linux-kernel - Re: [PATCH 5/6] dma-mapping: Allow batched DMA sync operations if supported by the arch

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Message-ID: <CAGsJ_4x9YOT2kG0iWz_5NzRVt2iDN9YeUTknZ2MKg7SwmOGOqg@mail.gmail.com>
Date: Wed, 24 Dec 2025 14:29:13 +1300
From: Barry Song <21cnbao@...il.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: ada.coupriediaz@....com, anshuman.khandual@....com, ardb@...nel.org, 
	catalin.marinas@....com, iommu@...ts.linux.dev, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	m.szyprowski@...sung.com, maz@...nel.org, robin.murphy@....com, 
	ryan.roberts@....com, surenb@...gle.com, v-songbaohua@...o.com, 
	will@...nel.org, zhengtangquan@...o.com
Subject: Re: [PATCH 5/6] dma-mapping: Allow batched DMA sync operations if
 supported by the arch

On Wed, Dec 24, 2025 at 3:14 AM Leon Romanovsky <leon@...nel.org> wrote:
>
> On Tue, Dec 23, 2025 at 01:02:55PM +1300, Barry Song wrote:
> > On Mon, Dec 22, 2025 at 9:49 PM Leon Romanovsky <leon@...nel.org> wrote:
> > >
> > > On Mon, Dec 22, 2025 at 03:24:58AM +0800, Barry Song wrote:
> > > > On Sun, Dec 21, 2025 at 7:55 PM Leon Romanovsky <leon@...nel.org> wrote:
> > > > [...]
> > > > > > +
> > > > >
> > > > > I'm wondering why you don't implement this batch‑sync support inside the
> > > > > arch_sync_dma_*() functions. Doing so would minimize changes to the generic
> > > > > kernel/dma/* code and reduce the amount of #ifdef‑based spaghetti.
> > > > >
> > > >
> > > > There are two cases: mapping an sg list and mapping a single
> > > > buffer. The former can be batched with
> > > > arch_sync_dma_*_batch_add() and flushed via
> > > > arch_sync_dma_batch_flush(), while the latter requires all work to
> > > > be done inside arch_sync_dma_*(). Therefore,
> > > > arch_sync_dma_*() cannot always batch and flush.
> > >
> > > Probably in all cases you can call the _batch_ variant, followed by _flush_,
> > > even when handling a single page. This keeps the code consistent across all
> > > paths. On platforms that do not support _batch_, the _flush_ operation will be
> > > a NOP anyway.
> >
> > We have a lot of code outside kernel/dma that also calls
> > arch_sync_dma_for_* such as arch/arm, arch/mips, drivers/xen,
> > I guess we don’t want to modify so many things?
>
> Aren't they using internal, arch specific, arch_sync_dma_for_* implementations?

for arch/arm, arch/mips, they are arch-specific implementations.
xen is an exception:

static void xen_swiotlb_unmap_phys(struct device *hwdev, dma_addr_t dev_addr,
                size_t size, enum dma_data_direction dir, unsigned long attrs)
{
        phys_addr_t paddr = xen_dma_to_phys(hwdev, dev_addr);
        struct io_tlb_pool *pool;

        BUG_ON(dir == DMA_NONE);

        if (!dev_is_dma_coherent(hwdev) && !(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
                if (pfn_valid(PFN_DOWN(dma_to_phys(hwdev, dev_addr))))
                        arch_sync_dma_for_cpu(paddr, size, dir);
                else
                        xen_dma_sync_for_cpu(hwdev, dev_addr, size, dir);
        }

        /* NOTE: We use dev_addr here, not paddr! */
        pool = xen_swiotlb_find_pool(hwdev, dev_addr);
        if (pool)
                __swiotlb_tbl_unmap_single(hwdev, paddr, size, dir,
                                           attrs, pool);
}

>
> >
> > for kernel/dma, we have two "single" callers only:
> > kernel/dma/direct.h, kernel/dma/swiotlb.c.  and they looks quite
> > straightforward:
> >
> > static inline void dma_direct_sync_single_for_device(struct device *dev,
> >                 dma_addr_t addr, size_t size, enum dma_data_direction dir)
> > {
> >         phys_addr_t paddr = dma_to_phys(dev, addr);
> >
> >         swiotlb_sync_single_for_device(dev, paddr, size, dir);
> >
> >         if (!dev_is_dma_coherent(dev))
> >                 arch_sync_dma_for_device(paddr, size, dir);
> > }
> >
> > I guess moving to arch_sync_dma_for_device_batch + flush
> > doesn’t really look much better, does it?
> >
> > >
> > > I would also rename arch_sync_dma_batch_flush() to arch_sync_dma_flush().
> >
> > Sure.
> >
> > >
> > > You can also minimize changes in dma_direct_map_phys() too, by extending
> > > it's signature to provide if flush is needed or not.
> >
> > Yes. I have
> >
> > static inline dma_addr_t __dma_direct_map_phys(struct device *dev,
> >                 phys_addr_t phys, size_t size, enum dma_data_direction dir,
> >                 unsigned long attrs, bool flush)
>
> My suggestion is to use it directly, without wrappers.
>
> >
> > and two wrappers:
> > static inline dma_addr_t dma_direct_map_phys(struct device *dev,
> >                 phys_addr_t phys, size_t size, enum dma_data_direction dir,
> >                 unsigned long attrs)
> > {
> >         return __dma_direct_map_phys(dev, phys, size, dir, attrs, true);
> > }
> >
> > static inline dma_addr_t dma_direct_map_phys_batch_add(struct device *dev,
> >                 phys_addr_t phys, size_t size, enum dma_data_direction dir,
> >                 unsigned long attrs)
> > {
> >         return __dma_direct_map_phys(dev, phys, size, dir, attrs, false);
> > }
> >
> > If you prefer exposing "flush" directly in dma_direct_map_phys()
> > and updating its callers with flush=true, I think that’s fine.
>
> Yes
>

OK. Could you take a look at [1] and see if any further
improvements are needed before I send v2?

[1] https://lore.kernel.org/lkml/20251223023648.31614-1-21cnbao@gmail.com/

Thanks
Barry