linux-kernel - RE: [RFC 1/1] swiotlb: Reduce calls to swiotlb_find

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Date: Sat, 29 Jun 2024 15:55:58 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Petr Tesařík <petr@...arici.cz>, "hch@....de"
	<hch@....de>
CC: "robin.murphy@....com" <robin.murphy@....com>, "joro@...tes.org"
	<joro@...tes.org>, "will@...nel.org" <will@...nel.org>, "jgross@...e.com"
	<jgross@...e.com>, "sstabellini@...nel.org" <sstabellini@...nel.org>,
	"oleksandr_tyshchenko@...m.com" <oleksandr_tyshchenko@...m.com>,
	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: RE: [RFC 1/1] swiotlb: Reduce calls to swiotlb_find_pool()

From: Petr Tesařík <petr@...arici.cz> Sent: Friday, June 28, 2024 12:47 AM
> 
> V Fri, 28 Jun 2024 08:01:29 +0200
> "hch@....de" <hch@....de> napsáno:
> 
> > On Thu, Jun 27, 2024 at 04:02:59PM +0000, Michael Kelley wrote:
> > > > > Conceptually, it's still being used as a boolean function based on
> > > > > whether the return value is NULL.  Renaming it to swiotlb_get_pool()
> > > > > more accurately describes the return value, but obscures the
> > > > > intent of determining if it is a swiotlb buffer.  I'll think about it.
> > > > > Suggestions are welcome.
> > > >
> > > > Just keep is_swiotlb_buffer as a trivial inline helper that returns
> > > > bool.
> > >
> > > I don't understand what you are suggesting.  Could you elaborate a bit?
> > > is_swiotlb_buffer() can't be trivial when CONFIG_SWIOTLB_DYNAMIC
> > > is set.
> >
> > Call the main function that finds and retuns the pool swiotlb_find_pool,
> > and then have a is_swiotlb_buffer wrapper that just returns bool.
> >
> 
> I see. That's not my point. After applying Michael's patch, the return
> value is always used, except here:
> 
> bool dma_direct_need_sync(struct device *dev, dma_addr_t dma_addr)
> {
> 	return !dev_is_dma_coherent(dev) ||
> 	       is_swiotlb_buffer(dev, dma_to_phys(dev, dma_addr));
> }
> 
> I don't think this one occurrence in the entire source tree is worth a
> separate inline function.
> 
> If nobody has a better idea, I'm not really offended by keeping the
> original name, is_swiotlb_buffer(). It would just become the only
> function which starts with "is_" and provides more information in the
> return value than a simple yes/no, and I thought there must be an
> unwritten convention about that.
> 

Unless there is further discussion on this point, I'll just keep the original
"is_swiotlb_buffer()" in v2.

Michael