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

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Feb 02, 2017 at 11:31:56AM +0100, Maxime Ripard 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>
> Signed-off-by: Maxime Ripard <maxime.ripard@free-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) "]");

Pure bikshed: Should this be an overallocation %? 200 for
double-buffering, 100 as the default?

Slightly less bikesheddy: Can't we do this in the core helpers somehow?
I'd be nice if this is not cma specific. If it's not possible, I'd at
least move the symbol to drm_fb_helper.c, and add some kernel-doc around
it. That allows any other non-cma driver to at least implement support for
this. That also has the benefit of featuring it more prominently, in our
docs.

Besides these bikesheds/question, looks all reasonable to me. If you can
get some more acks would be great, but will merge anyway.
-Daniel

> +
>  /*
>   * 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);
> -- 
> git-series 0.8.11
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch