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

[Maxime Ripard]

Hi,

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.

Maxime

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