linux-kernel - Re: [PATCH v4 5/8] [Media] vcodec: mediatek: Add Mediatek V4L2 Video Encoder Driver

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Message-ID: <1455697992.26202.13.camel@mtksdaap41>
Date:	Wed, 17 Feb 2016 16:33:12 +0800
From:	tiffany lin <tiffany.lin@...iatek.com>
To:	Hans Verkuil <hverkuil@...all.nl>
CC:	Hans Verkuil <hans.verkuil@...co.com>,
	<daniel.thompson@...aro.org>, "Rob Herring" <robh+dt@...nel.org>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Daniel Kurtz <djkurtz@...omium.org>,
	Pawel Osciak <posciak@...omium.org>,
	Eddie Huang <eddie.huang@...iatek.com>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-media@...r.kernel.org>,
	<linux-mediatek@...ts.infradead.org>, <PoChun.Lin@...iatek.com>,
	Andrew-CT Chen <andrew-ct.chen@...iatek.com>
Subject: Re: [PATCH v4 5/8] [Media] vcodec: mediatek: Add Mediatek V4L2
 Video Encoder Driver

Hi Hans,

On Wed, 2016-02-17 at 08:47 +0100, Hans Verkuil wrote:
> On 02/16/2016 07:37 AM, tiffany lin wrote:
> >>> +static int fops_vcodec_open(struct file *file)
> >>> +{
> >>> +	struct video_device *vfd = video_devdata(file);
> >>> +	struct mtk_vcodec_dev *dev = video_drvdata(file);
> >>> +	struct mtk_vcodec_ctx *ctx = NULL;
> >>> +	int ret = 0;
> >>> +
> >>> +	mutex_lock(&dev->dev_mutex);
> >>> +
> >>> +	ctx = devm_kzalloc(&dev->plat_dev->dev, sizeof(*ctx), GFP_KERNEL);
> >>> +	if (!ctx) {
> >>> +		ret = -ENOMEM;
> >>> +		goto err_alloc;
> >>> +	}
> >>> +
> >>> +	if (dev->num_instances >= MTK_VCODEC_MAX_ENCODER_INSTANCES) {
> >>> +		mtk_v4l2_err("Too many open contexts\n");
> >>> +		ret = -EBUSY;
> >>> +		goto err_no_ctx;
> >>
> >> Hmm. I never like it if you can't open a video node because of a reason like this.
> >>
> >> I.e. a simple 'v4l2-ctl -D' (i.e. calling QUERYCAP) should never fail.
> >>
> >> If there are hardware limitation that prevent more than X instances from running at
> >> the same time, then those limitations typically kick in when you start to stream
> >> (or possibly when calling REQBUFS). But before that it should always be possible to
> >> open the device.
> >>
> >> Having this check at open() is an indication of a poor design.
> >>
> >> Is this is a hardware limitation at all?
> >>
> > This is to make sure performance meet requirements, such as bitrate and
> > framerate.
> 
> Is it the driver's job to make enforce this? What if the application only
> deals with low-res video, but wants to encode a lot of those? Or is encoding
> a video off-line?
> 
> The driver generally doesn't know the use-case, so if this is an artificial
> limitation as opposed to a hardware limitation, then I would just drop this.
> 
We got your point, we will remove this artificial limitation in next
version.

best regards,
Tiffany


> Regards,
> 
> 	Hans
> 
> > We got your point. We will remove this and move limitation control to
> > start_streaming or REQBUFS.
> > Appreciated for your suggestion.:)
> > 
> > 
> >>> +	}
>