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soha

writing device register has no effect

Question

 

I have a Ubuntu Linux (4.6 kernel) running on a Zedboard, which is based on an ARM architecture. The architecture comes with a hardware module called Coresight, which enables HW-based program tracing. In my setup, the Coresight component is registered at physical address 0xf889c000. I want to configure the component through the device register etmcr, whose offset is 0x0. I tried to run devmem2 (a program helps manipulate physical memory via mmap) as a root to write some custom value to this register. The mmap threw no error, the write succeeded since the immediate readback value was the expected value. However, when I read this address again, the value didn't actually change. (Please see the attached screenshot).

I've checked the Technical Manual of the Coresight and this register is for sure a RW register. So what could be the reason of what happened here? Why the register write succeeds but has no effect actually? Thanks in advance.

 

Capture.JPG

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4 answers to this question

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1 hour ago, jpeyron said:

Hi @soha,

I have reached out to a more experienced embedded Linux engineer to see if they have any input for you. 

cheers,

Jon

Thanks @jpeyron . I'm looking forward to it.

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21 hours ago, jpeyron said:

Hi @soha,

I also was wondering if you have had a chance to look at the information about coresight on linaro here?

cheers,

Jon

Hi @jpeyron,

Yes I checked the link but it seems there is no tutorial on how to manipulate these device registers... I've also submitted a request to the Linaro community but haven't received any reply... Anyway, if we regard the coresight component as a normal hardware device, there might be (I guess) some common solution for my case?

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