Re: [PATCH v8 2/2] iommu/amd: Add basic debugfs infrastructure for AMD IOMMU

[Gary R Hook]

On 06/05/2018 12:06 PM, Greg KH wrote:
> On Tue, Jun 05, 2018 at 11:58:13AM -0500, Gary R Hook wrote:
> > On 05/29/2018 01:39 PM, Greg KH wrote:
> > > On Tue, May 29, 2018 at 01:23:23PM -0500, Gary R Hook wrote:
> > > > Implement a skeleton framework for debugfs support in the
> > > > AMD IOMMU. Add a hidden boolean to Kconfig that is defined
> > > > for the AMD IOMMU when general IOMMY DebugFS support is
> > > > enabled.
> > > >
> > > > Signed-off-by: Gary R Hook <gary.hook@xxxxxxx>
> > > > ---
> > > >    drivers/iommu/Kconfig             |    4 ++++
> > > >    drivers/iommu/Makefile            |    1 +
> > > >    drivers/iommu/amd_iommu_debugfs.c |   39 +++++++++++++++++++++++++++++++++++++
> > > >    drivers/iommu/amd_iommu_init.c    |    6 ++++--
> > > >    drivers/iommu/amd_iommu_proto.h   |    6 ++++++
> > > >    drivers/iommu/amd_iommu_types.h   |    5 +++++
> > > >    6 files changed, 59 insertions(+), 2 deletions(-)
> > > >    create mode 100644 drivers/iommu/amd_iommu_debugfs.c
> > > >
> > > > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > > > index f9af25ac409f..ec223f6f4ad4 100644
> > > > --- a/drivers/iommu/Kconfig
> > > > +++ b/drivers/iommu/Kconfig
> > > > @@ -137,6 +137,10 @@ config AMD_IOMMU
> > > >    	  your BIOS for an option to enable it or if you have an IVRS ACPI
> > > >    	  table.
> > > > +config AMD_IOMMU_DEBUGFS
> > > > +	def_bool y
> > >
> > > Why default y?  Can you not boot a box without this?  If not, it should
> > > not be Y.
> >
> > Again, apologies for not seeing this sooner.
> >
> > Yes, the system can boot without this. The idea of a hidden option was
> > surfaced by Robin, and after my first approach was shot down, I tried this.
> >
> > Logic: If the over-arching IOMMU debugfs option is enabled, then
> > AMD_IOMMU_DEBUGFS gets defined, and AMD IOMMU code gets included.
> >
> > This issue was discussed a few weeks ago. No single approach appears to
> > satisfy everyone. I like this because it depends upon one switch: Do you
> > want DebugFS support enabled in the IOMMU driver, period? Vendor-specific
> > code can then choose to implement support or not, and a builder doesn't have
> > to worry about enabling/disabling multiple Kconfig options.
> >
> > At least, that was my line of reasoning.
> >
> > I'm not married to any approach, and I don't find clever use of Kconfig
> > options too terribly challenging. And I'm not defending, I'm just
> > explaining.
>
> The issue is, no one sets Kconfig options except a very tiny subset of
> kernel developers.  Distros allways enable everything, as they have to
> do that.
>
> If you are creating something here that is so dangerous that you spam
> the kernel log with big warning messages, you should not be making it
> easy to enable, let alone be enabled by default :)

Okay, I get that. Totally understand.

> Just make it an option, have it rely on the kernel debugging option, and
> say "DO NOT ENABLE THIS UNLESS YOU REALLY REALLY REALLY KNOW WHAT YOU
> ARE DOING!"

Nah, Randy voted for separate options per device, on top of the IOMMU 
option. So I'll go with that. With loud messages, of course.