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ICSR RETTOBASE bit indicates whether there is any active exception other than the exception indicated by current value of IPSR. Existing code already makes use of it to find whether interrupt occurred during user(thread) mode. Add preemption support in the code path that executes upon finding an active exception other than the current interrupt. Having an active exception during interrupt exception indicates that the interrupt occurred during SVC or PendSV with current Linux exception setup. On PendSV, interrupts are disabled at exception entry (applicable to all) & enabled back just before returning from exception. Hence it has been assumed that active exception during interrupt is indicating SVC interruption. This would have to be revisited. With this change, there is some effect, but not enough to make it usable, this is being debugged. A simple character driver that does infinite while loop during read was used to test it as follows (/dev/mymisc corresponds to the node for the character driver), A. Before this change, A.1 ~ # cat /dev/mymisc [ 28.099225] mymisc_open [ 28.101625] mymisc_read: enter command prompt is not usable (expected), interrupts do happen during this time. A.2 ~ # cat /dev/mymisc & [1] 39 cat /dev/mymisc ~ # [ 11.699880] mymisc_open [ 11.702274] mymisc_read: enter command prompt is not usable (expected as preemption not supported). B. After this change, B.1 ~ # cat /dev/mymisc [ 27.374821] mymisc_open [ 27.377349] mymisc_read: enter though user will not get control back (as expected as it is fg process), entering on prompt causes new line, doesn't know what to make out of this behaviour, this doesn't happen in the A.1 case. Interrupts happen here as well. B.2 ~ # cat /dev/mymisc & [1] 41 cat /dev/mymisc ~ # [ 44.836417] mymisc_open [ 44.838814] mymisc_read: enter though prompt is available under the control of user, upon typing anything on the prompt (typed character doesn't get echoed), it crashes as follows, [ 44.838814] mymisc_read: enter [ 51.710314] [ 51.710314] Unhandled exception: IPSR = 00000006 LR = fffffffd [ 51.717576] CPU: 0 PID: 37 Comm: sh Not tainted 5.5.0-rc4-00004-g2328d01dbd85 torvalds#105 [ 51.725078] Hardware name: STM32 (Device Tree Support) [ 51.730206] PC is at 0x90195958 [ 51.733329] LR is at 0x901c4df3 [ 51.736471] pc : [<90195958>] lr : [<901c4df3>] psr: 21000000 [ 51.742713] sp : 901e5a58 ip : 00000000 fp : 901d89fc [ 51.747911] r10: 00000000 r9 : 00000000 r8 : 00000001 [ 51.753143] r7 : 000000a8 r6 : 901e5a58 r5 : 901e5b08 r4 : ffffffff [ 51.759643] r3 : 000000a8 r2 : ffffffff r1 : 00000001 r0 : 00000001 [ 51.766122] xPSR: 21000000 [ 51.768866] CPU: 0 PID: 37 Comm: sh Not tainted 5.5.0-rc4-00004-g2328d01dbd85 torvalds#105 [ 51.776369] Hardware name: STM32 (Device Tree Support) [ 51.781594] [<0800c0c9>] (unwind_backtrace) from [<0800b25b>] (show_stack+0xb/0xc) [ 51.789166] [<0800b25b>] (show_stack) from [<0800b9eb>] (__invalid_entry+0x4b/0x4c) It is a Usage Fault happening while in thread(user) mode. PC & LR in the dump is strange in the sense that they do not point to text section. The change here has been made based on how Cortex-A handles preemption. Since PAN is not applicable and since it seemed that saving & restoring thread_info::addr_limit does not make difference on the !MMU Cortex-M due to single address space being used, there was no additional save & restore of 'dacr' & 'addr_limit' as done in the case of interrupt during system call on Cortex-A. Also as DUT was UP Cortex-M4, it seemed that cache related would not affect in anyway w.r.t preemption, as currently cache is not enabled for M4 in Linux. As mentioned above, it was assumed that active exception during interrupt meant SVC was interrupted, though it could have been PendSV as well. To ensure that issue is not due to interrupt during PendSV, the PendSV handling of pending work was lifted & sticked onto __irq_entry and setting PendSV removed as well. Still the behaviour is same. Signed-off-by: afzal mohammed <afzal.mohd.ma@gmail.com>
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