linux-kernel - Re: [PATCH v2 5/8] dma-mapping: Support batch mode for dma_direct_sync_sg_for

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Message-ID: <20251228145041.GS11869@unreal>
Date: Sun, 28 Dec 2025 16:50:41 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Barry Song <21cnbao@...il.com>
Cc: catalin.marinas@....com, m.szyprowski@...sung.com, robin.murphy@....com,
	will@...nel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xenproject.org,
	Ada Couprie Diaz <ada.coupriediaz@....com>,
	Ard Biesheuvel <ardb@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Ryan Roberts <ryan.roberts@....com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Tangquan Zheng <zhengtangquan@...o.com>
Subject: Re: [PATCH v2 5/8] dma-mapping: Support batch mode for
 dma_direct_sync_sg_for_*

On Sun, Dec 28, 2025 at 09:52:05AM +1300, Barry Song wrote:
> On Sun, Dec 28, 2025 at 9:09 AM Leon Romanovsky <leon@...nel.org> wrote:
> >
> > On Sat, Dec 27, 2025 at 11:52:45AM +1300, Barry Song wrote:
> > > From: Barry Song <baohua@...nel.org>
> > >
> > > Instead of performing a flush per SG entry, issue all cache
> > > operations first and then flush once. This ultimately benefits
> > > __dma_sync_sg_for_cpu() and __dma_sync_sg_for_device().
> > >
> > > Cc: Leon Romanovsky <leon@...nel.org>
> > > Cc: Catalin Marinas <catalin.marinas@....com>
> > > Cc: Will Deacon <will@...nel.org>
> > > Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> > > Cc: Robin Murphy <robin.murphy@....com>
> > > Cc: Ada Couprie Diaz <ada.coupriediaz@....com>
> > > Cc: Ard Biesheuvel <ardb@...nel.org>
> > > Cc: Marc Zyngier <maz@...nel.org>
> > > Cc: Anshuman Khandual <anshuman.khandual@....com>
> > > Cc: Ryan Roberts <ryan.roberts@....com>
> > > Cc: Suren Baghdasaryan <surenb@...gle.com>
> > > Cc: Tangquan Zheng <zhengtangquan@...o.com>
> > > Signed-off-by: Barry Song <baohua@...nel.org>
> > > ---
> > >  kernel/dma/direct.c | 14 +++++++-------
> > >  1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > <...>
> >
> > > -             if (!dev_is_dma_coherent(dev)) {
> > > +             if (!dev_is_dma_coherent(dev))
> > >                       arch_sync_dma_for_device(paddr, sg->length,
> > >                                       dir);
> > > -                     arch_sync_dma_flush();
> > > -             }
> > >       }
> > > +     if (!dev_is_dma_coherent(dev))
> > > +             arch_sync_dma_flush();
> >
> > This patch should be squashed into the previous one. You introduced
> > arch_sync_dma_flush() there, and now you are placing it elsewhere.
> 
> Hi Leon,
> 
> The previous patch replaces all arch_sync_dma_for_* calls with
> arch_sync_dma_for_* plus arch_sync_dma_flush(), without any
> functional change. The subsequent patches then implement the
> actual batching. I feel this is a better approach for reviewing
> each change independently. Otherwise, the previous patch would
> be too large.

Don't worry about it. Your patches are small enough.

> 
> Thanks
> Barry