linux-kernel - Re: [PATCH v2 5/8] drm/client: Convert drm_client_buffer_addfb() to drm_mode

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Date:   Thu, 31 Aug 2023 10:15:23 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Daniel Stone <daniel@...ishbar.org>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 5/8] drm/client: Convert drm_client_buffer_addfb() to
 drm_mode_addfb2()

Hi

Am 24.08.23 um 17:22 schrieb Geert Uytterhoeven:
> Hi Daniel,
> 
> On Thu, Aug 24, 2023 at 5:12 PM Daniel Stone <daniel@...ishbar.org> wrote:
>> On Thu, 24 Aug 2023 at 16:09, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>>>          struct drm_client_dev *client = buffer->client;
>>> -       struct drm_mode_fb_cmd fb_req = { };
>>> -       const struct drm_format_info *info;
>>> +       struct drm_mode_fb_cmd2 fb_req = { };
>>>          int ret;
>>>
>>> -       info = drm_format_info(format);
>>> -       fb_req.bpp = drm_format_info_bpp(info, 0);
>>> -       fb_req.depth = info->depth;
>>>          fb_req.width = width;
>>>          fb_req.height = height;
>>> -       fb_req.handle = handle;
>>> -       fb_req.pitch = buffer->pitch;
>>> +       fb_req.pixel_format = format;
>>> +       fb_req.handles[0] = handle;
>>> +       fb_req.pitches[0] = buffer->pitch;
>>>
>>> -       ret = drm_mode_addfb(client->dev, &fb_req, client->file);
>>> +       ret = drm_mode_addfb2(client->dev, &fb_req, client->file);
>>>          if (ret)
>>>                  return ret;
>>
>> This should explicitly set the LINEAR modifier (and the modifier flag)
>> if the driver supports modifiers.
> 
> Thanks for your comment!
> 
> I have no idea how to do that, and I do not know what the impact
> would be.  All I know is that the current implementation of
> drm_client_buffer_addfb() does not do that, so changing that in this
> patch would mean that this would no longer be a trivial conversion.

I agree. That change is too large and invasive for this patchset. Rather 
add a comment or TODO item.

The correct way for adding the modifier is to extend the 
drm_client_buffer_addfb() and pass in the information from the caller. 
That caller is fbdev code. We currently detect the format from (bpp, 
depth) values and forward the format to _addfb(). [1] Here we'd have to 
check the driver for a modifier as well and forward it to _addfb()

[1] 
https://elixir.bootlin.com/linux/v6.5/source/drivers/gpu/drm/drm_fbdev_generic.c#L88

The pair of (bpp, depth) values comes from __drm_fb_helper_find_sizes, 
[2] which looks through the planes' formats and tries to find something 
that fits the user's requested color mode. It would have to look for 
modifiers as well. At some point, I want the helper to return formats 
directly, but that's still a bit away.

[2] 
https://elixir.bootlin.com/linux/v6.5/source/drivers/gpu/drm/drm_fb_helper.c#L1504

Best regards
Thomas

> 
> Gr{oetje,eeting}s,
> 
>                          Geert
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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