Re: [PATCH] ACPI/IORT: Add 'smmu=off' command line option
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From: Robin Murphy <robin.murphy@arm.com>
Date: 2021-09-15 20:13:06
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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?
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Signed-off-by: Yao Hongbo <redacted> --- 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.
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+ 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 portdiff --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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