drm/sitronix/st7586: fix bad pixel data due to byte swap

Correctly set dbi->write_memory_bpw for the ST7586 driver. This driver
is for a monochrome display that has an unusual data format, so the
default value set in mipi_dbi_spi_init() is not correct simply because
this controller is non-standard.

Previously, we were using dbi->swap_bytes to make the same sort of
workaround, but it was removed in the same commit that added
dbi->write_memory_bpw, so we need to use the latter now to have the
correct behavior.

This fixes every 3 columns of pixels being swapped on the display. There
are 3 pixels per byte, so the byte swap caused this effect.

Fixes: df3fb27a74a4 ("drm/mipi-dbi: Make bits per word configurable for pixel transfers")
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20260228-drm-mipi-dbi-fix-st7586-byte-swap-v1-1-e78f6c24cd28@baylibre.com

diff --git a/drivers/gpu/drm/sitronix/st7586.c b/drivers/gpu/drm/sitronix/st7586.c
index b57ebf37a664c92652141715a7387273b7df34b2..16b6b4e368af85319cffba8ca1732a5514de1559 100644 (file)
--- a/drivers/gpu/drm/sitronix/st7586.c
+++ b/drivers/gpu/drm/sitronix/st7586.c
@@ -347,6 +347,12 @@ static int st7586_probe(struct spi_device *spi)
        if (ret)
                return ret;
 
+       /*
+        * Override value set by mipi_dbi_spi_init(). This driver is a bit
+        * non-standard, so best to set it explicitly here.
+        */
+       dbi->write_memory_bpw = 8;
+
        /* Cannot read from this controller via SPI */
        dbi->read_commands = NULL;
 
@@ -356,15 +362,6 @@ static int st7586_probe(struct spi_device *spi)
        if (ret)
                return ret;
 
-       /*
-        * we are using 8-bit data, so we are not actually swapping anything,
-        * but setting mipi->swap_bytes makes mipi_dbi_typec3_command() do the
-        * right thing and not use 16-bit transfers (which results in swapped
-        * bytes on little-endian systems and causes out of order data to be
-        * sent to the display).
-        */
-       dbi->swap_bytes = true;
-
        drm_mode_config_reset(drm);
 
        ret = drm_dev_register(drm, 0);