[PATCH 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@xxxxxxxx>
---
 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