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

I have found no feedback on that patch though, which makes me wonder if
I'm perhaps barking up the wronig tree?


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 @@


+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);


@@ -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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