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

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Date:   Fri, 10 Feb 2017 16:27:45 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     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, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 1/2] drm/cma-helper: Add multi buffer support for cma
 fbdev

Hi Daniel,

On Thu, Feb 09, 2017 at 06:04:38PM +0100, Daniel Vetter wrote:
> On Thu, Feb 02, 2017 at 11:31:56AM +0100, 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.
> > 
> > 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) "]");
> 
> Pure bikshed: Should this be an overallocation %? 200 for
> double-buffering, 100 as the default?

I'm not against it, but would it make sense to allocate something like
120% of a buffer?

> Slightly less bikesheddy: Can't we do this in the core helpers somehow?

I guess that would move it to drm_fb_helper_single_fb_probe then? It
makes sense.

> 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.

I'll move it and add the kerneldoc.

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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