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vsergeev / python-periphery

A pure Python 2/3 library for peripheral I/O (GPIO, LED, PWM, SPI, I2C, MMIO, Serial) in Linux.
MIT License
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Writing to an MMIO has no effect --- what do I the wrong way? #15

Closed alpintrekker closed 4 years ago

alpintrekker commented 6 years ago

Hi @vsergeev

I'd power down USB0 PHY on an AM437x, so I was doing this as root from ipython console: from periphery import MMIO USB0_CTRL_MMIO = MMIO(0x44E10620, 0x1000) hex(USB0_CTRL_MMIO.read32(0x00)) the last line gives output 0x3c186004 then I executed: USB0_CTRL_MMIO.write32(0x00, USB0_CTRL_MMIO.read32(0x00) | 0x3) but reading back hex(USB0_CTRL_MMIO.read32(0x00)) gives the same: 0x3c186004 instead of 0x3c186007

Could you please help me, what do I the wrong way?

ukleinek commented 4 years ago

Hello,

I didn't check the relevant hardware manual, but I guess the issue is just that the MMIO register you write to is readonly (or at least bits 0 and 1 are). Or maybe the register is not writable in user mode?

To rule out problems with python-periphery, you might want to use a program like memtool. The equivalent of your program is:

memtool md 0x44E10620
memtool mw 0x44E10620 0x3c186007
memtool md 0x44E10620

Best regards Uwe

vsergeev commented 4 years ago

I found the register on page 676 of http://www.ti.com/lit/ug/spruhl7h/spruhl7h.pdf. It looks like those bits are R/W and should retain their value. I second @ukleinek's recommendation of using a tool like memtool or devmem to write it. In addition, you could try reading and writing a scratchpad register RTCSS_SCRATCH0_REG.

It's also possible (but maybe not likely, in this case) that a kernel driver is overwriting the register behind the scene (e.g. if the power down caused an event that cause the driver to overwrite it).

I'm going to close this issue for now, but if you do happen to try again and see an issue with python-periphery writing data that can't be reproduced with the other tools, please re-open.