Re: [RFC 0/3] Frame synchronisation events and support for them in the OMAP 3 ISP driver

[Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>]

Sylwester Nawrocki wrote:
> Hi Sakari,

Hi Sylwester,

Thanks for the comments.

> On 07/19/2011 03:37 PM, Sakari Ailus wrote:
>> Hi all,
>>
>> The OMAP 3 ISP driver implements an HS_VS event which is triggered when
>> the reception of a frame begins. This functionality is very, very likely
>> not specific to OMAP 3 ISP so it should be standardised.
>>
>> I have a few patches to do that. Additionally the next expected buffer
>> sequence number is provided with the event, unlike earlier.
>>
>> There are a few open questions, however, and this is why I'm sending the
>> set as RFC.
>>
>>
>> 1) Other frame synchronisation events. The CCDC block in the OMAP 3 ISP
>> is able to trigger interrupts at two chosen lines of the image. These
>> naturally can be translated to events. The driver uses both of them
>> internally at specific points of the frame. Nevertheless, there might be
>> some use for these in user space. Other hardware might implement a
>> number of these which wouldn't be used by the driver itself, but I don't
>> know of that at the moment. On the other hand high resolution timers are
>> also available in user space, so doing timing based on ISP provided
>> events is not quite as important as before --- as long as there's one
>> frame based event produced at a known time, such as V4L2_EVENT_FRAME_START.
> 
> I'm curious, have you perhaps tried to measure latency of such up calls
> to a user space process? I mean this is going to be a real time stuff,
> with HSYNC periods of 50 us order. Could a user space thread be receiving
> such periodic events reliably ? From my experience I doubt this can work
> reliably outside of an interrupt handler even with high priority real time
> threads.
> 
> V4L2_EVENT_FRAME_START event seems OK, but HSYNC events in user space
> sound rather tricky to me :-)

I think the user space could be interested in just one or two of these
per frame, not for every line. But how to subscribe them --- if they are
needed?

Perhaps it'd be better to start with just one and add more once necessary?

> Also HS_VS looks a bit more descriptive than FRAME_START for me.

HS_VS doesn't really tell which one it is (horizontal or vertical), and
we already have a VSYNC event but it's used for a different purpose.
HS_VS is specific to the CCDC block and doesn't have that meaning in
context of serial interfaces.

This is why I proposed FRAME_START.

> But unfortunately I can't come up with a better name, e.g. something like
> V4L2_EVENT_FRAME_AV_START - frame active video start. Just in case in
> future there are more specific events added.

What additional information would AV add which isn't evident from
FRAME_START?

I admit that there could be differencies in terminology used in this
area; terms that are meaningful to some might not be to others, or they
could mean different things to them.

Regards,

-- 
Sakari Ailus
sakari.ailus@maxwell.research.nokia.com