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

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Message-ID: <20251222084921.GA13529@unreal>
Date: Mon, 22 Dec 2025 10:49:21 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Barry Song <21cnbao@...il.com>
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 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.

I would also rename arch_sync_dma_batch_flush() to arch_sync_dma_flush().

You can also minimize changes in dma_direct_map_phys() too, by extending
it's signature to provide if flush is needed or not.

dma_direct_map_phys(....) -> dma_direct_map_phys(...., bool flush):

static inline dma_addr_t dma_direct_map_phys(...., bool flush)
{
	....

	if (dma_addr != DMA_MAPPING_ERROR && !dev_is_dma_coherent(dev) &&
	    !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_MMIO)))
        {
	    	arch_sync_dma_for_device(phys, size, dir);
		if (flush)
			arch_sync_dma_flush();
	}
}

Thanks