linux-kernel - Re: [PATCH] drm/atmel-hlcdc: add command line option to specify preferred depth

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Date:   Thu, 29 Mar 2018 09:10:25 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Boris Brezillon <boris.brezillon@...tlin.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, Peter Rosin <peda@...ntia.se>,
        Egbert Eich <eich@...e.de>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        David Airlie <airlied@...ux.ie>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Takashi Iwai <tiwai@...e.com>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] drm/atmel-hlcdc: add command line option to specify
 preferred depth

On Wed, Mar 28, 2018 at 02:25:12PM +0200, Boris Brezillon wrote:
> On Wed, 28 Mar 2018 14:22:36 +0200
> Daniel Vetter <daniel@...ll.ch> wrote:
> 
> > On Wed, Mar 28, 2018 at 09:34:54AM +0200, Boris Brezillon wrote:
> > > Hi Peter,
> > > 
> > > On Mon, 26 Mar 2018 09:35:02 +0200
> > > Peter Rosin <peda@...ntia.se> wrote:
> > >   
> > > > I have an sama5d31-based system with 64MB of memory and a 1920x1080
> > > > LVDS display wired for 16-bpp. When I enable legacy fbdev support,
> > > > the contiguous memory allocator invariably fails with the order-11
> > > > allocation for a 1920x1080@...bpp buffer (~6MB). But this HW can never
> > > > make any good use of RGB888, so that is a wasted attempt anyway that
> > > > would also waste precious memory should it succeed.
> > > > 
> > > > Sure, I could rewrite user-space to go directly to KMS etc, and that
> > > > makes the (attempted) order-11 allocation go away, replacing it with
> > > > one order-10 allocation per application restart for a 1920x1080@...bpp
> > > > buffer (<4MB). But after a few restarts, order-10 allocations start to
> > > > fail as well, which is only to be expected AFAIU.
> > > > 
> > > > So, I'd rather not change user-space (which was originally written
> > > > to target a smaller display) so that I at the same time get the
> > > > benefit of an early pre-allocated fbdev frame-buffer that can be
> > > > reused over and over. But to do that I need to tell the driver that
> > > > 16-bpp is the preferred depth. Add a module parameter to do just that.
> > > > 
> > > > Signed-off-by: Peter Rosin <peda@...ntia.se>
> > > > ---
> > > >  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c | 18 +++++++++++++++++-
> > > >  1 file changed, 17 insertions(+), 1 deletion(-)
> > > > 
> > > > I found some inspiration regarding naming and implementation here:
> > > > https://patchwork.kernel.org/patch/9848631/
> > > > 
> > > > I have found no feedback on that patch though, which makes me wonder if
> > > > I'm perhaps barking up the wronig tree?  
> > > 
> > > Hm, isn't that something you can already overload with the video=
> > > parameter?
> > > 
> > > 	video=<output>:<resolution>[-<bpp>]
> > > 
> > > AFAIR, <bpp> encodes the color depth, so what is the benefit of adding
> > > this new property to overload the default depth?
> > > 
> > > Maybe I'm wrong and the default depth param is actually useful, but in
> > > this case we should probably make it generic since other drivers seems
> > > to need it too, and we might want to attach it to a specific display
> > > engine instance.  
> > 
> > I think for the drm's fbdev emulation we ignore the bpp ...
> 
> Nope, it's already parsed [1].
> 
> [1]https://elixir.bootlin.com/linux/v4.16-rc3/source/drivers/gpu/drm/drm_fb_helper.c#L1812

Oh, totally missed this. I honestly wonder why we then have all the
various bpp module options in different drivers ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch