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A comment on Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen resolution. Parent comment

Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by abainbridge on Thursday November 28, 2019 @05:02AM (#21655971)
5GHz WiFi has more bandwidth than 2.4GHz, so typically will involve larger IO buffers in the driver, which could easily be enough to expose a memory scribbler (I imagine there's a bunch of other features that are enabled/disabled by the frequency band switch too). However, I think asdfasgasdgasdg's answer is the correct reason not to suspect a memory scribbler - ie a memory scribbler would cause the driver to crash/fail and the kernel would log a message.
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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by londons_explore on Thursday November 28, 2019 @11:19AM (#21658379)
Remember the Pi has an odd architecture and all the IO passes through the GPU. The GPU doesn't log human readable messages anywhere. There's a good chance the GPU did log a crash or failure, but only broadcoms engineers can see it.
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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by abainbridge on Thursday November 28, 2019 @11:51AM (#21658659)
Er, really, wow. I didn't know that. Can you point me at some more info for that? Surely the GPIOs don't go via the GPU.
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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by londons_explore on Thursday November 28, 2019 @12:12PM (#21658812)
It's a BCM2711, and the datasheet is NDA only - typical Broadcom!

The VideoCore (Broadcoms GPU) is the main processor on the thing, and the cluster of ARM cores that run Linux are more of a coprocessor which can only see some of RAM.

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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by StavrosK on Thursday November 28, 2019 @05:18AM (#21656042)
But 5 GHz doesn't fail, only 2.4 GHz does.
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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by wolfgke on Thursday November 28, 2019 @07:15AM (#21656669)
This is exactly abainbridge's point.
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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by StavrosK on Thursday November 28, 2019 @07:46AM (#21656873)
How do you mean?

> 5GHz WiFi has more bandwidth than 2.4GHz, so typically will involve larger IO buffers in the driver, which could easily be enough to expose a memory scribbler

He's saying 5 GHz will expose the scribbler, and the opposite is happening, only 2.4 GHz fails.

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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by abainbridge on Thursday November 28, 2019 @11:48AM (#21658645)
@StavrosK Thanks for wading in in my defence, but I had actually mis-understood the situation :-)

Although, if my theory that the IO buffers are different sizes is true, then that could perturb memory layout enough to expose/hide the bug in either direction.

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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by StavrosK on Thursday November 28, 2019 @11:51AM (#21658666)
Haha, I was actually the attacker in this instance :)

I do agree that the different IO buffers might hide the bug in one instance, but I think this is just plain old RF noise.

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Re:Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen r... by abainbridge on Thursday November 28, 2019 @12:54PM (#21659066)
> Haha, I was actually the attacker in this instance :)

I need more sleep.

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