Virtualization - Nov 2022 - [PATCH 0/7] drm: Fix the color-depth/bpp confusion

A number of drivers mix up the meaning of bits-per-pixel and color
depth. For each of them, set the correct values. As a rule of thumb,
the color depth is the number of color and alpha bits that affect
image composition. The bpp value is the color depth in the pixel
plus the filler bits.

The color depth is exported to userspace, while the bpp value only
affects fbdev emulation. Currently, fbdev fails if it selects a color
format that is unsupported by the driver. The fix would be to fall
back to a driver default value for the bpp. Getting the default fixed
in drivers will then allow us to fix the fbdev format selection.

Thomas Zimmermann (7):
  drm/hisilicon/hibmc: Fix preferred depth and bpp
  drm/logicvc: Fix preferred fbdev cpp
  drm/cirrus: Decouple fbdev bpp value from color depth
  drm/ofdrm: Set preferred depth from format of scanout buffer
  drm/simpledrm: Set preferred depth from format of scanout buffer
  drm/solomon: Set preferred color depth and bpp to the correct values
  drm/fb-helper: Don't use the preferred depth for the BPP default

 drivers/gpu/drm/drm_fbdev_generic.c             | 15 +++++++++------
 drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c |  4 ++--
 drivers/gpu/drm/logicvc/logicvc_drm.c           | 14 +++++++++++++-
 drivers/gpu/drm/solomon/ssd130x.c               |  4 ++--
 drivers/gpu/drm/tiny/cirrus.c                   |  2 +-
 drivers/gpu/drm/tiny/ofdrm.c                    | 13 +------------
 drivers/gpu/drm/tiny/simpledrm.c                |  4 ++--
 7 files changed, 30 insertions(+), 26 deletions(-)

-- 
2.38.1

Set the preferred color depth to 24 bits and the fbdev bpp to 32
bits. This will signal XRGB8888 as default format to clients.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c
b/drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c
index 22053c613644a..0c4aa4d9b0a77 100644
--- a/drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c
+++ b/drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c
@@ -106,7 +106,7 @@ static int hibmc_kms_init(struct hibmc_drm_private *priv)
 	dev->mode_config.max_width = 1920;
 	dev->mode_config.max_height = 1200;
 
-	dev->mode_config.preferred_depth = 32;
+	dev->mode_config.preferred_depth = 24;
 	dev->mode_config.prefer_shadow = 1;
 
 	dev->mode_config.funcs = (void *)&hibmc_mode_funcs;
@@ -340,7 +340,7 @@ static int hibmc_pci_probe(struct pci_dev *pdev,
 		goto err_unload;
 	}
 
-	drm_fbdev_generic_setup(dev, dev->mode_config.preferred_depth);
+	drm_fbdev_generic_setup(dev, 32);
 
 	return 0;
 
-- 
2.38.1

Logicvc can have different values for the preferred color depth. Set
the fbdev bpp value depending on the runtime value.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/logicvc/logicvc_drm.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/logicvc/logicvc_drm.c
b/drivers/gpu/drm/logicvc/logicvc_drm.c
index 9de24d9f0c963..d9cd5d967e31f 100644
--- a/drivers/gpu/drm/logicvc/logicvc_drm.c
+++ b/drivers/gpu/drm/logicvc/logicvc_drm.c
@@ -301,6 +301,7 @@ static int logicvc_drm_probe(struct platform_device *pdev)
 	struct regmap *regmap = NULL;
 	struct resource res;
 	void __iomem *base;
+	unsigned int preferred_bpp;
 	int irq;
 	int ret;
 
@@ -438,7 +439,18 @@ static int logicvc_drm_probe(struct platform_device *pdev)
 		goto error_mode;
 	}
 
-	drm_fbdev_generic_setup(drm_dev, drm_dev->mode_config.preferred_depth);
+	switch (drm_dev->mode_config.preferred_depth) {
+	case 15:
+	case 16:
+		preferred_bpp = 16;
+		break;
+	case 24:
+	case 32:
+	default:
+		preferred_bpp = 32;
+		break;
+	}
+	drm_fbdev_generic_setup(drm_dev, preferred_bpp);
 
 	return 0;
 
-- 
2.38.1

Cirrus has a preferred color depth of 16 bit; also use it as fbdev
bpp value. Don't use the color depth directly. It has a different
meaning than bpp and both cannot be used interchangeably.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/tiny/cirrus.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/tiny/cirrus.c b/drivers/gpu/drm/tiny/cirrus.c
index 678c2ef1cae70..cf35b60905032 100644
--- a/drivers/gpu/drm/tiny/cirrus.c
+++ b/drivers/gpu/drm/tiny/cirrus.c
@@ -604,7 +604,7 @@ static int cirrus_pci_probe(struct pci_dev *pdev,
 	if (ret)
 		return ret;
 
-	drm_fbdev_generic_setup(dev, dev->mode_config.preferred_depth);
+	drm_fbdev_generic_setup(dev, 16);
 	return 0;
 }
 
-- 
2.38.1

Set the preferred depth from the format of the scanout buffer. The
value cannot be hardcoded, as the scanout buffer's is only known at
runtime. Keeping the existing switch statement just duplicates the
driver's existing logic for format detection.

Also remove the FIXME comment from the call to drm_fbdev_generic_setup()
as the driver now handles color depth and bpp values correctly.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/tiny/ofdrm.c | 13 +------------
 1 file changed, 1 insertion(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/tiny/ofdrm.c b/drivers/gpu/drm/tiny/ofdrm.c
index dc9e4d71b12ae..33eefeba437c5 100644
--- a/drivers/gpu/drm/tiny/ofdrm.c
+++ b/drivers/gpu/drm/tiny/ofdrm.c
@@ -1284,14 +1284,7 @@ static struct ofdrm_device *ofdrm_device_create(struct
drm_driver *drv,
 	dev->mode_config.min_height = height;
 	dev->mode_config.max_height = max_height;
 	dev->mode_config.funcs = &ofdrm_mode_config_funcs;
-	switch (depth) {
-	case 32:
-		dev->mode_config.preferred_depth = 24;
-		break;
-	default:
-		dev->mode_config.preferred_depth = depth;
-		break;
-	}
+	dev->mode_config.preferred_depth = format->depth;
 	dev->mode_config.quirk_addfb_prefer_host_byte_order = true;
 
 	/* Primary plane */
@@ -1390,10 +1383,6 @@ static int ofdrm_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
-	/*
-	 * FIXME: 24-bit color depth does not work reliably with a 32-bpp
-	 * value. Force the bpp value of the scanout buffer's format.
-	 */
 	drm_fbdev_generic_setup(dev, drm_format_info_bpp(odev->format, 0));
 
 	return 0;
-- 
2.38.1

Set the preferred depth from the format of the scanout buffer. The
value cannot be hardcoded, as the scanout buffer's is only known at
runtime. Also derive the fbdev emulations bpp value from the scanout
format.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/tiny/simpledrm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/tiny/simpledrm.c b/drivers/gpu/drm/tiny/simpledrm.c
index 162eb44dcba89..30e928d627e8f 100644
--- a/drivers/gpu/drm/tiny/simpledrm.c
+++ b/drivers/gpu/drm/tiny/simpledrm.c
@@ -739,7 +739,7 @@ static struct simpledrm_device
*simpledrm_device_create(struct drm_driver *drv,
 	dev->mode_config.max_width = max_width;
 	dev->mode_config.min_height = height;
 	dev->mode_config.max_height = max_height;
-	dev->mode_config.preferred_depth = format->cpp[0] * 8;
+	dev->mode_config.preferred_depth = format->depth;
 	dev->mode_config.funcs = &simpledrm_mode_config_funcs;
 
