[PATCH v2 0/4] drm/bridge: samsung-dsim: Stop controlling vsync display FIFO flush in panels

[PATCH v2 0/4] drm/bridge: samsung-dsim: Stop controlling vsync display FIFO flush in panels

[Philipp Zabel]

This series enables the vsync flush feature in the samsung-dsim driver
unconditionally and removes the MIPI_DSI_MODE_VSYNC_FLUSH flag.

Background: I've recently seen shifted display issues on two different
i.MX8MM boards (mxsfb + samsung-dsim) with different DSI panels.
The symptoms were horizonally shifted display contents, with a stable
offset, in about 0.1 to 0.6 percent of modesets.
Enabling the MIPI_DSI_MODE_VSYNC_FLUSH flag in the panels' mode_flags
fixed the issue in both cases.

The samsung-dsim driver is the only DSI bridge driver that uses this
flag: If the flag is absent, the driver sets the DSIM_MFLUSH_VS bit in
the DSIM_CONFIG_REG register, which disables the vsync flush feature.
The reset value of this bit is cleared (vsync flush is default-enabled).
According to the i.MX8MM reference manual,

    "It needs that Main display FIFO should be flushed for deleting
     garbage data."

This appears to match the comment in mxsfb_reset_block() in mxsfb_kms.c:

    /*
     * It seems, you can't re-program the controller if it is still
     * running. This may lead to shifted pictures (FIFO issue?), so
     * first stop the controller and drain its FIFOs.
     */

Now I wonder why the bit is controlled by a flag in the panel drivers.
Whether the display controller pushes up to a FIFO worth of garbage data
into the DSI bridge during initialization seems to be a property of the
display controller / DSI bridge integration (whether this is due to
hardware or driver bugs), not a specific requirement of the panel.
Surely no panel needs to receive a partial line of garbage data in front
of the first frame?

Instead of adding the flag to every panel connected to affected SoCs,
the vsync flush feature could just be enabled unconditionally.
Clearing an already-empty display FIFO should have no effect, unless
I'm missing something? With that, the MIPI_DSI_MODE_VSYNC_FLUSH flag
would not be used anymore and could be removed.

regards
Philipp

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
---
Changes in v2:
- Collect Marek's Acked-by.
- Drop RFC tag.
- No further changes.
- Link to v1: https://lore.kernel.org/r/20250527-dsi-vsync-flush-v1-0-9b4ea4578729@pengutronix.de

---
Philipp Zabel (4):
      drm/bridge: samsung-dsim: Always flush display FIFO on vsync pulse
      drm/panel: samsung-s6d7aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag
      drm/panel: samsung-s6e8aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag
      drm/mipi-dsi: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

 drivers/gpu/drm/bridge/samsung-dsim.c         | 2 --
 drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c | 2 +-
 drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c | 2 +-
 include/drm/drm_mipi_dsi.h                    | 2 --
 4 files changed, 2 insertions(+), 6 deletions(-)
---
base-commit: b462b0ef4d788d56f0e575406e58450358dcbd96
change-id: 20250527-dsi-vsync-flush-8f8cc91a6f04

Best regards,
-- 
Philipp Zabel <p.zabel@pengutronix.de>

[PATCH v2 1/4] drm/bridge: samsung-dsim: Always flush display FIFO on vsync pulse

[Philipp Zabel]

Always flush the display FIFO on vsync pulse, even if not explicitly
requested by the panel via MIPI_DSI_MODE_VSYNC_FLUSH mode_flag.

The display FIFO should be empty at vsync. Flushing it at vsync pulses
helps to remove garbage that may have entered the FIFO during startup
(if synchronisation between upstream display controller and Samsung DSIM
is lacking) and that may persist in form of last frame's leftovers on
subsequent frames. Flushing the display FIFO if it is already empty
should have no effect.

This will allow to remove the MIPI_DSI_MODE_VSYNC_FLUSH flag, which is
only used by the Samsung DSIM bridge driver. Arguably this flag doesn't
belong in the panel configuration at all: flushing the display FIFO on
vsync is a workaround for issues with the integration between display
controller and DSI bridge, not a property of the DSI link between bridge
and panel. No panel actually has a requirement to receive garbage or old
frame content after vsync.

I wonder if host controller FIFO resets are mentioned by the MIPI DSI
specification at all. This patch is based on the assumption that the
MIPI_DSI_MODE_VSYNC_FLUSH flag only exists because the DSIM_MFLUSH_VS
bit happens to be located in the same register as the bits controlling
the DSI mode.

