Re: [PATCH v4 1/8] swiotlb: make io_tlb_default_mem local to swiotlb.c

[Christoph Hellwig <hch@lst.de>]

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.

> +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.