Re: [PATCH 2/2] drm/panel: Add driver for Visionox r66451 panel

From: Jessica Zhang
Date: Tue May 30 2023 - 13:44:00 EST

On 5/29/2023 5:09 AM, Marijn Suijten wrote:
On 2023-05-26 09:32:45, Neil Armstrong wrote:
+static int visionox_r66451_bl_update_status(struct backlight_device *bl)
+    struct mipi_dsi_device *dsi = bl_get_data(bl);
+    u16 brightness = backlight_get_brightness(bl);
+    return mipi_dsi_dcs_set_display_brightness(dsi, cpu_to_le16(brightness));

mipi_dsi_dcs_set_display_brightness() already converts the brightness,
so you don't need cpu_to_le16 here.

Tread carefully here: we've had the same issue and conversation on our
Sony panels where this extra inversion is required.
set_display_brightness() sends the bytes as little-endian to the panel
(and it even assumes little-endian in get_display_brightness()) but the
spec for 16-bit brightness values states that they have to be sent in
big-endian.  This is why c9d27c6be518b ("drm/mipi-dsi: Fix byte order of
16-bit DCS set/get brightness") added

Jessica, if you need to have the endian swap here (should be very easy
to test with a real panel, but it should be given the max_brightness
value being over 8 bits) please switch to the _large() variant.

Got it, thanks for the heads up!

Hi Marijn,

Just wanted to update this thread -- I've checked the backlight brightness values in the sysfs and it matches the value being given in the panel driver (255), so I think it should be fine to use *_set_display_brightness() without the _large() variant.

Sure, I was also misleaded by you using cpu_to_le16() but as Dmitry said it's already
done in mipi_dsi_dcs_set_display_brightness() and a no-op on LE arm64 platforms anyway.

Yuck, right, it's cpu_to_le16 here and not cpu_to_be16. @Jessica, can
you please remove this misleading conversion?
mipi_dsi_dcs_set_display_brightness() takes a native u16, not a specific

Hi Marijn,

Acked, will drop the conversion.


Jessica Zhang

- Marijn