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
---
 drivers/gpu/drm/bridge/samsung-dsim.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
index 8ec6f4e7952f1f0cc1fc977732d8e457d6eb5012..c4997795db18280903570646b0a5b2c03b666307 100644
--- a/drivers/gpu/drm/bridge/samsung-dsim.c
+++ b/drivers/gpu/drm/bridge/samsung-dsim.c
@@ -897,8 +897,6 @@ static int samsung_dsim_init_link(struct samsung_dsim *dsi)
 		 * The user manual describes that following bits are ignored in
 		 * command mode.
 		 */
-		if (!(dsi->mode_flags & MIPI_DSI_MODE_VSYNC_FLUSH))
-			reg |= DSIM_MFLUSH_VS;
 		if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE)
 			reg |= DSIM_SYNC_INFORM;
 		if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_BURST)

-- 
2.39.5

[PATCH v2 2/4] drm/panel: samsung-s6d7aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

[Philipp Zabel]

Drop the MIPI_DSI_MODE_VSYNC_FLUSH flag from DSI mode_flags.
It has no effect anymore.

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
---
 drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c b/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c
index b5b9e80690f66cc62acbd341865d8d47419e498b..692020081524ad9d1bc815464fa447b5a2d9241d 100644
--- a/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c
+++ b/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c
@@ -244,7 +244,7 @@ static const struct s6d7aa0_panel_desc s6d7aa0_lsl080al02_desc = {
 	.init_func = s6d7aa0_lsl080al02_init,
 	.off_func = s6d7aa0_lsl080al02_off,
 	.drm_mode = &s6d7aa0_lsl080al02_mode,
-	.mode_flags = MIPI_DSI_MODE_VSYNC_FLUSH | MIPI_DSI_MODE_VIDEO_NO_HFP,
+	.mode_flags = MIPI_DSI_MODE_VIDEO_NO_HFP,
 	.bus_flags = 0,
 
 	.has_backlight = false,

-- 
2.39.5

[PATCH v2 3/4] drm/panel: samsung-s6e8aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

[Philipp Zabel]

Drop the MIPI_DSI_MODE_VSYNC_FLUSH flag from DSI mode_flags.
It has no effect anymore.

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
---
 drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c b/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c
index 897df195f2f3437224d1fe9f42d0bfc761541ab2..1b5c500d4f4eb7d43dff4b452a0f1b1bc06f5a2c 100644
--- a/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c
+++ b/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c
@@ -992,7 +992,7 @@ static int s6e8aa0_probe(struct mipi_dsi_device *dsi)
 	dsi->lanes = 4;
 	dsi->format = MIPI_DSI_FMT_RGB888;
 	dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST
-		| MIPI_DSI_MODE_VSYNC_FLUSH | MIPI_DSI_MODE_VIDEO_AUTO_VERT;
+		| MIPI_DSI_MODE_VIDEO_AUTO_VERT;
 
 	ret = s6e8aa0_parse_dt(ctx);
 	if (ret < 0)

-- 
2.39.5

[PATCH v2 4/4] drm/mipi-dsi: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

[Philipp Zabel]

Drop the unused MIPI_DSI_MODE_VSYNC_FLUSH flag. Whether or not a display
FIFO flush on vsync is required to avoid sending garbage to the panel is
not a property of the DSI link, but of the integration between display
controller and DSI host bridge.

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
---
 include/drm/drm_mipi_dsi.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
index b37860f4a895c25ef8ba1c5b3f44827ef53aa100..369b0d8830c3d14a4fc1e8e38d5fa55f04ca143e 100644
--- a/include/drm/drm_mipi_dsi.h
+++ b/include/drm/drm_mipi_dsi.h
@@ -130,8 +130,6 @@ struct mipi_dsi_host *of_find_mipi_dsi_host_by_node(struct device_node *node);
 #define MIPI_DSI_MODE_VIDEO_NO_HBP	BIT(6)
 /* disable hsync-active area */
 #define MIPI_DSI_MODE_VIDEO_NO_HSA	BIT(7)
-/* flush display FIFO on vsync pulse */
-#define MIPI_DSI_MODE_VSYNC_FLUSH	BIT(8)
 /* disable EoT packets in HS mode */
 #define MIPI_DSI_MODE_NO_EOT_PACKET	BIT(9)
 /* device supports non-continuous clock behavior (DSI spec 5.6.1) */

-- 
2.39.5

Re: [PATCH v2 2/4] drm/panel: samsung-s6d7aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

[Neil Armstrong]

On 27/06/2025 13:45, Philipp Zabel wrote:
> Drop the MIPI_DSI_MODE_VSYNC_FLUSH flag from DSI mode_flags.
> It has no effect anymore.
> 
> Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
> ---
>   drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c b/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c
> index b5b9e80690f66cc62acbd341865d8d47419e498b..692020081524ad9d1bc815464fa447b5a2d9241d 100644
> --- a/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c
> +++ b/drivers/gpu/drm/panel/panel-samsung-s6d7aa0.c
> @@ -244,7 +244,7 @@ static const struct s6d7aa0_panel_desc s6d7aa0_lsl080al02_desc = {
>   	.init_func = s6d7aa0_lsl080al02_init,
>   	.off_func = s6d7aa0_lsl080al02_off,
>   	.drm_mode = &s6d7aa0_lsl080al02_mode,
> -	.mode_flags = MIPI_DSI_MODE_VSYNC_FLUSH | MIPI_DSI_MODE_VIDEO_NO_HFP,
> +	.mode_flags = MIPI_DSI_MODE_VIDEO_NO_HFP,
>   	.bus_flags = 0,
>   
>   	.has_backlight = false,
> 

Acked-by: Neil Armstrong <neil.armstrong@linaro.org>

Re: [PATCH v2 3/4] drm/panel: samsung-s6e8aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

[Neil Armstrong]

On 27/06/2025 13:45, Philipp Zabel wrote:
> Drop the MIPI_DSI_MODE_VSYNC_FLUSH flag from DSI mode_flags.
> It has no effect anymore.
> 
> Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
> ---
>   drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c b/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c
> index 897df195f2f3437224d1fe9f42d0bfc761541ab2..1b5c500d4f4eb7d43dff4b452a0f1b1bc06f5a2c 100644
> --- a/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c
> +++ b/drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c
> @@ -992,7 +992,7 @@ static int s6e8aa0_probe(struct mipi_dsi_device *dsi)
>   	dsi->lanes = 4;
>   	dsi->format = MIPI_DSI_FMT_RGB888;
>   	dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST
> -		| MIPI_DSI_MODE_VSYNC_FLUSH | MIPI_DSI_MODE_VIDEO_AUTO_VERT;
> +		| MIPI_DSI_MODE_VIDEO_AUTO_VERT;
>   
>   	ret = s6e8aa0_parse_dt(ctx);
>   	if (ret < 0)
> 

Acked-by: Neil Armstrong <neil.armstrong@linaro.org>

All patches can go with patch 1.

Neil

Re: [PATCH v2 4/4] drm/mipi-dsi: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

[Neil Armstrong]

On 27/06/2025 13:45, Philipp Zabel wrote:
> Drop the unused MIPI_DSI_MODE_VSYNC_FLUSH flag. Whether or not a display
> FIFO flush on vsync is required to avoid sending garbage to the panel is
> not a property of the DSI link, but of the integration between display
> controller and DSI host bridge.
> 
> Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
> ---
>   include/drm/drm_mipi_dsi.h | 2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
> index b37860f4a895c25ef8ba1c5b3f44827ef53aa100..369b0d8830c3d14a4fc1e8e38d5fa55f04ca143e 100644
> --- a/include/drm/drm_mipi_dsi.h
> +++ b/include/drm/drm_mipi_dsi.h
> @@ -130,8 +130,6 @@ struct mipi_dsi_host *of_find_mipi_dsi_host_by_node(struct device_node *node);
>   #define MIPI_DSI_MODE_VIDEO_NO_HBP	BIT(6)
>   /* disable hsync-active area */
>   #define MIPI_DSI_MODE_VIDEO_NO_HSA	BIT(7)
> -/* flush display FIFO on vsync pulse */
> -#define MIPI_DSI_MODE_VSYNC_FLUSH	BIT(8)
>   /* disable EoT packets in HS mode */
>   #define MIPI_DSI_MODE_NO_EOT_PACKET	BIT(9)
>   /* device supports non-continuous clock behavior (DSI spec 5.6.1) */
> 

Acked-by: Neil Armstrong <neil.armstrong@linaro.org>

Re: [PATCH v2 0/4] drm/bridge: samsung-dsim: Stop controlling vsync display FIFO flush in panels

[Philipp Zabel]

On Fr, 2025-06-27 at 13:45 +0200, Philipp Zabel wrote:
> This series enables the vsync flush feature in the samsung-dsim driver
> unconditionally and removes the MIPI_DSI_MODE_VSYNC_FLUSH flag.

Applied to drm-misc-next.

[1/4] drm/bridge: samsung-dsim: Always flush display FIFO on vsync pulse
      commit: a36c533ad3e1b8c0326d665d2d02d2ef7aea8f92
[2/4] drm/panel: samsung-s6d7aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag
      commit: f488640e51312fbca02f9229d238a2aed7a8aac9
[3/4] drm/panel: samsung-s6e8aa0: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag
      commit: 5c449ddbaba1c807c4674f92e5ebf6a887edb58c
[4/4] drm/mipi-dsi: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag
      commit: 12853b279100adc7074b51db23bf62a41cb84cb8