On Tue, 28 Jun 2022 at 16:47, Peter Maydell <peter.mayd...@linaro.org> wrote: > > In the M-profile Arm ARM, rule R_CVJS defines when an interrupt should > be set to the Pending state: > A) when the input line is high and the interrupt is not Active > B) when the input line transitions from low to high and the interrupt > is Active > (Note that the first of these is an ongoing condition, and the > second is a point-in-time event.) > > This can be rephrased as: > 1 when the line goes from low to high, set Pending > 2 when Active goes from 1 to 0, if line is high then set Pending > 3 ignore attempts to clear Pending when the line is high > and Active is 0 > > where 1 covers both B and one of the "transition into condition A" > cases, 2 deals with the other "transition into condition A" > possibility, and 3 is "don't drop Pending if we're already in > condition A". Transitions out of condition A don't affect Pending > state. > > We handle case 1 in set_irq_level(). For an interrupt (as opposed > to other kinds of exception) the only place where we clear Active > is in armv7m_nvic_complete_irq(), where we handle case 2 by > checking for whether we need to re-pend the exception. For case 3, > the only places where we clear Pending state on an interrupt are in > armv7m_nvic_acknowledge_irq() (where we are setting Active so it > doesn't count) and for writes to NVIC_CPSRn.
Should read "NVIC_ICPRn"... > It is the "write to NVIC_ICPRn" case that we missed: we must ignore > this if the input line is high and the interrupt is not Active. > (This required behaviour is differently and perhaps more clearly > stated in the v7M Arm ARM, which has pseudocode in section B3.4.1 > that implies it.) -- PMM
