Message-ID: <CAKMK7uEotYyRaa4WqNKRGc0nfwcyGccRpX2YzZmETrPdgXkAKA@mail.gmail.com>
Date:   Wed, 26 Jun 2019 21:30:26 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Brian Starkey <Brian.Starkey@....com>
Cc:     Raymond Smith <Raymond.Smith@....com>,
        "maarten.lankhorst@...ux.intel.com" 
        <maarten.lankhorst@...ux.intel.com>,
        "maxime.ripard@...tlin.com" <maxime.ripard@...tlin.com>,
        "sean@...rly.run" <sean@...rly.run>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yuq825@...il.com" <yuq825@...il.com>,
        Ayan Halder <Ayan.Halder@....com>,
        "malidp@...s.arm.com" <malidp@...s.arm.com>, nd <nd@....com>
Subject: Re: [PATCH] drm/fourcc: Add Arm 16x16 block modifier

On Mon, Jun 24, 2019 at 1:23 PM Brian Starkey <Brian.Starkey@....com> wrote:
>
> On Mon, Jun 24, 2019 at 11:58:59AM +0200, Daniel Vetter wrote:
> > On Mon, Jun 24, 2019 at 11:32 AM Brian Starkey <Brian.Starkey@....com> wrote:
> > >
> > > Hi Daniel,
> > >
> > > On Fri, Jun 21, 2019 at 05:27:00PM +0200, Daniel Vetter wrote:
> > > > On Fri, Jun 21, 2019 at 12:21 PM Raymond Smith <Raymond.Smith@....com> wrote:
> > > > >
> > > > > Add the DRM_FORMAT_MOD_ARM_16X16_BLOCK_U_INTERLEAVED modifier to
> > > > > denote the 16x16 block u-interleaved format used in Arm Utgard and
> > > > > Midgard GPUs.
> > > > >
> > > > > Signed-off-by: Raymond Smith <raymond.smith@....com>
> > > > > ---
> > > > >  include/uapi/drm/drm_fourcc.h | 10 ++++++++++
> > > > >  1 file changed, 10 insertions(+)
> > > > >
> > > > > diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> > > > > index 3feeaa3..8ed7ecf 100644
> > > > > --- a/include/uapi/drm/drm_fourcc.h
> > > > > +++ b/include/uapi/drm/drm_fourcc.h
> > > > > @@ -743,6 +743,16 @@ extern "C" {
> > > > >  #define AFBC_FORMAT_MOD_BCH     (1ULL << 11)
> > > > >
> > > > >  /*
> > > > > + * Arm 16x16 Block U-Interleaved modifier
> > > > > + *
> > > > > + * This is used by Arm Mali Utgard and Midgard GPUs. It divides the image
> > > > > + * into 16x16 pixel blocks. Blocks are stored linearly in order, but pixels
> > > > > + * in the block are reordered.
> > > > > + */
> > > > > +#define DRM_FORMAT_MOD_ARM_16X16_BLOCK_U_INTERLEAVED \
> > > > > +       fourcc_mod_code(ARM, ((1ULL << 55) | 1))
> > > >
> > > > This seems to be an extremely random pick for a new number. What's the
> > > > thinking here? Aside from "doesnt match any of the afbc combos" ofc.
> > > > If you're already up to having thrown away 55bits, then it's not going
> > > > to last long really :-)
> > > >
> > > > I think a good idea would be to reserve a bunch of the high bits as
> > > > some form of index (afbc would get index 0 for backwards compat). And
> > > > then the lower bits would be for free use for a given index/mode. And
> > > > the first mode is probably an enumeration, where possible modes simple
> > > > get enumerated without further flags or anything.
> > >
> > > Yup, that's the plan:
> > >
> > >         (0 << 55): AFBC
> > >         (1 << 55): This "non-category" for U-Interleaved
> > >         (1 << 54): Whatever the next category is
> > >         (3 << 54): Whatever comes after that
> > >         (1 << 53): Maybe we'll get here someday
> >
> > Uh, so the index would be encoded with least-significant bit first,
> > starting from bit55 downwards?
>
> Yeah.
>
> > Clever idea, but I think this needs a
> > macro (or at least a comment). Not sure there's a ready-made bitmask
> > mirror function for this stuff, works case we can hand-code it and
> > extend every time we need one more bit encoded. Something like:
> >
> > MIRROR_U32((u & (BIT(0)) << 31 | (u & BIT(1) << 30 | ...)
> >
>
> Is it really worth it? People can just use the definitions as written
> in drm_fourcc.h. I agree that we should have the high bits described
> in a comment though.
>
> > And then shift that to the correct place. Probably want an
> >
> > ARM_MODIFIER_ENCODE(space_idx, flags) macro which assembles everything.
> >
> > >         ...
> > >
> > > I didn't want to explicitly reserve some high bits, because we've no
> > > idea how many to reserve. This way, we can assign exactly as many
> > > high bits as we need, when we need them. If any of the "modes" start
> > > encroaching towards the high bits, we'll have to make a decision at
> > > that point.
> > >
> > > Also, this is the only U-Interleaved format (that I know of), so it's
> > > not worth calling bit 55 "The U-Interleaved bit" because that would be
> > > a waste of space. It's more like the "misc" bit, but that's not a
> > > useful name to enshrine in UAPI.
> >
> > Yeah that's what I meant. Also better to explicitly reserve this, i.e.
> >
> > #define ARM_FBC_MODIFIER_SPACE 0
> > #define ARM_MISC_MODIFIER_SPACE 1
> >
> > and then encode with the mirror trickery.
> >
>
> I don't really see the value in that either, it's just giving
> userspace the opportunity to depend on more stuff: more future
> headaches. So long as the 64-bit values are stable, that should be
> enough.

