DRM Format Modifiers in v4l2

[Nicolas Dufresne]

Le jeudi 24 août 2017 à 13:26 +0100, Brian Starkey a écrit :
> > What I mean was: an application can use the modifier to give buffers from
> > one device to another without needing to understand it.
> > 
> > But a generic video capture application that processes the video itself
> > cannot be expected to know about the modifiers. It's a custom HW specific
> > format that you only use between two HW devices or with software written
> > for that hardware.
> > 
> 
> Yes, makes sense.
> 
> > > 
> > > However, in DRM the API lets you get the supported formats for each
> > > modifier as-well-as the modifier list itself. I'm not sure how exactly
> > > to provide that in a control.
> > 
> > We have support for a 'menu' of 64 bit integers: V4L2_CTRL_TYPE_INTEGER_MENU.
> > You use VIDIOC_QUERYMENU to enumerate the available modifiers.
> > 
> > So enumerating these modifiers would work out-of-the-box.
> 
> Right. So I guess the supported set of formats could be somehow
> enumerated in the menu item string. In DRM the pairs are (modifier +
> bitmask) where bits represent formats in the supported formats list
> (commit db1689aa61bd in drm-next). Printing a hex representation of
> the bitmask would be functional but I concede not very pretty.

The problem is that the list of modifiers depends on the format
selected. Having to call S_FMT to obtain this list is quite
inefficient.

Also, be aware that DRM_FORMAT_MOD_SAMSUNG_64_32_TILE modifier has been
implemented in V4L2 with a direct format (V4L2_PIX_FMT_NV12MT). I think
an other one made it the same way recently, something from Mediatek if
I remember. Though, unlike the Intel one, the same modifier does not
have various result depending on the hardware revision.

regards,
Nicolas

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