[PATCH RESEND 0/2] drm/tiny: sharp-memory: fix TX buffer corruption on partial update

[PATCH RESEND 0/2] drm/tiny: sharp-memory: fix TX buffer corruption on partial update

[Tobias Johansson]

Users running applications that submit partial framebuffer updates
(such as LVGL with dirty-region tracking) can observe persistent
flickering on the display when using animations.

The flickering is caused by two bugs that corrupt the TX buffer on
partial updates. The first patch fixes incorrect line address
assignment in sharp_memory_set_tx_buffer_addresses(). The second
patch fixes stale data transmission in
sharp_memory_update_display(). Both patches are needed to fully
eliminate the flickering.

Signed-off-by: Tobias Johansson <tobias.johansson@axis.com>
---
Tobias Johansson (2):
      drm/tiny: sharp-memory: fix line address assignment on partial update
      drm/tiny: sharp-memory: avoid transmitting stale TX buffer data

 drivers/gpu/drm/tiny/sharp-memory.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
---
base-commit: ba683f774299d89d17cde03bb1bdb13f3513cd20
change-id: 20260316-flickering-f9df09243d2f

Best regards,
-- 
Tobias Johansson <tobias.johansson@axis.com>

[PATCH RESEND 1/2] drm/tiny: sharp-memory: fix line address assignment on partial update

[Tobias Johansson]

When only a subset of lines is dirty, the TX buffer sent to the
panel contains incorrect line addresses, resulting in visible
flickering on the display.

sharp_memory_set_tx_buffer_addresses() iterates from line 0 to the
last damaged line, assigning addresses sequentially from 1. When
only lines 10-20 are dirty, line 10's pixel data is written to the
slot with address 1 instead of address 11, corrupting the address-
to-data mapping.

Fix sharp_memory_set_tx_buffer_addresses() to iterate over only the
damaged line count and offset assigned addresses by the clip start,
so that addresses match the pixel data that follows.

Fixes: b8f9f21716fec ("drm/tiny: Add driver for Sharp Memory LCD")
Signed-off-by: Tobias Johansson <tobias.johansson@axis.com>
---
 drivers/gpu/drm/tiny/sharp-memory.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/tiny/sharp-memory.c b/drivers/gpu/drm/tiny/sharp-memory.c
index cbf69460ebf3..595926ed660e 100644
--- a/drivers/gpu/drm/tiny/sharp-memory.c
+++ b/drivers/gpu/drm/tiny/sharp-memory.c
@@ -120,8 +120,8 @@ static inline void sharp_memory_set_tx_buffer_addresses(u8 *buffer,
 							struct drm_rect clip,
 							u32 pitch)
 {
-	for (u32 line = 0; line < clip.y2; ++line)
-		buffer[line * pitch] = line + 1;
+	for (u32 line = 0; line < drm_rect_height(&clip); ++line)
+		buffer[line * pitch] = clip.y1 + line + 1;
 }
 
 static void sharp_memory_set_tx_buffer_data(u8 *buffer,

-- 
2.43.0

[PATCH RESEND 2/2] drm/tiny: sharp-memory: avoid transmitting stale TX buffer data

[Tobias Johansson]

When only a subset of lines is dirty, the TX buffer sent to the
panel contains stale line data from previous updates, resulting
in visible flickering on the display.

sharp_memory_update_display() transmits the entire TX buffer
regardless of how many lines were updated. Entries written by a
previous larger update linger in the buffer and
are retransmitted on every subsequent smaller update, overwriting
the newly written data with stale content.

Fix sharp_memory_update_display() to transmit only the buffer
entries populated by the current update.

Fixes: b8f9f21716fec ("drm/tiny: Add driver for Sharp Memory LCD")
Signed-off-by: Tobias Johansson <tobias.johansson@axis.com>
---
 drivers/gpu/drm/tiny/sharp-memory.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/tiny/sharp-memory.c b/drivers/gpu/drm/tiny/sharp-memory.c
index 595926ed660e..e7521fc6a010 100644
--- a/drivers/gpu/drm/tiny/sharp-memory.c
+++ b/drivers/gpu/drm/tiny/sharp-memory.c
@@ -155,7 +155,7 @@ static int sharp_memory_update_display(struct sharp_memory_device *smd,
 	u32 pitch = smd->pitch;
 	u8 vcom = smd->vcom;
 	u8 *tx_buffer = smd->tx_buffer;
-	u32 tx_buffer_size = smd->tx_buffer_size;
+	u32 tx_len = 1 + (drm_rect_height(&clip) * pitch);
 
 	mutex_lock(&smd->tx_mutex);
 
@@ -165,7 +165,7 @@ static int sharp_memory_update_display(struct sharp_memory_device *smd,
 	sharp_memory_set_tx_buffer_addresses(&tx_buffer[1], clip, pitch);
 	sharp_memory_set_tx_buffer_data(&tx_buffer[2], fb, vmap, clip, pitch, fmtcnv_state);
 
-	ret = sharp_memory_spi_write(smd->spi, tx_buffer, tx_buffer_size);
+	ret = sharp_memory_spi_write(smd->spi, tx_buffer, tx_len);
 
 	mutex_unlock(&smd->tx_mutex);
 

-- 
2.43.0