Re: DRM Format Modifiers in v4l2

[Brian Starkey <brian.starkey@arm.com>]

On Wed, Aug 30, 2017 at 11:53:58AM +0200, Hans Verkuil wrote:
>On 30/08/17 11:36, Brian Starkey wrote:
>> On Wed, Aug 30, 2017 at 10:10:01AM +0200, Hans Verkuil wrote:
>>> On 30/08/17 09:50, Daniel Vetter wrote:
>>>> On Tue, Aug 29, 2017 at 10:47:01AM +0100, Brian Starkey wrote:
>>>>> The fact is, adding special formats for each combination is
>>>>> unmanageable - we're talking dozens in the case of our hardware.
>>>>
>>>> Hm right, we can just remap the special combos to the drm-fourcc +
>>>> modifier style. Bonus point if v4l does that in the core so not everyone
>>>> has to reinvent that wheel :-)
>>>
>>> Probably not something we'll do: there are I believe only two drivers that
>>> are affected (exynos & mediatek), so they can do that in their driver.
>>>
>>> Question: how many modifiers will typically apply to a format? I ask
>>> because I realized that V4L2 could use VIDIOC_ENUMFMT to make the link
>>> between a fourcc and modifiers:
>>>
>>> https://hverkuil.home.xs4all.nl/spec/uapi/v4l/vidioc-enum-fmt.html
>>>
>>> The __u32 reserved[4] array can be used to provide a bitmask to modifier
>>> indices (for the integer menu control). It's similar to what drm does,
>>> except instead of modifiers mapping to fourccs it is the other way around.
>>>
>>> This would avoid having to change the modifiers control whenever a new
>>> format is set and it makes it easy to enumerate all combinations.
>>>
>>> But this only works if the total number of modifiers used by a single driver
>>> is expected to remain small (let's say no more than 64).
>>
>> In our current (yet to be submitted) description, we've got around a
>> dozen modifiers for any one format to describe our compression
>> variants. We have a lot of on/off toggles which leads to combinatorial
>> expansion, so it can grow pretty quickly (though I am trying to limit
>> the valid combinations as much as possible).
>>
>> How about if the mask fills up then VIDIOC_ENUM_FMT can return another
>> fmtdsc with the same FourCC and different modifier bitmask, where the
>> second one's modifier bitmask is for the next "N" modifiers?
>
>I was thinking along similar lines, but it could cause some problems with
>the ABI since applications currently assume that no fourcc will appear
>twice when enumerating formats. Admittedly, we never explicitly said in
>the spec that that can't happen, but it is kind of expected.
>
>There are ways around that, but if possible I'd like to avoid that.
>
>In theory there are up to 128 bits available but I can't help thinking
>that if you create more than, say, 64 modifiers for a HW platform you
>have a big mess anyway.
>
>If I am wrong, then I need to know because then I can prepare for it
>(or whoever is going to actually implement this...)

You're probably right, but I can't speak for everyone. From the
current state of drm_fourcc.h it looks like 64 would be plenty (there
aren't anywhere near 64 modifiers even defined right now). Adding in
the Arm compression formats will expand it a lot, but still not to 64
(yet).

>
>If the number of modifiers is expected to be limited then making 64 bits
>available would be good enough, at least for now.
>
>BTW, is a modifier always optional? I.e. for all fourccs, is the unmodified
>format always available? Or are there fourccs that require the use of a
>modifier?

We do actually have one or two formats which are only supported with a
modifier (on our HW).

-Brian

>
>Regards,
>
>	Hans