linux-kernel - Re: [RFC PATCH v3 2/3] drm: add support modifiers for drivers whose planes only support linear layout

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Date:   Wed, 19 Jan 2022 11:35:22 +0900
From:   Esaki Tomohito <etom@...l.co.jp>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     dri-devel@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Ben Skeggs <bskeggs@...hat.com>,
        Michel Dänzer <mdaenzer@...hat.com>,
        Simon Ser <contact@...rsion.fr>,
        Qingqing Zhuo <qingqing.zhuo@....com>,
        Bas Nieuwenhuizen <bas@...nieuwenhuizen.nl>,
        Mark Yacoub <markyacoub@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        Evan Quan <evan.quan@....com>, Petr Mladek <pmladek@...e.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Lee Jones <lee.jones@...aro.org>,
        Abhinav Kumar <abhinavk@...eaurora.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Rob Clark <robdclark@...omium.org>,
        amd-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        nouveau@...ts.freedesktop.org, Daniel Stone <daniel@...ishbar.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Damian Hobson-Garcia <dhobsong@...l.co.jp>,
        Takanari Hayama <taki@...l.co.jp>
Subject: Re: [RFC PATCH v3 2/3] drm: add support modifiers for drivers whose
 planes only support linear layout

On 2022/01/18 18:53, Andy Shevchenko wrote:
> On Mon, Jan 17, 2022 at 02:15:48PM +0900, Esaki Tomohito wrote:
>> On 2022/01/14 23:16, Andy Shevchenko wrote:
>>> On Fri, Jan 14, 2022 at 07:17:52PM +0900, Tomohito Esaki wrote:
>>>> The LINEAR modifier is advertised as default if a driver doesn't specify
>>>> modifiers.
>>>
>>> ...
>>>
>>>> +	const uint64_t default_modifiers[] = {
>>>> +		DRM_FORMAT_MOD_LINEAR,
>>>> +		DRM_FORMAT_MOD_INVALID
>>>
>>> + Comma?
>>
>> There is no mention in the coding style about adding/removing a comma to the
>> last element of an array. Is there a policy in drm driver?
>>
>> I think the advantage of adding a comma to the last element of an array is
>> that diff is only one line when an element is added to the end.
>> However since INVALID is always the last element in the modifiers array, I
>> think it can be either in this case.
>> If there is a policy, I will match it.
> 
> Indeed, but there is a common sense. The idea behind (multi-line) definitions
> that when next time somebody will add an element in the array, there are will
> be:
> 
> a) no additional churn (like in case of this patch, if the item will be added
>     at the bottom;
> 
> b) an element that may not be added behind the terminator, which will look
>     weird.
> 
> That said, the question is if the element is terminator one or not, if not,
> comma is better than no comma and vise versa.
> 

Ah I see. In this case, DRM_FORMAT_MOD_INVALID is terminator, so it
should not have a comma.

Thanks
Tomohito Esaki