Re: [PATCH] misra: comment default case in single-clause switch

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2025-08-11 19:36, Dmytro Prokopchuk1 wrote:
> MISRA Rule 16.4: Every switch statement shall have a default label. The
> default clause must contain either a statement or a comment prior to 
> its
> terminating break statement.
> 
> This change adds comments in default cases in single-clause switches to
> avoid violations of the rule where the `default` clause lacks a 
> suitable
> comment or statement.
> 

If you want to go this way then at least this config wants dropping:

-doc_begin="A switch statement with a single switch clause and no 
default label may be used in place of an equivalent if statement if it 
is considered to improve readability."
-config=MC3A2.R16.4,switch_clauses+={deliberate,"switch(1)&&default(0)"}
-doc_end

and probably this one too (haven't checked):

-doc_begin="A switch statement with a single switch clause and no 
default label may be used in place of an equivalent if statement if it 
is considered to improve readability."
-config=MC3A2.R16.6,switch_clauses+={deliberate, "default(0)"}
-doc_end

In the end it's a tradeoff; placing a comment with a suitable 
argumentation might be strictly better than simply tagging the tool 
report, at least from a developer's perspective. From a MISRA compliance 
standpoint in my opinion both are fine. Let's see what the maintainers 
think.

> Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>
> ---
>  xen/arch/arm/cpuerrata.c | 5 +++++
>  xen/arch/arm/gic.c       | 6 ++++++
>  xen/arch/arm/mmu/p2m.c   | 6 ++++++
>  xen/arch/arm/time.c      | 6 ++++++
>  xen/common/kexec.c       | 6 ++++++
>  5 files changed, 29 insertions(+)
> 
> diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
> index 17cf134f1b..e31c72694b 100644
> --- a/xen/arch/arm/cpuerrata.c
> +++ b/xen/arch/arm/cpuerrata.c
> @@ -735,6 +735,11 @@ static int cpu_errata_callback(struct 
> notifier_block *nfb,
>          rc = enable_nonboot_cpu_caps(arm_errata);
>          break;
>      default:
> +        /*
> +         * Default case left empty as other notifier actions do not 
> require handling here.
> +         * The single CPU_STARTING action in the switch is sufficient 
> because this function
> +         * specifically handles enabling errata workarounds during CPU 
> startup.
> +         */
>          break;
>      }
> 
> diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
> index e80fe0ca24..dfbb0dfe08 100644
> --- a/xen/arch/arm/gic.c
> +++ b/xen/arch/arm/gic.c
> @@ -436,6 +436,12 @@ static int cpu_gic_callback(struct notifier_block 
> *nfb,
>          release_irq(gic_hw_ops->info->maintenance_irq, NULL);
>          break;
>      default:
> +        /*
> +         * Default case left empty as other notifier actions do not 
> require handling here.
> +         * The CPU_DYING action specifically handles releasing 
> resources acquired by
> +         * init_maintenance_interrupt(), when the CPU is being taken 
> offline. Other CPU
> +         * actions do not require GIC-specific handling.
> +         */
>          break;
>      }
> 
> diff --git a/xen/arch/arm/mmu/p2m.c b/xen/arch/arm/mmu/p2m.c
> index 51abf3504f..d1f0ae0226 100644
> --- a/xen/arch/arm/mmu/p2m.c
> +++ b/xen/arch/arm/mmu/p2m.c
> @@ -1828,6 +1828,12 @@ static int cpu_virt_paging_callback(struct 
> notifier_block *nfb,
>          setup_virt_paging_one(NULL);
>          break;
>      default:
> +        /*
> +         * Default case left empty as other notifier actions do not 
> require handling here.
> +         * The CPU_STARTING action is specifically handled because 
> this function sets up
> +         * virtual paging for CPUs, particularly for non-boot CPUs 
> during hotplug. Other
> +         * actions do not involve virtual paging setup.
> +         */
>          break;
>      }
> 
> diff --git a/xen/arch/arm/time.c b/xen/arch/arm/time.c
> index e74d30d258..4776110d97 100644
> --- a/xen/arch/arm/time.c
> +++ b/xen/arch/arm/time.c
> @@ -382,6 +382,12 @@ static int cpu_time_callback(struct notifier_block 
> *nfb,
>          deinit_timer_interrupt();
>          break;
>      default:
> +        /*
> +         * Default case left empty as other notifier actions do not 
> require handling here.
> +         * The CPU_DYING action is specifically handled to revert 
> actions done in
> +         * init_timer_interrupt() and properly disable timer 
> interrupts on the CPU being
> +         * taken offline. Other actions do not involve timer 
> deinitialization.
> +         */
>          break;
>      }
> 
> diff --git a/xen/common/kexec.c b/xen/common/kexec.c
> index 84fe8c3597..0f78087a4a 100644
> --- a/xen/common/kexec.c
> +++ b/xen/common/kexec.c
> @@ -549,6 +549,12 @@ static int cf_check cpu_callback(
>          kexec_init_cpu_notes(cpu);
>          break;
>      default:
> +        /*
> +         * Default case left empty as other notifier actions do not 
> require handling here.
> +         * The CPU_UP_PREPARE action is specifically handled to 
> allocate crash note buffers
> +         * for a newly onlined CPU. Other actions do not pertain to 
> crash note allocation
> +         * or memory preservation for kexec.
> +         */
>          break;
>      }
>      return NOTIFY_DONE;

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253