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Subject: Re: [PATCH v3 1/2] iommu - Enable debugfs exposure of the IOMMU
To:     "Hook, Gary" <ghook@amd.com>, Gary R Hook <gary.hook@amd.com>,
        iommu@lists.linux-foundation.org
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 <9123cd44-e01f-bff6-8441-792acbde10bc@amd.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <a24f9f4c-0c65-a77c-a18b-7a6bf5a33b85@arm.com>
Date:   Tue, 17 Apr 2018 19:13:52 +0100
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On 17/04/18 18:46, Hook, Gary wrote:
> On 4/6/2018 9:17 AM, Gary R Hook wrote:
>> Provide base enablement for using debugfs to expose internal data of
>> an IOMMU driver. When called, create the /sys/kernel/debug/iommu
>> directory.  Emit a strong warning at boot time to indicate that this
>> feature is enabled.
>>
>> This patch adds a top-level function that will create the (above)
>> directory, under which a driver may create a hw-specific directory for
>> its use. The function
>>
>>     iommu_debugfs_setup()
>>
>> returns a pointer to the new dentry structure created for
>> /sys/kernel/debug/iommu, or NULL in the event of a failure. An IOMMU
>> driver should call this function first, and then create a directory
>> beneath it. A driver implementation might look something like:
>>
>> static struct dentry *my_debugfs;
>>
>>     struct dentry *d_top;
>>     if (!my_debugfs) {
>>         d_top = iommu_debugfs_setup();
>>         if (d_top)
>>             my_debugfs = debugfs_create_dir("vendor", d_top);
>>     }
>>
>> Since the IOMMU driver can not be removed from the running system, this
>> patch only provides an "on" function.
>>
>> Signed-off-by: Gary R Hook <gary.hook@amd.com>
>> ---
>>   drivers/iommu/Kconfig         |   11 ++++++++
>>   drivers/iommu/Makefile        |    1 +
>>   drivers/iommu/iommu-debugfs.c |   58 
>> +++++++++++++++++++++++++++++++++++++++++
>>   include/linux/iommu.h         |    4 +++
>>   4 files changed, 74 insertions(+)
>>   create mode 100644 drivers/iommu/iommu-debugfs.c
>>
>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>> index f3a21343e636..c1e39dabfec2 100644
>> --- a/drivers/iommu/Kconfig
>> +++ b/drivers/iommu/Kconfig
>> @@ -60,6 +60,17 @@ config IOMMU_IO_PGTABLE_ARMV7S_SELFTEST
>>   endmenu
>> +config IOMMU_DEBUG
>> +    bool "Enable IOMMU internals in DebugFS"
>> +    depends on DEBUG_FS
>> +    default n
>> +    help
>> +      Allows exposure of IOMMU device internals. This option enables
>> +      the use of debugfs by IOMMU drivers as required. Devices can,
>> +      at initialization time, cause the IOMMU code to create a top-level
>> +      debug/iommu directory, and then populate a subdirectory with
>> +      entries as required.
> 
> I should explicitly ask about this:
> 
> Joerg had suggested IOMMU_DEBUGFS, but here I've changed to IOMMU_DEBUG. 
> I'm not seeing a lot of CONFIG options that use DEBUGFS for debugfs 
> options, so I chose to follow an implied convention.
> 
> Question: should this indeed be IOMMU_DEBUGFS?
>                                  ^^^^^^^^^^^^^

Personally I'd say yes, since there is at least some precedent for 
*_DEBUGFS already, and it does help make the intent that much clearer 
(*_DEBUG often just means lots of dmesg spam rather than an actual 
debugfs interface).

Robin.