[RFC PATCH] drm/ssd130x: Allocate buffer in the CRTC's .atomic_check() callback

[Geert Uytterhoeven]

Hi Maxime,

On Fri, Sep 1, 2023 at 2:00 PM Maxime Ripard <mripard at kernel.org> wrote:
> On Fri, Sep 01, 2023 at 10:36:17AM +0200, Geert Uytterhoeven wrote:
> > On Fri, Sep 1, 2023 at 10:22 AM Maxime Ripard <mripard at kernel.org> wrote:
> > > On Wed, Aug 30, 2023 at 08:25:08AM +0200, Javier Martinez Canillas wrote:
> > > > The commit 45b58669e532 ("drm/ssd130x: Allocate buffer in the plane's
> > > > .atomic_check() callback") moved the allocation of the intermediate and
> > > > HW buffers from the encoder's .atomic_enable callback to primary plane's
> > > > .atomic_check callback.
> > > >
> > > > This was suggested by Maxime Ripard because drivers aren't allowed to fail
> > > > after drm_atomic_helper_swap_state() has been called, and the encoder's
> > > > .atomic_enable happens after the new atomic state has been swapped.
> > > >
> > > > But that change caused a performance regression in very slow platforms,
> > > > since now the allocation happens for every plane's atomic state commit.
> > > > For example, Geert Uytterhoeven reports that is the case on a VexRiscV
> > > > softcore (RISC-V CPU implementation on an FPGA).
> > >
> > > I'd like to have numbers on that. It's a bit surprising to me that,
> > > given how many objects we already allocate during a commit, two small
> > > additional allocations affect performances so dramatically, even on a
> > > slow platform.
> >
> > To be fair, I didn't benchmark that.  Perhaps it's just too slow due to
> > all these other allocations (and whatever else happens).
> >
> > I just find it extremely silly to allocate a buffer over and over again,
> > while we know that buffer is needed for each and every display update.
>
> Maybe it's silly, but I guess it depends on what you want to optimize
> for. You won't know the size of that buffer before you're in
> atomic_check. So it's a different trade-off than you would like, but I
> wouldn't call it extremely silly.

The size of ssd130x_plane_state.data_array[] is fixed, and depends
on the actual display connected.
The size of ssd130x_plane_state.buffer[]  is also fixed, and depends
on the plane's size (which is currently fixed to the display size).

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds