Re: Documentation issues with some MBUS flags

Sakari Ailus
2016-07-20T20:45:49Z

Hi Mauro,

Mauro Carvalho Chehab wrote:
> Hi Sylwester/Sakari,
>
> While checking the docs for the V4L2 framework, I noticed something weird
> Related to those two flags:
>
> #define V4L2_MBUS_FRAME_DESC_FL_LEN_MAX (1U << 0)
> #define V4L2_MBUS_FRAME_DESC_FL_BLOB (1U << 1)
>
> They were originally introduced by this changeset:
>
> commit 291031192426bfc6ad4ab2eb9fa986025a926598
> Author: Sylwester Nawrocki <s.nawrocki@samsung.com>
> Date: Thu May 17 14:33:30 2012 -0300
>
> [media] V4L: Add [get/set]_frame_desc subdev callbacks
>
> Add subdev callbacks for setting up parameters of the frame on media bus
> that are not exposed to user space directly. This is just an initial,
> mostly stub implementation. struct v4l2_mbus_frame_desc is intended
> to be extended with sub-structures specific to a particular hardware media
> bus. For now these new callbacks are used only to query or specify maximum
> size of a compressed or hybrid (container) media bus frame in octets.
>
> Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
>
> And the comments were modified by this one:
>
> commit 174d6a39b07f51212c23a8e30a0440598d18392c
> Author: Sakari Ailus <sakari.ailus@linux.intel.com>
> Date: Wed Dec 18 08:40:28 2013 -0300
>
> [media] v4l: V4L2_MBUS_FRAME_DESC_FL_BLOB is about 1D DMA
>
> V4L2_MBUS_FRAME_DESC_FL_BLOB intends to say the receiver must use 1D DMA to
> receive the image, as the format does not have line offsets. This typically
> includes all compressed formats.
>
> Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>
> Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
>
> The problem is that the description of V4L2_MBUS_FRAME_DESC_FL_LEN_MAX
> says that:
> Indicates the @length field specifies maximum data length.
>
> But the description of the @length field says otherwise:
> * @length: number of octets per frame, valid if V4L2_MBUS_FRAME_DESC_FL_BLOB
>
> So, I decided to take a look on what the heck is that:
>
> $ git grep V4L2_MBUS_FRAME_DESC_FL_BLOB
> include/media/v4l2-subdev.h:#define V4L2_MBUS_FRAME_DESC_FL_BLOB (1U << 1)
> include/media/v4l2-subdev.h: * %V4L2_MBUS_FRAME_DESC_FL_BLOB.
> include/media/v4l2-subdev.h: * @length: number of octets per frame, valid if V4L2_MBUS_FRAME_DESC_FL_BLOB
>
> (nobody is using it)
>
> And:
>
> $ git grep V4L2_MBUS_FRAME_DESC_FL_LEN_MAX
> drivers/media/i2c/m5mols/m5mols_core.c: fd->entry[0].flags = V4L2_MBUS_FRAME_DESC_FL_LEN_MAX;
> drivers/media/i2c/m5mols/m5mols_core.c: fd->entry[0].flags = V4L2_MBUS_FRAME_DESC_FL_LEN_MAX;
> include/media/v4l2-subdev.h:#define V4L2_MBUS_FRAME_DESC_FL_LEN_MAX (1U << 0)
>
> Only one driver is using it.
>
> So, I'm thinking if are there any reason why to keep the
> V4L2_MBUS_FRAME_DESC_FL_BLOB, and what's the expected behavior
> when V4L2_MBUS_FRAME_DESC_FL_LEN_MAX is found.
>
> Could you shed some light?

There isn't really a problem that I could see here, except that the
m5mols driver should perhaps set the BLOB flag to indicate it's using
1-dimensional DMA.

What comes to the flags, the LEN_MAX flag indicates that the length
specified in the frame descriptor is the maximum length whereas the real
amount of data could be less than that. The BLOB flag indicates
1-dimensional DMA, but no driver (yet) uses two-dimensional DMA with
frame descriptors. The patch adding two-dimensional support is here:

<URL:http://git.retiisi.org.uk/?p=~sailus/linux.git;a=commitdiff;h=b701bd4160410739b165e19327bb64ce25fc509d>

AFAIR I posted it to linux-media long time ago but the set currently is
still unfinished. It will be needed to add support for sensor embedded
data, for instance. Certain newer sensors also provide more distinct
types of data than the usual (embedded data and image data).

--
Kind regards,

Sakari Ailus
sakari.ailus@linux.intel.com