Re: [PATCH] drm/sun4i: sun8i: Add the ability to keep scaler enabled for VI layer

From: Maxime Ripard <maxime@cerno.tech>
To: Roman Stratiienko <r.stratiienko@gmail.com>
Cc: wens@csie.org, jernej.skrabec@gmail.com, airlied@linux.ie,
	daniel@ffwll.ch, samuel@sholland.org,
	dri-devel@lists.freedesktop.org,
	linux-arm-kernel@lists.infradead.org,
	linux-sunxi@lists.linux.dev, linux-kernel@vger.kernel.org,
	megi@xff.cz
Subject: Re: [PATCH] drm/sun4i: sun8i: Add the ability to keep scaler enabled for VI layer
Date: Fri, 3 Jun 2022 10:24:16 +0200	[thread overview]
Message-ID: <20220603082416.ukohug3mwzu43csu@penduick> (raw)
In-Reply-To: <20220602180118.66170-1-r.stratiienko@gmail.com>

Hi,

On Thu, Jun 02, 2022 at 06:01:18PM +0000, Roman Stratiienko wrote:
> According to DE2.0/DE3.0 manual VI scaler enable register is double
> buffered, but de facto it doesn't, or the hardware has the shadow
> register latching issues which causes single-frame picture corruption
> after changing the state of scaler enable register.
> 
> Allow the user to keep the scaler always enabled, preventing the UI
> glitches on the transition from scaled to unscaled state.
> 
> NOTE:
> UI layer scaler has more registers with double-buffering issue and can't
> be workarounded in the same manner.
> 
> You may find a python test and a demo video for this issue at [1]
> 
> [1]: https://github.com/GloDroid/glodroid_tests/issues/4

Please describe the issue entirely here. The commit log must be self-sufficient.

> Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
> ---
>  drivers/gpu/drm/sun4i/sun8i_mixer.c    | 12 ++++++++++++
>  drivers/gpu/drm/sun4i/sun8i_vi_layer.c |  4 +++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/sun4i/sun8i_mixer.c b/drivers/gpu/drm/sun4i/sun8i_mixer.c
> index 71ab0a00b4de..15cad0330f66 100644
> --- a/drivers/gpu/drm/sun4i/sun8i_mixer.c
> +++ b/drivers/gpu/drm/sun4i/sun8i_mixer.c
> @@ -27,6 +27,18 @@
>  #include "sun8i_vi_layer.h"
>  #include "sunxi_engine.h"
>  
> +/* According to DE2.0/DE3.0 manual VI scaler enable register is double
> + * buffered, but de facto it doesn't, or the hardware has the shadow
> + * register latching issues which causes single-frame picture corruption
> + * after changing the state of scaler enable register.
> + * Allow the user to keep the scaler always enabled, preventing the UI
> + * glitches on the transition from scaled to unscaled state.
> + */
> +int sun8i_vi_keep_scaler_enabled;
> +MODULE_PARM_DESC(keep_vi_scaler_enabled,
> +		 "Keep VI scaler enabled (1 = enabled, 0 = disabled (default))");
> +module_param_named(keep_vi_scaler_enabled, sun8i_vi_keep_scaler_enabled, int, 0644);
> +

It's not clear to me why we would want to make that a parameter?

If it never works, we should fix it once and for all and not allow a broken setup at all.

Maxime

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