Re: [PATCH 1/3] drm/cma-helper: Add multi buffer support for cma fbdev

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jul 13, 2016 at 10:11:46AM +0200, Stefan Christ wrote:
> From: Xinliang Liu <xinliang.liu@linaro.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.
> 
> Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
> [s.christ@phytec.de: Picking patch from
>                      https://lkml.org/lkml/2015/9/14/188]
> Signed-off-by: Stefan Christ <s.christ@phytec.de>

We first need to discuss whether we want to do this as a high level idea,
but meanwhile also some comments about details on the patches.

> ---
>  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 fc35731..56cf34d 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -106,6 +106,14 @@ 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"
>  
>  config DRM_TDFX
> diff --git a/drivers/gpu/drm/drm_fb_cma_helper.c b/drivers/gpu/drm/drm_fb_cma_helper.c
> index 5075fae..be66042 100644
> --- a/drivers/gpu/drm/drm_fb_cma_helper.c
> +++ b/drivers/gpu/drm/drm_fb_cma_helper.c
> @@ -27,6 +27,12 @@
>  
>  #define DEFAULT_FBDEFIO_DELAY_MS 50
>  
> +#ifdef CONFIG_DRM_CMA_FBDEV_BUFFER_NUM
> +#define FBDEV_BUFFER_NUM CONFIG_DRM_CMA_FBDEV_BUFFER_NUM
> +#else
> +#define FBDEV_BUFFER_NUM 1
> +#endif

Imo this should be done in the core helpers, we could do this by
multiplying the height of buffers. And I think we should also expose this
as a module option, with Kconfig simply setting the default.
-Daniel

> +
>  struct drm_fb_cma {
>  	struct drm_framebuffer		fb;
>  	struct drm_gem_cma_object	*obj[4];
> @@ -389,7 +395,7 @@ 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_BUFFER_NUM;
>  	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);
> -- 
> 1.9.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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