[PATCH 03/18] media controller: use MEDIA_ENT_T_AV_DMA for A/V DMA engines

Hans Verkuil hverkuil at xs4all.nl
Fri May 8 05:57:02 PDT 2015

Hi Mauro,

On 05/08/2015 02:32 PM, Mauro Carvalho Chehab wrote:
>>> and may also not be true on embedded devices
>>> that, due to DRM reasons, don't allow writing unencrypted
>>> data on a memory that could be seen by the CPU.
>> This actually might still work by using opaque DMABUF handles. But that's
>> under discussion right now in the Secure Data Path thread.
> Well, a DMABUF opaque handler like that actually refers to either a
> buffer that is shared only between 2 devices or to a device-to-device
> DMA transfer.
> Such dataflow is different than the usual meaning of the video devnode,
> where the devnode is used to do I/O transfers. So, it may actually 
> be mapped as a different type of entity.
> We'll need to discuss further when we start mapping this via MC.

Yes, this is quite theoretical at the moment.

>>> So, we'll eventually need to add another entity for such
>>> bridge chipsets that have a video/vbi/radio device node
>>> associated, but don't have DMA engines on (some) devnodes.
>>> As, currently, we don't have any such case,
>> ??? Radio devices are exactly that.
> I actually meant to say:
> 	"As, currently, no driver uses Media Controller on such cases,"

Ah, OK. That makes more sense :-)

>>> let's for now
>>> just rename the device nodes that are associated with a
>>> DMA engine as MEDIA_ENT_T_AV_DMA.
>>> So,
>>> PS.: This is not actually true for USB devices, as the
>>> DMA engine is an internal component, as it is up to the
>>> Kernel to strip the stream payload from the URB packages.
>> How about MEDIA_ENT_T_DATA_STREAMING? Or perhaps DATA_IO? Perhaps even just
>> "IO"?
> Almost entities do I/O and data streaming (exceptions are flash controller,
> SEC and similar control subdevices). So, DATA_STREAMING, DATA_IO or IO
> are a way too generic.

For some of our products we have lots of video nodes that just control the
receiver or transmitter (and possibly other related blocks), but leave the
streaming to vendor code where we are forced to use that. So this can be
a lot more common than you might think, although you won't see that appearing
in the kernel.

> DMA is a little bit more restrictive than we wanted.
> Actually, I originally named those as MEDIA_ENT_T_AV_BRIDGE, because
> the hardware component that implements the device->CPU I/O is typically
> a bridge. But then I went into the drivers, and I noticed that several
> devices with just one bridge have several different entities for I/O.
> So, I ran this script:
> 	$ git filter-branch -f --msg-filter 'cat && sed s,MEDIA_ENT_T_AV_BRIDGE,MEDIA_ENT_T_AV_DMA,g' origin/patchwork..
> 	$ git filter-branch -f --tree-filter 'for i in $(git grep -l MEDIA_ENT_T_AV_BRIDGE); do sed s,MEDIA_ENT_T_AV_BRIDGE,MEDIA_ENT_T_AV_DMA,g $i >a && mv a $i; done' origin/patchwork..
> To replace the name everywere. Provided that we decide a better name,
> this can be easily fixable by doing the above scripting.
> Perhaps MEDIA_ENT_T_DEV_CPU_AV_IO would be a better name?
>> That would cover USB as well, and I dislike the use of "AV", since the
>> data might contain other things besides audio and/or video.
> True, but how to distinguish such device from an ALSA DEV/CPU IO?
> Answering myself, I see one alternative for that... we could use
> MEDIA_ENT_T_DEV_CPU_IO for all device->CPU I/O devices, and use
> properties to tell what APIs are valid on such entity. The bad thing
> is that someone could add multiple "incompatible" APIs at the same
> device (like ALSA, V4L and DVB - all on the dame sevnode).
> I'm running out of ideas here ;)
> In the lack of a better name, I would keep MEDIA_ENT_T_AV_DMA, as
> it is the closest one to what's provided by such entities (or the
> less wrong one).

See my reply to patch 10/18: I see whether streaming is supported or not as a
function (or feature) of the entity, just like a subdev entity can be both
a tuner and a video decoder. I.e. these are properties.

I think the naming issue here shows the problem with your approach. Whereas
having a 'streaming' property simplifies this IMHO.



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