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

[Daniel Vetter]

On Wed, Mar 28, 2018 at 12:03:39PM +0200, Peter Rosin wrote:
> On 2018-03-28 09:34, 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>]
> 
> Heh, you are right...
> 
> > AFAIR, <bpp> encodes the color depth, so what is the benefit of adding
> > this new property to overload the default depth?
> 
> ...but hhhmmm, I think I will have to have to encode the display size
> in the bootargs so I will have to use distinct barebox environments
> depending on the panel, but that's no biggie.
> 
> Splendid, please throw away the patch!

I think we could fix the parsing to allow -bpp without the resolution ...
Not sure how to best do that though. Maybe state that 0x0-bpp means to not
change the resolution from the default?
-Daniel

> 
> Cheers,
> Peter
> 
> > 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.
> > 
> > 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);
> > 
> > 
> > 
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch