linux-kernel - Re: [PATCH v2 2/3] doc: swiotlb: Document SWIOTLB areas parameter

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Date: Thu, 9 May 2024 22:01:47 +0200
From: Petr Tesařík <petr@...arici.cz>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet
 <corbet@....net>, Tianyu Lan <Tianyu.Lan@...rosoft.com>,
 mhklinux@...look.com, robin.murphy@....com, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 2/3] doc: swiotlb: Document SWIOTLB areas parameter

On Thu, 9 May 2024 09:04:16 -0700
"T.J. Mercier" <tjmercier@...gle.com> wrote:

> On Wed, May 8, 2024 at 11:14 PM Petr Tesařík <petr@...arici.cz> wrote:
> >
> > On Tue,  7 May 2024 01:34:59 +0000
> > "T.J. Mercier" <tjmercier@...gle.com> wrote:
> >  
> > > Commit 20347fca71a3 ("swiotlb: split up the global swiotlb lock") added
> > > the ability to specify the number of SWIOTLB areas, but boot-options.rst
> > > was not updated as part of that commit.
> > >
> > > Reported-by: Michael Kelley <mhklinux@...look.com>
> > > Fixes: 20347fca71a3 ("swiotlb: split up the global swiotlb lock")
> > > Signed-off-by: T.J. Mercier <tjmercier@...gle.com>
> > > ---
> > >  Documentation/arch/x86/x86_64/boot-options.rst | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/Documentation/arch/x86/x86_64/boot-options.rst b/Documentation/arch/x86/x86_64/boot-options.rst
> > > index a37139d1752f..18161657b301 100644
> > > --- a/Documentation/arch/x86/x86_64/boot-options.rst
> > > +++ b/Documentation/arch/x86/x86_64/boot-options.rst
> > > @@ -287,9 +287,11 @@ iommu options only relevant to the AMD GART hardware IOMMU:
> > >  iommu options only relevant to the software bounce buffering (SWIOTLB) IOMMU
> > >  implementation:
> > >
> > > -    swiotlb=<slots>[,force,noforce]
> > > +    swiotlb=<slots>[,<areas>,force,noforce]  
> >
> > This is not quite how this parameter is parsed. All the following
> > specifications are valid:
> >
> > - swiotlb=512        // only slots
> > - swiotlb=,4         // only areas
> > - swiotlb=512,4      // slots and areas
> > - swiotlb=force      // default size
> > - swiotlb=512,force  // default areas
> > - swiotlb=512,4,force // explicitly specify everything
> >
> > I believe the syntax should be somethig like:
> >
> >   swiotlb={ | [<slots>][,<areas>],}{ force | noforce }
> >
> > Petr T
> >  
> What does the leading | mean in front of slots?  How about brackets

I wanted to mark somehow that "force" and "noforce" alone do not
require a leading comma, but if you specify slots and/or areas, the
comma is required, like you write below.

> around force/noforce since they're also optional and mutually
> exclusive?

Right. I missed that.

> The Rebooting section uses double brackets instead of
> braces for groupings like that.

TBH I don't know what syntax is used here. Use anything that can capture
the variants I listed above.

> Only weird thing here is the
> force/noforce comma being potentially unneeded, but that's getting
> pretty picky and I think the intent is clear.
> 
> swiotlb=[<slots>][,<areas>][, [force] | [noforce] ]

No objections from my side.

Petr T