linux-kernel - Re: [PATCH] drm/arcpgu: use .mode_fixup instead of .atomic

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Date:   Thu, 2 Mar 2017 20:54:38 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:     dri-devel@...ts.freedesktop.org,
        linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Jose Abreu <Jose.Abreu@...opsys.com>
Subject: Re: [PATCH] drm/arcpgu: use .mode_fixup instead of .atomic_check

On Thu, Mar 02, 2017 at 08:27:54PM +0300, Alexey Brodkin wrote:
> Since we cannot always generate exactly requested pixel clock
> there's not much sense in checking requested_clock == clk_round_rate().
> In that case for quite some modes we'll be getting -EINVAL and no video
> output at all.
> 
> But given there's some tolerance to real pixel clock in TVs/monitors
> we may still give it a try with the clock as close to requested one as
> PLL on the board may generate. So we just do a fixup to what current
> board may provide.
> 
> Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: David Airlie <airlied@...ux.ie>
> Cc: Jose Abreu <joabreu@...opsys.com>
> ---
>  drivers/gpu/drm/arc/arcpgu_crtc.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/arc/arcpgu_crtc.c b/drivers/gpu/drm/arc/arcpgu_crtc.c
> index ad9a95916f1f..3f2823c1efc3 100644
> --- a/drivers/gpu/drm/arc/arcpgu_crtc.c
> +++ b/drivers/gpu/drm/arc/arcpgu_crtc.c
> @@ -129,18 +129,16 @@ static void arc_pgu_crtc_disable(struct drm_crtc *crtc)
>  			      ~ARCPGU_CTRL_ENABLE_MASK);
>  }
>  
> -static int arc_pgu_crtc_atomic_check(struct drm_crtc *crtc,
> -				     struct drm_crtc_state *state)
> +static bool arc_pgu_crtc_mode_fixup(struct drm_crtc *crtc,
> +				    const struct drm_display_mode *mode,
> +				    struct drm_display_mode *adjusted_mode)

This isn't required at all, see drm_crtc_state.adjusted_mode. Just update
that and you're good - .mode_fixup is the backwards compatibility function
for old kms drivers, atomic_check is strictly more powerful.

Please also make sure the documentation properly explains this, and if
not, please submit a patch to improve it.
-Daniel

>  {
>  	struct arcpgu_drm_private *arcpgu = crtc_to_arcpgu_priv(crtc);
> -	struct drm_display_mode *mode = &state->adjusted_mode;
> -	long rate, clk_rate = mode->clock * 1000;
>  
> -	rate = clk_round_rate(arcpgu->clk, clk_rate);
> -	if (rate != clk_rate)
> -		return -EINVAL;
> +	adjusted_mode->clock =
> +		clk_round_rate(arcpgu->clk, mode->clock * 1000) / 1000;
>  
> -	return 0;
> +	return true;
>  }
>  
>  static void arc_pgu_crtc_atomic_begin(struct drm_crtc *crtc,
> @@ -165,8 +163,8 @@ static const struct drm_crtc_helper_funcs arc_pgu_crtc_helper_funcs = {
>  	.disable	= arc_pgu_crtc_disable,
>  	.prepare	= arc_pgu_crtc_disable,
>  	.commit		= arc_pgu_crtc_enable,
> -	.atomic_check	= arc_pgu_crtc_atomic_check,
>  	.atomic_begin	= arc_pgu_crtc_atomic_begin,
> +	.mode_fixup	= arc_pgu_crtc_mode_fixup,
>  };
>  
>  static void arc_pgu_plane_atomic_update(struct drm_plane *plane,
> -- 
> 2.7.4
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch