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Subject: Re: [PATCH v9 11/15] vb2: add in-fence support to QBUF
To:     Ezequiel Garcia <ezequiel@collabora.com>,
        linux-media@vger.kernel.org
Cc:     kernel@collabora.com,
        Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
        Shuah Khan <shuahkh@osg.samsung.com>,
        Pawel Osciak <pawel@osciak.com>,
        Alexandre Courbot <acourbot@chromium.org>,
        Sakari Ailus <sakari.ailus@iki.fi>,
        Brian Starkey <brian.starkey@arm.com>,
        linux-kernel@vger.kernel.org,
        Gustavo Padovan <gustavo.padovan@collabora.com>
References: <20180504200612.8763-1-ezequiel@collabora.com>
 <20180504200612.8763-12-ezequiel@collabora.com>
 <5fd5d7a9-5b74-fe2a-6148-59b90cabb9e8@xs4all.nl>
 <5541e08b048b932789db1c58438c2a2c2b6da7ce.camel@collabora.com>
From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <0a4f2701-a699-3c28-559d-2bf638178b94@xs4all.nl>
Date:   Wed, 9 May 2018 09:04:40 +0200
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On 05/08/2018 09:16 PM, Ezequiel Garcia wrote:
> On Mon, 2018-05-07 at 14:07 +0200, Hans Verkuil wrote:
>> On 04/05/18 22:06, Ezequiel Garcia wrote:
>>> @@ -1421,15 +1505,40 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb)
>>>  	trace_vb2_qbuf(q, vb);
>>>  
>>>  	/*
>>> -	 * If already streaming, give the buffer to driver for processing.
>>> -	 * If not, the buffer will be given to driver on next streamon.
>>> +	 * For explicit synchronization: If the fence didn't signal
>>> +	 * yet we setup a callback to queue the buffer once the fence
>>> +	 * signals and then return successfully. But if the fence
>>> +	 * already signaled we lose the reference we held and queue the
>>> +	 * buffer to the driver.
>>
>> What happens if the fence signaled an error? Is that error returned to userspace?
>> (i.e. VIDIOC_QBUF will fail in that case)
>>
> 
> Hm, good question. If the fence signals with an error, we won't catch it apparently.
> We should fix dma_fence_add_callback to know about signaled vs. error signaled.

OK, so in the meantime we need a comment explaining this in the code. Perhaps as
a FIXME or TODO.

Regards,

	Hans