Em 06-07-2011 08:31, Hans Verkuil escreveu:
>> Em 05-07-2011 10:20, Hans Verkuil escreveu:
>>
>>>> I failed to see what information is provided by the "presets" name. If
>>>> this were removed
>>>> from the ioctl, and fps would be added instead, the API would be
>>>> clearer. The only
>>>> adjustment would be to use "index" as the preset selection key. Anyway,
>>>> it is too late
>>>> for such change. We need to live with that.
>>>
>>> Adding the fps solves nothing. Because that still does not give you
>>> specific timings.
>>> You can have 1920x1080P60 that has quite different timings from the
>>> CEA-861 standard
>>> and that may not be supported by a TV.
>>>
>>> If you are working with HDMI, then you may want to filter all supported
>>> presets to
>>> those of the CEA standard.
>>>
>>> That's one thing that is missing at the moment: that presets belonging
>>> to a certain
>>> standard get their own range. Since we only do CEA861 right now it
>>> hasn't been an
>>> issue, but it will.
>>
>> I prepared a long email about that, but then I realized that we're
>> investing our time into
>> something broken, at the light of all DV timing standards. So, I've
>> dropped it and
>> started from scratch.
>>
>> From what I've got, there are some hardware that can only do a limited set
>> of DV timings.
>> If this were not the case, we could simply just use the
>> VIDIOC_S_DV_TIMINGS/VIDIOC_G_DV_TIMINGS,
>> and put the CEA 861 and VESA timings into some userspace library.
>>
>> In other words, the PRESET API is meant to solve the case where hardware
>> only support
>> a limited set of frequencies, that may or may not be inside the CEA
>> standard.
>>
>> Let's assume we never added the current API, and discuss how it would
>> properly fulfill
>> the user needs. An API that would likely work is:
>>
>> struct v4l2_dv_enum_preset2 {
>> __u32 index;
>> __u8 name[32]; /* Name of the preset timing */
>>
>> struct v4l2_fract fps;
>>
>> #define DV_PRESET_IS_PROGRESSIVE 1<<31
>> #define DV_PRESET_SPEC(flag) (flag && 0xff)
>> #define DV_PRESET_IS_CEA861 1
>> #define DV_PRESET_IS_DMT 2
>> #define DV_PRESET_IS_CVF 3
>> #define DV_PRESET_IS_GTF 4
>> #define DV_PRESET_IS_VENDOR_SPECIFIC 5
>>
>> __u32 flags; /* Interlaced/progressive, DV specs, etc */
>>
>> __u32 width; /* width in pixels */
>> __u32 height; /* height in lines */
>> __u32 polarities; /* Positive or negative polarity */
>> __u64 pixelclock; /* Pixel clock in HZ. Ex. 74.25MHz->74250000 */
>> __u32 hfrontporch; /* Horizpontal front porch in pixels */
>> __u32 hsync; /* Horizontal Sync length in pixels */
>> __u32 hbackporch; /* Horizontal back porch in pixels */
>> __u32 vfrontporch; /* Vertical front porch in pixels */
>> __u32 vsync; /* Vertical Sync length in lines */
>> __u32 vbackporch; /* Vertical back porch in lines */
>> __u32 il_vfrontporch; /* Vertical front porch for bottom field of
>> * interlaced field formats
>> */
>> __u32 il_vsync; /* Vertical sync length for bottom field of
>> * interlaced field formats
>> */
>> __u32 il_vbackporch; /* Vertical back porch for bottom field of
>> * interlaced field formats
>> */
>> __u32 reserved[4];
>> };
>>
>> #define VIDIOC_ENUM_DV_PRESETS2 _IOWR('V', 83, struct
>> v4l2_dv_enum_preset2)
>> #define VIDIOC_S_DV_PRESET2 _IOWR('V', 84, u32 index)
>> #define VIDIOC_G_DV_PRESET2 _IOWR('V', 85, u32 index)
>>
>> Such preset API seems to work for all cases. Userspace can use any DV
>> timing
>> information to select the desired format, and don't need to have a switch
>> for
>> a preset macro to try to guess what the format actually means. Also,
>> there's no
>> need to touch at the API spec every time a new DV timeline is needed.
>>
>> Also, it should be noticed that, since the size of the data on the above
>> definitions
>> are different than the old ones, _IO macros will provide a different magic
>> number,
>> so, adding these won't break the existing API.
>>
>> So, I think we should work on this proposal, and mark the existing one as
>> deprecated.
>
> This proposal makes it very hard for applications to directly select a
> format like 720p50 because the indices can change at any time.
Why? All the application needs to do is to call VIDIOC_ENUM_DV_PRESETS2,
check what line it wants, and do a S_DV_PRESET2, just like any other place
where V4L2 defines an ENUM function.
The enum won't change during application runtime, so, they can be stored
if the application would need to switch to other formats latter.
> I think
> this is a very desirable feature, particularly for apps running on
> embedded systems where the hardware is known. This was one of the design
> considerations at the time this API was made.
This is a very weak argument. With just one ENUM loop, the application can
quickly get the right format(s), and associate them with any internal
namespace.
> But looking at this I wonder if we shouldn't just make a
> VIDIOC_G_PRESET_TIMINGS function? You give it the preset ID and you get
> all the timing information back. No need to deprecate anything. I'm not
> even sure if with this change we need to modify struct v4l2_dv_enum_preset
> as I proposed in my RFC, although I think we should.
Won't solve the issue: one new #define is needed for each video timing,
namespaces will be confusing, no support for VESA GVF/ VESA CVT timings
(or worse: we'll end by having thousands of formats at the end of the day),
instead of just one ENUM ioctl, an extra ioctl will be required for each
returned value, etc.
Cheers,
Mauro.
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