linux-kernel - Re: [PATCH v4 1/8] swiotlb: make io_tlb_default

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Message-ID: <20230720095410.16832a9e@meshulam.tesarici.cz>
Date:   Thu, 20 Jul 2023 09:54:10 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Christoph Hellwig <hch@....de>
Cc:     Petr Tesarik <petrtesarik@...weicloud.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Petr Tesarik <petr.tesarik.ext@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        James Seo <james@...iv.tech>,
        James Clark <james.clark@....com>,
        Kees Cook <keescook@...omium.org>,
        "moderated list:XEN HYPERVISOR ARM" <xen-devel@...ts.xenproject.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        "open list:XEN SWIOTLB SUBSYSTEM" <iommu@...ts.linux.dev>,
        Roberto Sassu <roberto.sassu@...weicloud.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH v4 1/8] swiotlb: make io_tlb_default_mem local to
 swiotlb.c

On Thu, 20 Jul 2023 08:37:44 +0200
Christoph Hellwig <hch@....de> wrote:

> On Thu, Jul 13, 2023 at 05:23:12PM +0200, Petr Tesarik wrote:
> > From: Petr Tesarik <petr.tesarik.ext@...wei.com>
> > 
> > SWIOTLB implementation details should not be exposed to the rest of the
> > kernel. This will allow to make changes to the implementation without
> > modifying non-swiotlb code.
> > 
> > To avoid breaking existing users, provide helper functions for the few
> > required fields.
> > 
> > As a bonus, using a helper function to initialize struct device allows to
> > get rid of an #ifdef in driver core.
> > 
> > Signed-off-by: Petr Tesarik <petr.tesarik.ext@...wei.com>
> > ---
> >  arch/arm/xen/mm.c          |  2 +-
> >  arch/mips/pci/pci-octeon.c |  2 +-
> >  arch/x86/kernel/pci-dma.c  |  2 +-
> >  drivers/base/core.c        |  4 +---
> >  drivers/xen/swiotlb-xen.c  |  2 +-
> >  include/linux/swiotlb.h    | 25 +++++++++++++++++++++++-
> >  kernel/dma/swiotlb.c       | 39 +++++++++++++++++++++++++++++++++++++-
> >  7 files changed, 67 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
> > index 3d826c0b5fee..0f32c14eb786 100644
> > --- a/arch/arm/xen/mm.c
> > +++ b/arch/arm/xen/mm.c
> > @@ -125,7 +125,7 @@ static int __init xen_mm_init(void)
> >  		return 0;
> >  
> >  	/* we can work with the default swiotlb */
> > -	if (!io_tlb_default_mem.nslabs) {
> > +	if (!is_swiotlb_allocated()) {
> >  		rc = swiotlb_init_late(swiotlb_size_or_default(),
> >  				       xen_swiotlb_gfp(), NULL);
> >  		if (rc < 0)  
> 
> I'd much rather move the already initialized check into
> swiotlb_init_late, which is a much cleaer interface.
> 
> >  	/* we can work with the default swiotlb */
> > -	if (!io_tlb_default_mem.nslabs) {
> > +	if (!is_swiotlb_allocated()) {
> >  		int rc = swiotlb_init_late(swiotlb_size_or_default(),
> >  					   GFP_KERNEL, xen_swiotlb_fixup);
> >  		if (rc < 0)  
> 
> .. and would take care of this one as well.

Oh, you're right! These are the only two places that look at
io_tlb_default_mem.nslabs, and all they need is to avoid double
initialization. Makes perfect sense to move it inside
swiotlb_init_late().

> > +bool is_swiotlb_allocated(void)
> > +{
> > +	return !!io_tlb_default_mem.nslabs;  
> 
> Nit: no need for the !!, we can rely on the implicit promotion to
> bool.  But with the suggestion above the need for this helper
> should go away anyway.

Eh, yes. I initially declared the return type as int and then forgot to
change the return statement. But as you say, the whole function will go
away entirely.

Petr T