Re: [PATCH RFC 5/6] media: cedrus: Make the slice_params array size limitation more explicit

From: Boris Brezillon <boris.brezillon@collabora.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: "Nicolas Dufresne" <nicolas@ndufresne.ca>,
	"Jernej Škrabec" <jernej.skrabec@siol.net>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Hans Verkuil" <hans.verkuil@cisco.com>,
	"Laurent Pinchart" <laurent.pinchart@ideasonboard.com>,
	"Sakari Ailus" <sakari.ailus@iki.fi>,
	linux-media@vger.kernel.org, "Tomasz Figa" <tfiga@chromium.org>,
	kernel@collabora.com,
	"Paul Kocialkowski" <paul.kocialkowski@bootlin.com>,
	"Ezequiel Garcia" <ezequiel@collabora.com>,
	"Jonas Karlman" <jonas@kwiboo.se>,
	"Alexandre Courbot" <acourbot@chromium.org>
Subject: Re: [PATCH RFC 5/6] media: cedrus: Make the slice_params array size limitation more explicit
Date: Tue, 4 Jun 2019 10:28:46 +0200	[thread overview]
Message-ID: <20190604102846.7045846d@collabora.com> (raw)
In-Reply-To: <20190604081210.GA9048@ulmo>

On Tue, 4 Jun 2019 10:12:10 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Mon, Jun 03, 2019 at 07:55:48PM -0400, Nicolas Dufresne wrote:
> > Le lundi 03 juin 2019 à 23:48 +0200, Jernej Škrabec a écrit :  
> > > Dne ponedeljek, 03. junij 2019 ob 13:09:45 CEST je Boris Brezillon napisal(a):  
> > > > The driver only supports per-slice decoding, and in that mode
> > > > decode_params->num_slices must be set to 1 and the slice_params array
> > > > should contain only one element.  
> > > 
> > > What Cedrus actually needs to know is if this is first slice in frame or not. I 
> > > imagine it resets some stuff internally when first slice is processed.
> > > 
> > > So if driver won't get all slices of one frame at once, it can't know if this 
> > > is first slice in frame or not. I guess we need additional flag for this.  
> > 
> > A first slice of a frame comes with a new timestamp, so you don't need
> > a flag for that.  
> 
> But slices for the same frame will all have the same timestamp, so we
> can't use the timestamp to tell the individual slices apart.

I think Nicolas was suggesting to keep the last decoded slice timestamp
around and check it against the new slice one. If they are different,
that means we are dealing with a new frame, and this slice is the first
in the frame.

> 
> I mentioned this in that other thread, but I think it'd be useful to
> pass along the number of each of the slices. Drivers can use this in
> order to conceal errors when corrupt slices are detected during the
> decode operation.
> 
> So if we also passed a slice index along with the offset of the slice in
> the bitstream, that should give us enough information to achieve both. A
> slice with index 0 is obviously going to be the first slice in a frame.

There's also the ->first_mb_in_slice field which should be 0 for the
first slice and > 0 for others assuming userspace pass slices in order
(might be an issue if we want to support ASO).