If you think you need to save the few bits this potentially saves you
over just encoding 8bit enum like in Qiang's original patch I think
you get to type a few macros and comments ...

> > > Note that isn't the same as the "not-AFBC bit", because we may well
> > > have something in the future which is neither AFBC nor "misc".
> > >
> > > We've been very careful in our code to enforce all
> > > undefined/unrecognised bits to be zero, to ensure that this works.
> > >
> > > >
> > > > The other bit: Would be real good to define the format a bit more
> > > > precisely, including the layout within the tile.
> > >
> > > It's U-Interleaved, obviously ;-)
> >
> > :-) I mean full code exists in panfrost/lima, so this won't change
> > anything really ...
>
> Yeah, so for us to provide a more detailed description would require
> another lengthy loop through our legal approval process, and I'm not
> sure we can make a strong business case (which is what we need) for
> why this is needed.
>
> Of course, if someone happens to know the layout and wants to
> contribute to this file... Then I don't know how ack/r-b would work in
> that case, but I imagine the subsystem maintainer(s) might take issue
> with us attempting to block that contribution.

Well can't really take a modifier without knowing what it's for, I
guess this is up to lima/panfrost folks then to figure out :-P
-Daniel

>
> Thanks,
> -Brian
>
> >
> > Cheers, Daniel
> >
> > >
> > > -Brian
> > >
> > > >
> > > > Also ofc needs acks from lima/panfrost people since I assume they'll
> > > > be using this, too.
> > > >
> > > > Thanks, Daniel
> > > >
> > > > > +
> > > > > +/*
> > > > >   * Allwinner tiled modifier
> > > > >   *
> > > > >   * This tiling mode is implemented by the VPU found on all Allwinner platforms,
> > > > > --
> > > > > 2.7.4
> > > > >
> > > >
> > > >
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > +41 (0) 79 365 57 48 - http://blog.ffwll.ch
> >
> >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > +41 (0) 79 365 57 48 - http://blog.ffwll.ch



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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