linux-kernel - Re: [PATCH v2 1/2] drm/cma-helper: Add multi buffer support for cma fbdev

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Date:   Sun, 12 Feb 2017 14:28:11 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Stefan Christ <s.christ@...tec.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] drm/cma-helper: Add multi buffer support for cma fbdev

Hi Maxime,

Thank you for the patch.

On Thursday 02 Feb 2017 11:31:56 Maxime Ripard wrote:
> From: Xinliang Liu <xinliang.liu@...aro.org>
> 
> This patch add a config to support to create multi buffer for cma fbdev.
> Such as double buffer and triple buffer.
> 
> Cma fbdev is convient to add a legency fbdev. And still many Android
> devices use fbdev now and at least double buffer is needed for these
> Android devices, so that a buffer flip can be operated. It will need
> some time for Android device vendors to abondon legency fbdev. So multi
> buffer for fbdev is needed.

How exactly do we expect Android to move away from fbdev if we add features to 
the fbdev compat layer ? I'd much rather make it clear to them that fbdev is a 
thing from the past and that they'd better migrate now.

> Signed-off-by: Xinliang Liu <xinliang.liu@...aro.org>
> [s.christ@...tec.de: Picking patch from
>                      https://lkml.org/lkml/2015/9/14/188]
> Signed-off-by: Stefan Christ <s.christ@...tec.de>
> Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> ---
>  drivers/gpu/drm/Kconfig             | 8 ++++++++
>  drivers/gpu/drm/drm_fb_cma_helper.c | 8 +++++++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index ebfe8404c25f..2ca9bb26a4e4 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -121,6 +121,14 @@ config DRM_KMS_CMA_HELPER
>  	help
>  	  Choose this if you need the KMS CMA helper functions
> 
> +config DRM_CMA_FBDEV_BUFFER_NUM
> +	int "Cma Fbdev Buffer Number"
> +	depends on DRM_KMS_CMA_HELPER
> +	default 1
> +	help
> +	  Defines the buffer number of cma fbdev.  Default is one buffer.
> +	  For double buffer please set to 2 and 3 for triple buffer.
> +
>  source "drivers/gpu/drm/i2c/Kconfig"
> 
>  source "drivers/gpu/drm/arm/Kconfig"
> diff --git a/drivers/gpu/drm/drm_fb_cma_helper.c
> b/drivers/gpu/drm/drm_fb_cma_helper.c index 81b3558302b5..e3f8b9e720a0
> 100644
> --- a/drivers/gpu/drm/drm_fb_cma_helper.c
> +++ b/drivers/gpu/drm/drm_fb_cma_helper.c
> @@ -411,6 +411,12 @@ static void drm_fbdev_cma_defio_fini(struct fb_info
> *fbi) kfree(fbi->fbops);
>  }
> 
> +static int fbdev_num_buffers = CONFIG_DRM_CMA_FBDEV_BUFFER_NUM;
> +module_param(fbdev_num_buffers, int, 0444);
> +MODULE_PARM_DESC(fbdev_num_buffers,
> +		 "Number of frame buffers to allocate [default="
> +		 __MODULE_STRING(CONFIG_DRM_CMA_FBDEV_BUFFER_NUM) "]");
> +
>  /*
>   * For use in a (struct drm_fb_helper_funcs *)->fb_probe callback function
> that * needs custom struct drm_framebuffer_funcs, like dirty() for
> deferred_io use. @@ -437,7 +443,7 @@ int
> drm_fbdev_cma_create_with_funcs(struct drm_fb_helper *helper,
> bytes_per_pixel = DIV_ROUND_UP(sizes->surface_bpp, 8);
> 
>  	mode_cmd.width = sizes->surface_width;
> -	mode_cmd.height = sizes->surface_height;
> +	mode_cmd.height = sizes->surface_height * fbdev_num_buffers;
>  	mode_cmd.pitches[0] = sizes->surface_width * bytes_per_pixel;
>  	mode_cmd.pixel_format = drm_mode_legacy_fb_format(sizes->surface_bpp,
>  		sizes->surface_depth);

-- 
Regards,

Laurent Pinchart