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From:   Peter Rosin <peda@axentia.se>
To:     linux-kernel@vger.kernel.org
Cc:     Peter Rosin <peda@axentia.se>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        David Airlie <airlied@linux.ie>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Takashi Iwai <tiwai@suse.com>, Egbert Eich <eich@suse.de>,
        dri-devel@lists.freedesktop.org,
        linux-arm-kernel@lists.infradead.org
Subject: [PATCH] drm/atmel-hlcdc: add command line option to specify preferred depth
Date:   Mon, 26 Mar 2018 09:35:02 +0200
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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@axentia.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?

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);
-- 
2.11.0