linux-kernel - Re: [PATCH v4 3/7] drm/fb-helper: Add fb_deferred

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Date:	Fri, 29 Apr 2016 15:50:40 +0300
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Noralf Trønnes <noralf@...nnes.org>,
	<dri-devel@...ts.freedesktop.org>, <linux-fbdev@...r.kernel.org>
CC:	<daniel@...ll.ch>, <laurent.pinchart@...asonboard.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/7] drm/fb-helper: Add fb_deferred_io support

Hi,

On 28/04/16 18:18, Noralf Trønnes wrote:
> This adds deferred io support to drm_fb_helper.
> The fbdev framebuffer changes are flushed using the callback
> (struct drm_framebuffer *)->funcs->dirty() by a dedicated worker
> ensuring that it always runs in process context.
> 
> Signed-off-by: Noralf Trønnes <noralf@...nnes.org>
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
> ---

Thanks for the series! Unfortunately I haven't been able to follow the
discussions properly, so I hope my questions haven't been covered earlier.

> Changes since v3:
> - Don't use forward decl, move drm_fb_helper_dirty_work()
> - Use DIV_ROUND_UP in drm_fb_helper_deferred_io()
> 
> Changes since v2:
> - FB_DEFERRED_IO is now always selected by DRM_KMS_FB_HELPER, ifdef removed
> - The drm_clip_rect utility functions are dropped, so open code it
> - docs: use & to denote structs
> 
> Changes since v1:
> - Use a dedicated worker to run the framebuffer flushing like qxl does
> - Add parameter descriptions to drm_fb_helper_deferred_io
> 
>  drivers/gpu/drm/Kconfig         |   1 +
>  drivers/gpu/drm/drm_fb_helper.c | 103 +++++++++++++++++++++++++++++++++++++++-
>  include/drm/drm_fb_helper.h     |  15 ++++++
>  3 files changed, 118 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 9e4f2f1..8e6f34b 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -52,6 +52,7 @@ config DRM_KMS_FB_HELPER
>  	select FB_CFB_FILLRECT
>  	select FB_CFB_COPYAREA
>  	select FB_CFB_IMAGEBLIT
> +	select FB_DEFERRED_IO
>  	help
>  	  FBDEV helpers for KMS drivers.
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 855108e..62f849f 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -84,6 +84,15 @@ static LIST_HEAD(kernel_fb_helper_list);
>   * and set up an initial configuration using the detected hardware, drivers
>   * should call drm_fb_helper_single_add_all_connectors() followed by
>   * drm_fb_helper_initial_config().
> + *
> + * If CONFIG_FB_DEFERRED_IO is enabled and &drm_framebuffer_funcs ->dirty is
> + * set, the drm_fb_helper_{cfb,sys}_{write,fillrect,copyarea,imageblit}
> + * functions will accumulate changes and schedule &fb_helper .dirty_work to run
> + * right away. This worker then calls the dirty() function ensuring that it
> + * will always run in process context since the fb_*() function could be
> + * running in atomic context. If drm_fb_helper_deferred_io() is used as the

Who's calling {write,fillrect,copyarea,imageblit} in an atomic context?
That sounds like a very bad idea to me...

If this is only for accumulating changes, I think it may be better to
leave that to the driver as it may have better idea of how to accumulate.

But, of course, this is a helper, so if all the drivers use this kind of
accumulation, it makes sense =).

 Tomi



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