On Samstag, 27. Juli 2019 22:53:03 CEST Lucas Jenss wrote: > Hey folks, > > I'm currently trying to get JTAG debugging of a Raspberry Pi 3 to work > properly, and ran into an issue. When issuing debug commands with GDB, > such as single step while the processor is in user mode (0x10), I'm > getting: > > error: cannot read system control register in this mode > > While the single-step is still executed (the program counter advances), > GDB acts as if the single step is never completed, so I have to hit ^C > again to see the new state. > > My question is: why does "aarch64_post_debug_entry" _need_ to read the > system control register. I do see that further down in the function > there's internal state being updated, but I have no idea of what the > implications are of not being able to update this state. As an > experiment, I changed the "return ERROR_FAIL" to "return ERROR_OK" after > the "error: cannot read system control register in this mode" message, > and stepping is now working properly for my limited use case. > > Some insight into why this is written this way, or where I can learn > more would be greatly appreciated!
The system control register is needed to determine the internal state of the MMU, which is required for breakpoint handling and virt2phys conversion in certain cases. So it needs to be read after entering debug state. It cannot be read in user space because it's a privileged register (don't want userspace to mess with the MMU of course). If the target is stopped in 64bit userspace mode (EL0T), the debugger will switch the target to EL1T read the SCTLR register and switch back to EL0T. However, in your case the target seems to stop in 32bit user mode and there's no code to cope with that. BR, Matthias > > Cheers and have a great weekend, > Lucas > > > > > > > > _______________________________________________ > OpenOCD-devel mailing list > OpenOCD-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/openocd-devel -- Mit freundlichen Grüßen/Best regards, Matthias Welwarsky Project Engineer SYSGO AG Office Mainz Am Pfaffenstein 14 / D-55270 Klein-Winternheim / Germany Phone: +49-6136-9948-0 / Fax: +49-6136-9948-10 VoIP: SIP:m...@sysgo.com E-mail: matthias.welwar...@sysgo.com / Web: http://www.sysgo.com _________________________________________________________________________________ Web: https://www.sysgo.com Blog: https://www.sysgo.com/blog Events: https://www.sysgo.com/events Newsletter: https://www.sysgo.com/newsletter _________________________________________________________________________________ Handelsregister/Commercial Registry: HRB Mainz 90 HRB 48884 Geschäftsführung/ Managing Directors: Etienne Butery (CEO), Kai Sablotny (COO) USt-Id-Nr./VAT-Id-No.: DE 149062328
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