 	/* Primary plane */
@@ -834,7 +834,7 @@ static int simpledrm_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
-	drm_fbdev_generic_setup(dev, 0);
+	drm_fbdev_generic_setup(dev, drm_format_info_bpp(sdev->format, 0));
 
 	return 0;
 }
-- 
2.38.1

Set the preferred color depth to 24 bits and the fbdev bpp to 32
bits. This will signal XRGB8888 as default format to clients.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/solomon/ssd130x.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/solomon/ssd130x.c
b/drivers/gpu/drm/solomon/ssd130x.c
index 53464afc2b9ac..c3bf3a18302ea 100644
--- a/drivers/gpu/drm/solomon/ssd130x.c
+++ b/drivers/gpu/drm/solomon/ssd130x.c
@@ -876,7 +876,7 @@ static int ssd130x_init_modeset(struct ssd130x_device
*ssd130x)
 	drm->mode_config.max_width = max_width;
 	drm->mode_config.min_height = mode->vdisplay;
 	drm->mode_config.max_height = max_height;
-	drm->mode_config.preferred_depth = 32;
+	drm->mode_config.preferred_depth = 24;
 	drm->mode_config.funcs = &ssd130x_mode_config_funcs;
 
 	/* Primary plane */
@@ -1006,7 +1006,7 @@ struct ssd130x_device *ssd130x_probe(struct device *dev,
struct regmap *regmap)
 	if (ret)
 		return ERR_PTR(dev_err_probe(dev, ret, "DRM device register
failed\n"));
 
-	drm_fbdev_generic_setup(drm, 0);
+	drm_fbdev_generic_setup(drm, 32);
 
 	return ssd130x;
 }
-- 
2.38.1

If no preferred value for bits-per-pixel has been given, fall back
to 32. Never use the preferred depth. The color depth is the number
of color/alpha bits per pixel, while bpp is the overall number of
bits in most cases.

Most noteworthy, XRGB8888 has a depth of 24 and a bpp value of 32.
Using depth for bpp would make the value 24 as well and format
selection in fbdev helpers fails. Unfortunately XRGB8888 is the most
common format and the old heuristic therefore fails for most of
the drivers (unless they implement the 24-bit RGB888 format).

Picking a bpp of 32 will lateron result in a default depth of 24
and the format XRGB8888. As XRGB8888 is the default format for most
of the current and legacy graphics stack, all drivers must support
it. So it is the safe choice.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/drm_fbdev_generic.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_fbdev_generic.c
b/drivers/gpu/drm/drm_fbdev_generic.c
index ab86956692795..0a4c160e0e58a 100644
--- a/drivers/gpu/drm/drm_fbdev_generic.c
+++ b/drivers/gpu/drm/drm_fbdev_generic.c
@@ -431,7 +431,6 @@ static const struct drm_client_funcs drm_fbdev_client_funcs
= {
  * drm_fbdev_generic_setup() - Setup generic fbdev emulation
  * @dev: DRM device
  * @preferred_bpp: Preferred bits per pixel for the device.
- *                 @dev->mode_config.preferred_depth is used if this is
zero.
  *
  * This function sets up generic fbdev emulation for drivers that supports
  * dumb buffers with a virtual address and that can be mmap'ed.
@@ -475,12 +474,16 @@ void drm_fbdev_generic_setup(struct drm_device *dev,
 	}
 
 	/*
-	 * FIXME: This mixes up depth with bpp, which results in a glorious
-	 * mess, resulting in some drivers picking wrong fbdev defaults and
-	 * others wrong preferred_depth defaults.
+	 * Pick a preferred bpp of 32 if no value has been given. This
+	 * will select XRGB8888 for the framebuffer formats. All drivers
+	 * have to support XRGB8888 for backwards compatibility with legacy
+	 * userspace, so it's the safe choice here.
+	 *
+	 * TODO: Replace struct drm_mode_config.preferred_depth and this
+	 *       bpp value with a preferred format that is given as struct
+	 *       drm_format_info. Then derive all other values from the
+	 *       format.
 	 */
-	if (!preferred_bpp)
-		preferred_bpp = dev->mode_config.preferred_depth;
 	if (!preferred_bpp)
 		preferred_bpp = 32;
 	fb_helper->preferred_bpp = preferred_bpp;
-- 
2.38.1