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SubjectRe: [PATCH] ACPI/IORT: Add 'smmu=off' command line option
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On 2021-09-15 13:00, Yao Hongbo wrote:
> Add a generic command line option to disable arm smmu drivers.
> iommu.passthrough can only bypass the IOMMU for DMA, but
> sometimes we need to ignore all available SMMUs.

Please elaborate on "sometimes" - what's the use-case for this which
can't already be achieved by other means like module_blacklist,
switching kernel images, ACPI table overrides, and so on?

> This patch is only used for acpi on arm64.

And yet the documentation implies that it works for all arm64 systems :/

And furthermore, why? If it's genuinely useful to disable SMMUs on
ACPI-based systems, surely it must be equally useful to disable SMMUs on
DT-based systems?

> Signed-off-by: Yao Hongbo <yaohongbo@linux.alibaba.com>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 4 ++++
> drivers/acpi/arm64/iort.c | 18 +++++++++++++++++-
> 2 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 91ba391f..6cffd91 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5198,6 +5198,10 @@
> smart2= [HW]
> Format: <io1>[,<io2>[,...,<io8>]]
>
> + smmu= [ARM64]
> + Format: {off}
> + off: Disable arm smmu driver.

There are at least two Arm SMMU drivers; as a user I might be wondering
about the ambiguity there.

> +
> smsc-ircc2.nopnp [HW] Don't use PNP to discover SMC devices
> smsc-ircc2.ircc_cfg= [HW] Device configuration I/O port
> smsc-ircc2.ircc_sir= [HW] SIR base I/O port
> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
> index 3b23fb7..70f92e7 100644
> --- a/drivers/acpi/arm64/iort.c
> +++ b/drivers/acpi/arm64/iort.c
> @@ -40,6 +40,22 @@ struct iort_fwnode {
> static LIST_HEAD(iort_fwnode_list);
> static DEFINE_SPINLOCK(iort_fwnode_lock);
>
> +static bool acpi_smmu_disabled;
> +
> +static int __init acpi_smmu_parse(char *str)
> +{
> + if (!str)
> + return -EINVAL;
> +
> + if (!strncmp(str, "off", 3)) {
> + acpi_smmu_disabled = true;
> + pr_info("SMMU disabled\n");
> + }
> +
> + return 0;
> +}
> +__setup("smmu=", acpi_smmu_parse);
> +
> /**
> * iort_set_fwnode() - Create iort_fwnode and use it to register
> * iommu data in the iort_fwnode_list
> @@ -1596,7 +1612,7 @@ static void __init iort_init_platform_devices(void)
> iort_enable_acs(iort_node);
>
> ops = iort_get_dev_cfg(iort_node);
> - if (ops) {
> + if (ops && !acpi_smmu_disabled) {

This will also effectively disable PMCGs, which is an undocumented
side-effect, and not necessarily desirable - PMCG functionality is
largely orthogonal, and may potentially be used to monitor traffic even
when the associated SMMU instance is disabled.

TBH there's really nothing SMMU-specific about this at all. I know I've
had a number of debugging situations where it would have been handy to
have a way to prevent a specific built-in driver from binding
automatically during boot, so if you really have got a compelling reason
to need it for SMMU, then you could still implement it generically in a
way that everyone could benefit from.

Thanks,
Robin.

> fwnode = acpi_alloc_fwnode_static();
> if (!fwnode)
> return;
>

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