On Wed, 29 Mar 2023 at 21:50, Wolfram Sang <wsa@xxxxxxxxxx> wrote: > Could you make sure please? Sure, I'll try. The check before bae1d3a was: in_atomic() || irqs_disabled() which boils down to: (preempt_count() != 0) || irqs_disabled() preemptible() is defined as: (preempt_count() == 0 && !irqs_disabled()) so this patch should behave the same as pre-v5.2, but with the additional system state check. From my point of view, the additional value of the in_atomic() check was that it activated atomic i2c xfers when preemption is disabled, like in the case of panic(). So reverting that commit would also re-activate atomic i2c transfers during emergency restarts. However, I think considering the system state makes sense here. >From my understanding, non-atomic i2c transfers require enabled IRQs, but atomic i2c transfers do not have any "requirements". So the irqs_disabled() check is not here to ensure that the following atomic i2c transfer works correctly, but to use non-atomic i2c xfer as long/often as possible. Unfortunately, I am not sure yet about !CONFIG_PREEMPTION. I looked into some i2c-bus implementations which implement both, atomic and non-atomic. As far as I saw, the basic difference is that the non-atomic variants usually utilize the DMA and then call a variant of wait_for_completion(), like in i2c_imx_dma_write() [1]. However, the documentation of wait_for_completion [2] states that: "wait_for_completion() and its variants are only safe in process context (as they can sleep) but not (...) [if] preemption is disabled". Therefore, I am not quite sure yet if !CONFIG_PREEMPTION uses the non-atomic variant at all or if this case is handled differently. > Asking Peter Zijlstra might be a good idea. > He helped me with the current implementation. Thanks for the hint! I wrote an extra email to him and added him to CC. Thanks & best regards, Benjamin [1] drivers/i2c/busses/i2c-imx.c [2] https://www.kernel.org/doc/Documentation/scheduler/completion.txt
