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

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Date:   Thu, 20 Jul 2023 08:37:44 +0200
From:   Christoph Hellwig <hch@....de>
To:     Petr Tesarik <petrtesarik@...weicloud.com>
Cc:     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>,
        Christoph Hellwig <hch@....de>,
        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>, petr@...arici.cz
Subject: Re: [PATCH v4 1/8] swiotlb: make io_tlb_default_mem local to
 swiotlb.c

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.

> +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.