The DSI frequency must be in the range:

  (CHA_DSI_CLK_RANGE * 5 MHz) <= DSI freq < ((CHA_DSI_CLK_RANGE + 1) * 5 MHz)

So the register value shouldpoint to the lower range value, but
DIV_ROUND_UP() rounds the division to the higher range value, resulting in
an excess of 1 (unless the frequency is an exact multiple of 5 MHz).

For example for a 437100000 MHz clock CHA_DSI_CLK_RANGE should be 87 (0x57):

  (87 * 5 = 435) <= 437.1 < (88 * 5 = 440)

but current code returns 88 (0x58).

Fix the computation by removing the DIV_ROUND_UP().

Fixes: ceb515ba29ba ("drm/bridge: ti-sn65dsi83: Add TI SN65DSI83 and SN65DSI84 driver")
Cc: stable@vger.kernel.org
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 drivers/gpu/drm/bridge/ti-sn65dsi83.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
index f6736b4457bb..d2a81175d279 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
@@ -351,9 +351,9 @@ static u8 sn65dsi83_get_dsi_range(struct sn65dsi83 *ctx,
 	 *  DSI_CLK = mode clock * bpp / dsi_data_lanes / 2
 	 * the 2 is there because the bus is DDR.
 	 */
-	return DIV_ROUND_UP(clamp((unsigned int)mode->clock *
-			    mipi_dsi_pixel_format_to_bpp(ctx->dsi->format) /
-			    ctx->dsi->lanes / 2, 40000U, 500000U), 5000U);
+	return clamp((unsigned int)mode->clock *
+		     mipi_dsi_pixel_format_to_bpp(ctx->dsi->format) /
+		     ctx->dsi->lanes / 2, 40000U, 500000U) / 5000U;
 }
 
 static u8 sn65dsi83_get_dsi_div(struct sn65dsi83 *ctx)

-- 
2.53.0

On 2/26/26 5:16 PM, Luca Ceresoli wrote:
> The DSI frequency must be in the range:
> 
>    (CHA_DSI_CLK_RANGE * 5 MHz) <= DSI freq < ((CHA_DSI_CLK_RANGE + 1) * 5 MHz)
> 
> So the register value shouldpoint to the lower range value, but

should point (missing space)

> DIV_ROUND_UP() rounds the division to the higher range value, resulting in
> an excess of 1 (unless the frequency is an exact multiple of 5 MHz).
> 
> For example for a 437100000 MHz clock CHA_DSI_CLK_RANGE should be 87 (0x57):
> 
>    (87 * 5 = 435) <= 437.1 < (88 * 5 = 440)
> 
> but current code returns 88 (0x58).
> 
> Fix the computation by removing the DIV_ROUND_UP().
> 
> Fixes: ceb515ba29ba ("drm/bridge: ti-sn65dsi83: Add TI SN65DSI83 and SN65DSI84 driver")
> Cc: stable@vger.kernel.org
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

Reviewed-by: Marek Vasut <marek.vasut@mailbox.org>

Thanks !