linux-kernel - Re: [PATCH v5 05/17] drm: bridge: analogix/dp: dynamic parse sync_pol & interlace & dynamic

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Date:	Wed, 30 Sep 2015 16:34:13 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Yakir Yang <ykk@...k-chips.com>, Inki Dae <inki.dae@...sung.com>,
	Andrzej Hajda <a.hajda@...sung.com>,
	Joonyoung Shim <jy0922.shim@...sung.com>,
	Seung-Woo Kim <sw0312.kim@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Mark Yao <mark.yao@...k-chips.com>,
	Thierry Reding <treding@...dia.com>, joe@...ches.com,
	Rob Herring <robh+dt@...nel.org>
Cc:	David Airlie <airlied@...ux.ie>,
	Russell King <linux@....linux.org.uk>, djkurtz@...omium.org,
	dianders@...omium.org, Sean Paul <seanpaul@...omium.org>,
	Kukjin Kim <kgene@...nel.org>,
	Kumar Gala <galak@...eaurora.org>, emil.l.velikov@...il.com,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	Kishon Vijay Abraham I <kishon@...com>,
	Pawel Moll <pawel.moll@....com>, ajaynumb@...il.com,
	robherring2@...il.com, Andy Yan <andy.yan@...k-chips.com>,
	dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
	linux-rockchip@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 05/17] drm: bridge: analogix/dp: dynamic parse sync_pol
 & interlace & dynamic_range

On 30.09.2015 16:19, Yakir Yang wrote:
> Hi Krzysztof,
> 
> On 09/30/2015 01:32 PM, Krzysztof Kozlowski wrote:
>> On 22.09.2015 16:37, Yakir Yang wrote:
>>> Both hsync/vsync polarity and interlace mode can be parsed from
>>> drm display mode, and dynamic_range and ycbcr_coeff can be judge
>>> by the video code.
>>>
>>> But presumably Exynos still relies on the DT properties, so take
>>> good use of mode_fixup() in to achieve the compatibility hacks.
>>>
>>> Signed-off-by: Yakir Yang <ykk@...k-chips.com>
>>> ---
>>> Changes in v5:
>>> - Switch video timing type to "u32", so driver could use "of_property_read_u32"
>>>   to get the backword timing values. 
>> Okay
>>
>>> Krzysztof suggest me that driver could use
>>>   the "of_property_read_bool" to get backword timing values, but that interfacs
>>>   would modify the original drm_display_mode timing directly (whether those
>>>   properties exists or not).
>> Hmm, I don't understand. You have a:
>> 	struct video_info {
>> 		bool h_sync_polarity;
>> 		bool v_sync_polarity;
>> 		bool interlaced;
>> 	};
>>
>> so what is wrong with:
>> 	dp_video_config->h_sync_polarity =
>> 		of_property_read_bool(dp_node, "hsync-active-high");
>>
>> Is it exactly the same binding as previously?
> 
> Yes, it is the same binding as previously. But just a note that we already
> mark those DT binding as deprecated.
> 
> +-interlaced:            deprecated prop that can parsed frm drm_display_mode.
> +-vsync-active-high:     deprecated prop that can parsed frm drm_display_mode.
> +-hsync-active-high:     deprecated prop that can parsed frm drm_display_mode.
> 
> 
> For now those values should come from "struct drm_display_mode",
> and we already parsed them out from "drm_display_mode" before
> driver provide the backward compatibility.
> 
> Let's used the "hsync-active-high" example:
>     As for now the code would like:
>     static void analogix_dp_bridge_mode_set(...)
>     {
>         // Parsed timing value from "drm_display_mode"
>         video->h_sync_polarity = !!(mode->flags & DRM_MODE_FLAG_NHSYNC);
> 
>         // Try to detect the deprecated property, providing
>         // the backward compatibility
>         of_property_read_u32(dp_node, "hsync-active-high",
>                                  &video->h_sync_polarity);    
> 
>         /*
>          * In this case, if "hsync-active-high" property haven't been
>          * found, then the video timing "h_sync_polarity" would  keep
>          * no change, keeping the parsed value from "drm_display_mode"
>          */     
>     }   
> 
>     But if keep the "of_property_read_bool", then code would like:
>     static void analogix_dp_bridge_mode_set(...)
>     {
>         // Parsed timing value from "drm_display_mode"
>         video->h_sync_polarity = !!(mode->flags & DRM_MODE_FLAG_NHSYNC);
> 
>         // Try to detect the deprecated property, providing
>         // the backward compatibility
>         video->h_sync_polarity =
>                     of_property_read_bool(dp_node, "hsync-active-high");
>    
> 
>         /*
>          * In this case, if "hsync-active-high" property haven't been
>          * found, then the video timing "h_sync_polarity" would just
>          * modify to "false". That is the place we don't want, cause
>          * it would always modify the timing value parsed from
>          * "drm_display_mode"
>          */  
>     }   
> 

OK, I see the point of overwriting values from drm_display_mode. However
I think you changed the binding. I believe the of_property_read_u32()
will behave differently for such DTS:

exynos_dp {
	...
	hsync-active-high;
}

It will return -EOVERFLOW which means it would be broken now...

Best regards,
Krzysztof


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