@...verdev.osuosl.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] media: imx: csi: Disable SMFC before disabling
 IDMA channel

On Mon, 2019-01-21 at 10:46 -0800, Steve Longerbeam wrote:
> 
> On 1/21/19 10:43 AM, Steve Longerbeam wrote:
> > 
> > 
> > On 1/21/19 3:49 AM, Philipp Zabel wrote:
> > > Also ipu_smfc_disable is refcounted, so if the other CSI is capturing
> > > simultaneously, this change has no effect.
> > 
> > Sigh, you're right. Let me go back to disabling the CSI before the 
> > channel, the CSI enable/disable is not refcounted (it doesn't need to 
> > be since it is single use) so it doesn't have this problem.
> > 
> > Should we drop this patch or keep it (with a big comment)? By only 
> > changing the disable order to "CSI then channel", the hang is reliably 
> > fixed from my and Gael's testing, but my concern is that by not 
> > disabling the SMFC before the channel, the SMFC could still empty its 
> > FIFO to the channel's internal FIFO and still create a hang.
> 
> Well, as you said it will have no effect if both CSI's are streaming 
> with the SMFC, in which case the danger would still exist. Perhaps it 
> would be best to just drop this patch.

Hm, if we can't guarantee the intended effect with this patch, and
stopping the CSI first helps reliably, it's indeed better to just do
that instead.

regards
Philipp