Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 20 Jul 2023 09:54:10 +0200
From:   Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Petr Tesarik <petrtesarik@huaweicloud.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Juergen Gross <jgross@suse.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Petr Tesarik <petr.tesarik.ext@huawei.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Hans de Goede <hdegoede@redhat.com>,
        James Seo <james@equiv.tech>,
        James Clark <james.clark@arm.com>,
        Kees Cook <keescook@chromium.org>,
        "moderated list:XEN HYPERVISOR ARM" <xen-devel@lists.xenproject.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@lists.infradead.org>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        "open list:XEN SWIOTLB SUBSYSTEM" <iommu@lists.linux.dev>,
        Roberto Sassu <roberto.sassu@huaweicloud.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>
Subject: Re: [PATCH v4 1/8] swiotlb: make io_tlb_default_mem local to
 swiotlb.c
Message-ID: <20230720095410.16832a9e@meshulam.tesarici.cz>
In-Reply-To: <20230720063744.GA3842@lst.de>
References: <cover.1689261692.git.petr.tesarik.ext@huawei.com>
        <7f64111986f4f361a2deb4a1a1b6f588e63a851b.1689261692.git.petr.tesarik.ext@huawei.com>
        <20230720063744.GA3842@lst.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

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

> On Thu, Jul 13, 2023 at 05:23:12PM +0200, Petr Tesarik wrote:
> > From: Petr Tesarik <petr.tesarik.ext@huawei.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@huawei.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