Quoting Vivek Gautam (2019-08-22 23:32:46)
> There are scnenarios where drivers are required to make a
> scm call in atomic context, such as in one of the qcom's
> arm-smmu-500 errata [1].
> 
> [1] ("https://source.codeaurora.org/quic/la/kernel/msm-4.9/
>       tree/drivers/iommu/arm-smmu.c?h=msm-4.9#n4842")
> 
> Signed-off-by: Vivek Gautam <vivek.gau...@codeaurora.org>
> Reviewed-by: Bjorn Andersson <bjorn.anders...@linaro.org>
> ---
>  drivers/firmware/qcom_scm-64.c | 136 
> ++++++++++++++++++++++++++++-------------
>  1 file changed, 92 insertions(+), 44 deletions(-)
> 
> diff --git a/drivers/firmware/qcom_scm-64.c b/drivers/firmware/qcom_scm-64.c
> index 91d5ad7cf58b..b6dca32c5ac4 100644
> --- a/drivers/firmware/qcom_scm-64.c
> +++ b/drivers/firmware/qcom_scm-64.c
> @@ -62,32 +62,71 @@ static DEFINE_MUTEX(qcom_scm_lock);
>  #define FIRST_EXT_ARG_IDX 3
>  #define N_REGISTER_ARGS (MAX_QCOM_SCM_ARGS - N_EXT_QCOM_SCM_ARGS + 1)
>  
> -/**
> - * qcom_scm_call() - Invoke a syscall in the secure world
> - * @dev:       device
> - * @svc_id:    service identifier
> - * @cmd_id:    command identifier
> - * @desc:      Descriptor structure containing arguments and return values
> - *
> - * Sends a command to the SCM and waits for the command to finish processing.
> - * This should *only* be called in pre-emptible context.
> -*/
> -static int qcom_scm_call(struct device *dev, u32 svc_id, u32 cmd_id,
> -                        const struct qcom_scm_desc *desc,
> -                        struct arm_smccc_res *res)
> +static void __qcom_scm_call_do(const struct qcom_scm_desc *desc,
> +                              struct arm_smccc_res *res, u32 fn_id,
> +                              u64 x5, u32 type)
> +{
> +       u64 cmd;
> +       struct arm_smccc_quirk quirk = {.id = ARM_SMCCC_QUIRK_QCOM_A6};

Nitpick: Put spaces around braces please.

> +
> +       cmd = ARM_SMCCC_CALL_VAL(type, qcom_smccc_convention,
> +                                ARM_SMCCC_OWNER_SIP, fn_id);
> +
> +       quirk.state.a6 = 0;
> +
> +       do {
> +               arm_smccc_smc_quirk(cmd, desc->arginfo, desc->args[0],
> +                                   desc->args[1], desc->args[2], x5,
> +                                   quirk.state.a6, 0, res, &quirk);
> +
> +               if (res->a0 == QCOM_SCM_INTERRUPTED)
> +                       cmd = res->a0;
> +
> +       } while (res->a0 == QCOM_SCM_INTERRUPTED);
> +}
> +
> +static void qcom_scm_call_do(const struct qcom_scm_desc *desc,
> +                            struct arm_smccc_res *res, u32 fn_id,
> +                            u64 x5, bool atomic)
> +{
> +       int retry_count = 0;
> +
> +       if (!atomic) {
> +               do {
> +                       mutex_lock(&qcom_scm_lock);
> +
> +                       __qcom_scm_call_do(desc, res, fn_id, x5,
> +                                          ARM_SMCCC_STD_CALL);
> +
> +                       mutex_unlock(&qcom_scm_lock);
> +
> +                       if (res->a0 == QCOM_SCM_V2_EBUSY) {
> +                               if (retry_count++ > QCOM_SCM_EBUSY_MAX_RETRY)
> +                                       break;
> +                               msleep(QCOM_SCM_EBUSY_WAIT_MS);
> +                       }
> +               }  while (res->a0 == QCOM_SCM_V2_EBUSY);
> +       } else {
> +               __qcom_scm_call_do(desc, res, fn_id, x5, ARM_SMCCC_FAST_CALL);
> +       }

To save on some indentation maybe you could write it like:

        if (atomic) {
                __qcom_scm_call_do(..)
                return;
        }

        do {
                mutex_lock(..)
                ...
        } while (..);

> +}
> +
> +static int ___qcom_scm_call(struct device *dev, u32 svc_id, u32 cmd_id,
> +                           const struct qcom_scm_desc *desc,
> +                           struct arm_smccc_res *res, bool atomic)
>  {
>         int arglen = desc->arginfo & 0xf;
> -       int retry_count = 0, i;
> +       int i;
>         u32 fn_id = QCOM_SCM_FNID(svc_id, cmd_id);
> -       u64 cmd, x5 = desc->args[FIRST_EXT_ARG_IDX];
> +       u64 x5 = desc->args[FIRST_EXT_ARG_IDX];
>         dma_addr_t args_phys = 0;
>         void *args_virt = NULL;
>         size_t alloc_len;
> -       struct arm_smccc_quirk quirk = {.id = ARM_SMCCC_QUIRK_QCOM_A6};
> +       gfp_t flag = atomic ? GFP_ATOMIC : GFP_KERNEL;
>  
>         if (unlikely(arglen > N_REGISTER_ARGS)) {
>                 alloc_len = N_EXT_QCOM_SCM_ARGS * sizeof(u64);
> -               args_virt = kzalloc(PAGE_ALIGN(alloc_len), GFP_KERNEL);
> +               args_virt = kzalloc(PAGE_ALIGN(alloc_len), flag);
>  
>                 if (!args_virt)
>                         return -ENOMEM;
> @@ -156,6 +169,41 @@ static int qcom_scm_call(struct device *dev, u32 svc_id, 
> u32 cmd_id,
>         return 0;
>  }
>  
> +/**
> + * qcom_scm_call() - Invoke a syscall in the secure world
> + * @dev:       device
> + * @svc_id:    service identifier
> + * @cmd_id:    command identifier
> + * @desc:      Descriptor structure containing arguments and return values
> + *
> + * Sends a command to the SCM and waits for the command to finish processing.
> + * This should *only* be called in pre-emptible context.

Add a might_sleep() then?

> + */
> +static int qcom_scm_call(struct device *dev, u32 svc_id, u32 cmd_id,
> +                        const struct qcom_scm_desc *desc,
> +                        struct arm_smccc_res *res)
> +{
> +       return ___qcom_scm_call(dev, svc_id, cmd_id, desc, res, false);
> +}
> +
> +/**
> + * qcom_scm_call_atomic() - atomic variation of qcom_scm_call()
> + * @dev:       device
> + * @svc_id:    service identifier
> + * @cmd_id:    command identifier
> + * @desc:      Descriptor structure containing arguments and return values
> + * @res:       Structure containing results from SMC/HVC call
> + *
> + * Sends a command to the SCM and waits for the command to finish processing.
> + * This should be called in atomic context only.
 
Maybe add a cant_sleep()?

> + */
> +static int qcom_scm_call_atomic(struct device *dev, u32 svc_id, u32 cmd_id,
> +                               const struct qcom_scm_desc *desc,
> +                               struct arm_smccc_res *res)
> +{
> +       return ___qcom_scm_call(dev, svc_id, cmd_id, desc, res, true);
> +}
> +
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