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

[Daniel Vetter]

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@xxxxxxxxxx> 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@24-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@16-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@xxxxxxxxxx>
> > ---
> >  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 ...

But yeah probably worth it to wire it up properly.
-Daniel
> 
> Thanks,
> 
> Boris
> 
> > 
> > Cheers,
> > Peter
> > 
> > diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c
> > index c1ea5c36b006..f0148627c221 100644
> > --- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c
> > +++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c
> > @@ -29,6 +29,11 @@
> >  
> >  #define ATMEL_HLCDC_LAYER_IRQS_OFFSET		8
> >  
> > +static int atmel_hlcdc_preferred_depth __read_mostly;
> > +
> > +MODULE_PARM_DESC(preferreddepth, "Set preferred bpp");
> > +module_param_named(preferreddepth, atmel_hlcdc_preferred_depth, int, 0400);
> > +
> >  static const struct atmel_hlcdc_layer_desc atmel_hlcdc_at91sam9n12_layers[] = {
> >  	{
> >  		.name = "base",
> > @@ -590,6 +595,7 @@ static int atmel_hlcdc_dc_modeset_init(struct drm_device *dev)
> >  	dev->mode_config.min_height = dc->desc->min_height;
> >  	dev->mode_config.max_width = dc->desc->max_width;
> >  	dev->mode_config.max_height = dc->desc->max_height;
> > +	dev->mode_config.preferred_depth = 24;
> >  	dev->mode_config.funcs = &mode_config_funcs;
> >  
> >  	return 0;
> > @@ -658,7 +664,7 @@ static int atmel_hlcdc_dc_load(struct drm_device *dev)
> >  
> >  	platform_set_drvdata(pdev, dev);
> >  
> > -	drm_fb_cma_fbdev_init(dev, 24, 0);
> > +	drm_fb_cma_fbdev_init(dev, atmel_hlcdc_preferred_depth, 0);
> >  
> >  	drm_kms_helper_poll_init(dev);
> >  
> > @@ -756,6 +762,16 @@ static int atmel_hlcdc_dc_drm_probe(struct platform_device *pdev)
> >  	struct drm_device *ddev;
> >  	int ret;
> >  
> > +	switch (atmel_hlcdc_preferred_depth) {
> > +	case 0: /* driver default */
> > +	case 8:
> > +	case 16:
> > +	case 24:
> > +		break;
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +
> >  	ddev = drm_dev_alloc(&atmel_hlcdc_dc_driver, &pdev->dev);
> >  	if (IS_ERR(ddev))
> >  		return PTR_ERR(ddev);
> 
> 
> 
> -- 
> Boris Brezillon, Bootlin (formerly Free Electrons)
> Embedded Linux and Kernel engineering
> https://bootlin.com
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch