On most MSM8939 devices, the bootloader already initializes the display to
show the boot splash screen. In this situation, MDSS is already configured
and left running when starting Linux. To avoid side effects from the
bootloader configuration, the MDSS reset can be specified in the device
tree to start again with a clean hardware state.

The reset for MDSS is currently missing in msm8939.dtsi, which causes
errors when the MDSS driver tries to re-initialize the registers:

 dsi_err_worker: status=6
 dsi_err_worker: status=6
 dsi_err_worker: status=6
 ...

It turns out that we have always indirectly worked around this by building
the MDSS driver as a module. Before v6.17, the power domain was temporarily
turned off until the module was loaded, long enough to clear the register
contents. In v6.17, power domains are not turned off during boot until
sync_state() happens, so this is no longer working. Even before v6.17 this
resulted in broken behavior, but notably only when the MDSS driver was
built-in instead of a module.

Cc: stable@vger.kernel.org
Fixes: 61550c6c156c ("arm64: dts: qcom: Add msm8939 SoC")
Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
---
 arch/arm64/boot/dts/qcom/msm8939.dtsi | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm64/boot/dts/qcom/msm8939.dtsi b/arch/arm64/boot/dts/qcom/msm8939.dtsi
index 68b92fdb996c26e7a1aadedf0f52e1afca85c4ab..eb64ec35e7f0e1c63b0b96f68d30006c2e440998 100644
--- a/arch/arm64/boot/dts/qcom/msm8939.dtsi
+++ b/arch/arm64/boot/dts/qcom/msm8939.dtsi
@@ -1249,6 +1249,8 @@ mdss: display-subsystem@1a00000 {
 
 			power-domains = <&gcc MDSS_GDSC>;
 
+			resets = <&gcc GCC_MDSS_BCR>;
+
 			#address-cells = <1>;
 			#size-cells = <1>;
 			#interrupt-cells = <1>;

-- 
2.50.1

On Mon, Sep 15, 2025 at 03:28:31PM +0200, Stephan Gerhold wrote:
> On most MSM8939 devices, the bootloader already initializes the display to
> show the boot splash screen. In this situation, MDSS is already configured
> and left running when starting Linux. To avoid side effects from the
> bootloader configuration, the MDSS reset can be specified in the device
> tree to start again with a clean hardware state.
> 
> The reset for MDSS is currently missing in msm8939.dtsi, which causes
> errors when the MDSS driver tries to re-initialize the registers:
> 
>  dsi_err_worker: status=6
>  dsi_err_worker: status=6
>  dsi_err_worker: status=6
>  ...
> 
> It turns out that we have always indirectly worked around this by building
> the MDSS driver as a module. Before v6.17, the power domain was temporarily
> turned off until the module was loaded, long enough to clear the register
> contents. In v6.17, power domains are not turned off during boot until
> sync_state() happens, so this is no longer working. Even before v6.17 this
> resulted in broken behavior, but notably only when the MDSS driver was
> built-in instead of a module.
> 
> Cc: stable@vger.kernel.org
> Fixes: 61550c6c156c ("arm64: dts: qcom: Add msm8939 SoC")
> Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
> ---
>  arch/arm64/boot/dts/qcom/msm8939.dtsi | 2 ++
>  1 file changed, 2 insertions(+)
> 

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>


-- 
With best wishes
Dmitry

On 9/15/25 3:28 PM, Stephan Gerhold wrote:
> On most MSM8939 devices, the bootloader already initializes the display to
> show the boot splash screen. In this situation, MDSS is already configured
> and left running when starting Linux. To avoid side effects from the
> bootloader configuration, the MDSS reset can be specified in the device
> tree to start again with a clean hardware state.
> 
> The reset for MDSS is currently missing in msm8939.dtsi, which causes
> errors when the MDSS driver tries to re-initialize the registers:
> 
>  dsi_err_worker: status=6
>  dsi_err_worker: status=6
>  dsi_err_worker: status=6
>  ...
> 
> It turns out that we have always indirectly worked around this by building
> the MDSS driver as a module. Before v6.17, the power domain was temporarily
> turned off until the module was loaded, long enough to clear the register
> contents. In v6.17, power domains are not turned off during boot until
> sync_state() happens, so this is no longer working. Even before v6.17 this
> resulted in broken behavior, but notably only when the MDSS driver was
> built-in instead of a module.
> 
> Cc: stable@vger.kernel.org
> Fixes: 61550c6c156c ("arm64: dts: qcom: Add msm8939 SoC")
> Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
> ---

Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>

Konrad