Re: [PATCH v2] drm: shmobile: Fix blank screen after resume when LCDC is stopped

[phucduc.bui@gmail.com]

Hi Geert,

> Thanks for your patch!

> What do you mean by "deep sleep"? s2ram? In upstream, s2ram behaves
> the same as s2idle, and the LCD works fine after resume from s2ram on
> my Amadillo, with and without your patch,

> What am I missing?
> Thanks!

Thank you for your feedback.

To clarify the "deep sleep" context, I have tested both s2idle and deep 
states using:
- echo freeze > /sys/power/state
- echo mem > /sys/power/state

On my hardware, the issue is consistently reproducible in both cases, 
resulting in a completely white screen after resume.

Testing on Stable and Upstream:
I have verified this behavior on both the stable and latest upstream 
kernels. While the patch was originally developed on a stable branch, I 
have confirmed that it applies cleanly to the current upstream tree and 
successfully resolves the issue there as well.

Context and Environment Difference:
I first tested several Armadillo-800 EVA boards more than 10 years ago, 
and this specific resume failure was already a known, persistent 
issue back then. After 10 years, I am returning to this platform and 
testing with my current hardware, and I can confirm the behavior 
remains exactly the same.

Regarding why it works on your side: I cannot be certain about the 
specific differences in our hardware configurations. However, on my 
setup, the debug traces clearly confirm that the LDSA1R register is 
reset to 0 after resume.

Video demonstration:
I have recorded a short video demonstrating the bug (white screen after 
pressing "Switch 3" to resume):

https://youtu.be/0n9NHeZ7MWU

Detailed Logs and Configuration:
I have uploaded the kernel config and dmesg logs (including debug 
traces for LDSA1R) to this Gist:

https://gist.github.com/BuiDucPhuc/b31fc7ee784e65d192b2f49a406c3326

Technical Analysis:
I focused my debug traces on the LDSA1R register. The logs confirm that 
during resume, LDSA1R is reset to 0. When the driver updates the 
registers:
1. It writes the correct address to Side B (Mirror).
2. However, according to Figure 38.13 ("Two-set register switching 
timing") in the R8A7740 Group Hardware Manual, the transfer from Side B 
to Side A requires a Frame End interrupt.
3. Since the controller is currently stopped (DO=0), no frame is 
processed and no Frame End pulse is generated. Consequently, Side A 
remains 0x00000000, leading to the DMA fetch error and white screen.

This priming patch ensures Side A is explicitly initialized while the 
switching logic is idle, making the driver robust against register 
context loss.

Best regards,
Bui Duc Phuc