linux-kernel - Re: [PATCH v3 2/2] media: platform: Add Aspeed Video Engine driver

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Message-ID: <373d2ca3-a6cd-8d87-2bbc-613d65305170@xs4all.nl>
Date:   Thu, 4 Oct 2018 15:12:34 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Eddie James <eajames@...ux.vnet.ibm.com>,
        Eddie James <eajames@...ux.ibm.com>,
        linux-kernel@...r.kernel.org
Cc:     mark.rutland@....com, devicetree@...r.kernel.org,
        linux-aspeed@...ts.ozlabs.org, andrew@...id.au,
        openbmc@...ts.ozlabs.org, robh+dt@...nel.org, mchehab@...nel.org,
        linux-media@...r.kernel.org
Subject: Re: [PATCH v3 2/2] media: platform: Add Aspeed Video Engine driver

On 10/03/18 22:43, Eddie James wrote:
> 
> 
> On 09/28/2018 06:30 AM, Hans Verkuil wrote:
>> On 09/25/2018 09:27 PM, Eddie James wrote:
>>> The Video Engine (VE) embedded in the Aspeed AST2400 and AST2500 SOCs
>>> can capture and compress video data from digital or analog sources. With
>>> the Aspeed chip acting a service processor, the Video Engine can capture
>>> the host processor graphics output.
>>>
>>> Add a V4L2 driver to capture video data and compress it to JPEG images.
>>> Make the video frames available through the V4L2 streaming interface.
>>>
>>> Signed-off-by: Eddie James <eajames@...ux.ibm.com>
> 
>>> +	}
>>> +
>>> +	video->height = (bottom - top) + 1;
>>> +	video->width = (right - left) + 1;
>>> +	size = video->height * video->width;
>> It looks like you can actually determine the blanking width/height and
>> possibly even more detailed information that would be very useful to
>> show with the DV_TIMINGS ioctls.
> 
> Hmm. This information is related to the video signal captured from the 
> host. That information has nothing to do with the buffer that is 
> compressed and grabbed by the driver and ultimately provided to 
> userspace. Isn't the timing information meaningless for JPEG frames?

It helps in debugging. Basically you are implementing a receiver for a
video signal. So if for some reason you cannot support the video timings
that the host sends, then it is very useful to have QUERY_DV_TIMINGS report
as much information about the signal as possible.

BTW, out of curiosity, how are the host video signals connected to the
aspeed? Is it still a VGA video signal?

Looking at product briefs it appears that it is VGA. So I guess the aspeed
'sniffs' the VGA signals from the host and can capture the video that way.
Is that correct?

If so, then this driver is a VGA receiver and should act like that.
The host can configure its VGA transmitter to invalid timings, or weird
values, and you need to be able to handle that in your driver.

> Forgot to include this question in my previous reply, sorry for the 
> additional mail.

No problem! Happy to help.

Regards,

	Hans