On 02/12/2013 08:59 AM, Hans Verkuil wrote:
On Mon February 11 2013 23:32:33 Sylwester Nawrocki wrote:
...
1. Is the event payload supposed to be relatively small and the interface
is deliberately defined to disallow passing anything with the payload
greater than 64 B ? In order to keep it a rather lightweight interface
and anything that needs more data should use other/new ioctls ?
Yes, that was the original design philisophy. In particular because events
can be generated from interrupt context and you cannot allocate memory in
interrupt context. Note that the original design had one event queue for
each filehandle containing all types of events. That made it basically
impossible to have variable sized payloads without having to allocate
memory for each payload.
I see, but I think Linux allows allocating memory in atomic contexts and
one example of it is [1] (function read_faces()). I'm not saying it is best
possible approach but with GFP_ATOMIC flag it is possible to use kmalloc
in interrupt handlers.
$ git grep -1 GFP_ATOMIC include/linux/slab.h
include/linux/slab.h- *
include/linux/slab.h: * %GFP_ATOMIC - Allocation will not sleep. May
use emergency pools.
include/linux/slab.h- * For example, use this inside interrupt handlers.
[1] http://patchwork.linuxtv.org/patch/8703
The idea that I had in mind was that if you need larger payloads then
the event should provide a cookie of some sort that you can use with another
ioctl to get hold of the full payload.
I was also considering something similar, but not with a generic ioctl to
dequeue payload of an event.
The later redesign (one queue per filehandle per event type) would have
made that much easier since you could allocate the needed payload data
when the event is subscribed, but by then the ioctl was already defined as
IOR.
2. If answer to 1. is 'no', then what would be a best way to proceed to
make the event's payload more flexible ? Would creating a new ioctl to
dequeue events be way to go ?
I am asking mostly in context of the face detection feature in the
Exynos4x12 SoC camera ISP. Similarly, the v4l2 event payload size was a
limitation during development of a driver for the face detection IP block
available in OMAP4 SoCs by Ming Lei [2]:
"From the start, I hope that the event interface can be used to retrieve
object detection result.
When I found it is difficult to fit 'struct v4l2_od_object' into 64 bytes,
I decide to introduce two IOCTLs for the purpose."
I thought it would have been better to make the event interface more
flexible and reuse the existing infrastructure, rather than inventing new
ioctls for the purpose and reimplementing similar set of features.
Any suggestions, thoughts are warm welcome.
I don't think changing DQEVENT is the right approach. While possible, it
would create more confusion than it solves IMHO. What might be better (just
brainstorming here) is to add a DQEVENT_PAYLOAD ioctl. The DQEVENT will give
you the required size of the payload and the sequence number can be used as
the cookie. Only the payload of the last dequeued event can be retrieved
that way, which shouldn't be an issue as far as I can tell.
It would be nice to be able to dequeue multiple event payloads in one go,
I guess if there are multiple events of same type it could be possible to
allow user space to dequeue further payloads. This would be useful in case
there are multiple objects detected and only one interrupt is raised.
This would save some syscalls, however I'm not sure yet if it is worth the
complication of the interface.
Hmm, strictly speaking you do not need the sequence number if you just return
the payload of the last event, but it's probably a good sanity check.
Internally this can be implemented by allocating the payload memory when the
event is subscribed or when the event is generated. The first method is best
if events need to be generated during interrupt context, the second method
is best if the payload can be large and differs in size for each event. Of
course, in that case the event can never be generated from interrupt context.
You probably want to have the choice which method to use.
Agreed, such an option sounds like something good to have.
I will likely give this idea a try at the beginning of next month.
Thank you for the pointers.
--
Regards,
Sylwester
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