Date:	Wed, 27 Apr 2016 11:45:31 +0200
From:	Noralf Trønnes <noralf@...nnes.org>
To:	dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
	laurent.pinchart@...asonboard.com, tomi.valkeinen@...com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] drm/fb-helper: Add fb_deferred_io support


Den 26.04.2016 19:19, skrev Daniel Vetter:
> On Tue, Apr 26, 2016 at 06:24:54PM +0200, Noralf Trønnes wrote:
>> Den 25.04.2016 11:09, skrev Daniel Vetter:
>>> On Sun, Apr 24, 2016 at 10:48:58PM +0200, Noralf Trønnes wrote:
>>>> This adds deferred io support if CONFIG_FB_DEFERRED_IO is enabled.
>>>> 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>
>>>> ---
>>>>
>>>> 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/drm_fb_helper.c | 127 +++++++++++++++++++++++++++++++++++++++-
>>>>   include/drm/drm_fb_helper.h     |  17 ++++++
>>>>   2 files changed, 143 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
>>>> index 855108e..46ee6f8 100644
>>>> --- a/drivers/gpu/drm/drm_fb_helper.c
>>>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>>>> @@ -40,6 +40,7 @@
>>>>   #include <drm/drm_crtc_helper.h>
>>>>   #include <drm/drm_atomic.h>
>>>>   #include <drm/drm_atomic_helper.h>
>>>> +#include <drm/drm_rect.h>
>>>>
>>>>   static bool drm_fbdev_emulation = true;
>>>>   module_param_named(fbdev_emulation, drm_fbdev_emulation, bool, 0600);
>>>> @@ -48,6 +49,10 @@ MODULE_PARM_DESC(fbdev_emulation,
>>>>
>>>>   static LIST_HEAD(kernel_fb_helper_list);
>>>>
>>>> +static void drm_fb_helper_dirty_init(struct drm_fb_helper *helper);
>>>> +static void drm_fb_helper_dirty(struct fb_info *info, u32 x, u32 y,
>>>> +				u32 width, u32 height);
>>>> +
>>>>   /**
>>>>    * DOC: fbdev helpers
>>>>    *
>>>> @@ -84,6 +89,16 @@ 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
>>>> + * (struct drm_framebuffer *)->funcs->dirty is set, the
>>>> + * drm_fb_helper_{cfb,sys}_{write,fillrect,copyarea,imageblit} functions
>>>> + * will accumulate changes and schedule (struct 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
>>>> + * deferred_io callback it will also schedule dirty_work with the damage
>>>> + * collected from the mmap page writes.
>>> One thing to consider (and personally I don't care either way) is whether
>>> we shouldn't just select CONFIG_FB_DEFERRED_IO if the fbdev helpers are
>>> enabled. Pushing that out to drivers is imo a bit fragile.
>>>
>>> But like I said I'm ok with either way.
>> My concern was adding code and data that only a few drivers would
>> actually use. But of course there's the tradeoff with complexity.
>> I use this to enable it:
>>          select FB_DEFERRED_IO if DRM_KMS_FB_HELPER
>>
>> I guess the maintainer has to make this choice between size and complexity
>> :-)
>> I can enable it by default if you want, drm is both huge and complex so I
>> don't know what's best.
>>
>> As a sidenote, I have also put all the fbdev code in a file of it's own to
>> make it simple with regards to the DRM_FBDEV_EMULATION user option:
>> tinydrm-$(CONFIG_DRM_KMS_FB_HELPER)     += tinydrm-fbdev.o
> Ok, if you ask maintainers then please nuke the #ifdef from .c files. If
> you select CONFIG_DRM_KMS_FB_HELPER, then you get hdmi, edid, dp aux, dp
> mst and whatever else helpers, even if you don't need them. Adding 3
> functions for defio when you select fbdev helpers and maybe don't need
> them is totally harmless. And removing the #ifdef will look so much better
> ;-)

Will do :-)
Kernel development is just my hobby so I'm not well versed in all of this.

>>>>    */
>>>>
>>>>   /**
>>>> @@ -401,11 +416,14 @@ backoff:
>>>>   static int restore_fbdev_mode(struct drm_fb_helper *fb_helper)
>>>>   {
>>>>   	struct drm_device *dev = fb_helper->dev;
>>>> +	struct fb_info *info = fb_helper->fbdev;
>>>>   	struct drm_plane *plane;
>>>>   	int i;
>>>>
>>>>   	drm_warn_on_modeset_not_all_locked(dev);
>>>>
>>>> +	drm_fb_helper_dirty(info, 0, 0, info->var.xres, info->var.yres);
>>> Why is this needed? If you do a modeset (or pageflip or whatever) drivers
>>> are supposed to re-upload the entire screen. We've talked about adding a
>>> dirty rectangle to atomic to allow userspace to optimize this, but there
>>> should _never_ be a need to do a dirtyfb call around a modeset. Probably
>>> just a driver bug in your panel drm drivers?
>> Ok, in tinydrm I now set a flag in &drm_simple_display_pipe_funcs
>> ->plane_update to indicate that the next dirty() should do the whole
>> framebuffer which seems to work fine.
>> Should I actually perform the update as well?
>> If so I would need to add a worker in tinydrm to do that.
> Yes, plane update should always do a full update. Not sure how you get
> away with delaying that to ->dirty, maybe modesetting isn't
> double-buffering when you don't have a GL that could do glamour.
>
> ->dirty is _only_ for frontbuffer rendering, not for page-flipping to an
> entirely new buffer. In short if someone calls ->dirty on a buffer which
> is currently not being displayed than a) they're silly b) drivers should
> treat it as a no-op. Maybe we need a helper to do that ...
> -Daniel

drm_fb_helper will call dirty() as long as there's fbdev activity, so the
driver needs to take that into account. For instance fbcon with a blinking
cursor will trigger calls even if a buffer has been set up on the drm side.
tinydrm checks the fb against the fb set on the plane and if it differs
it's a no-op.

Noralf.

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