Re: [PATCH net-next v2 2/7] dma: avoid redundant calls for sync operations

[Alexander Lobakin]

From: Christoph Hellwig <hch@xxxxxx>
Date: Tue, 13 Feb 2024 07:11:20 +0100

> On Mon, Feb 05, 2024 at 12:04:21PM +0100, Alexander Lobakin wrote:
>> Quite often, NIC devices do not need dma_sync operations on x86_64
>> at least.
> 
> This is a fundamental property of the platform being DMA coherent,
> and devices / platforms not having addressing limitations or other
> need for bounce buffering (like all those whacky trusted platform
> schemes).  Nothing NIC-specific here.

This sentence is from the original Eric's commit message, but I'll
reword it :D

> 
>> In case some device doesn't work with the shortcut:
>> * include <linux/dma-map-ops.h> to the driver source;
>> * call dma_set_skip_sync(dev, false) at the beginning of the probe
>>   callback. This will disable the shortcut and force DMA syncs.
> 
> No, drivers should never include dma-map-ops.h.  If we have a legit
> reason for drivers to ever call it it would have to move to
> dma-mapping.h.  But I see now reason why there would be such a need.
> For now I'd suggest simply dropping this paragraph from the commit
> message.

That's why I didn't move it to dma-mapping.h -- in general, drivers
should not call it, so it would be a workaround. I added this paragraph
in v2 as a couple folks asked "what if some weird device will break with
this optimization". I can drop it anyway.

> 
>>  	if (dma_map_direct(dev, ops))
>> +		/*
>> +		 * dma_skip_sync could've been set to false on first SWIOTLB
>> +		 * buffer mapping, but @dma_addr is not necessary an SWIOTLB
>> +		 * buffer. In this case, fall back to more granular check.
>> +		 */
>>  		return dma_direct_need_sync(dev, dma_addr);
>> +
> 
> Nit: with such a long block comment adding curly braces would make the
> code a bit more readable.
> 
>> +#ifdef CONFIG_DMA_NEED_SYNC
>> +void dma_setup_skip_sync(struct device *dev)
>> +{
>> +	const struct dma_map_ops *ops = get_dma_ops(dev);
>> +	bool skip;
>> +
>> +	if (dma_map_direct(dev, ops))
>> +		/*
>> +		 * dma_skip_sync will be set to false on first SWIOTLB buffer
>> +		 * mapping, if any. During the device initialization, it's
>> +		 * enough to check only for DMA coherence.
>> +		 */
>> +		skip = dev_is_dma_coherent(dev);
>> +	else if (!ops->sync_single_for_device && !ops->sync_single_for_cpu)
>> +		/*
>> +		 * Synchronization is not possible when none of DMA sync ops
>> +		 * is set. This check precedes the below one as it disables
>> +		 * the synchronization unconditionally.
>> +		 */
>> +		skip = true;
>> +	else if (ops->flags & DMA_F_CAN_SKIP_SYNC)
>> +		/*
>> +		 * Assume that when ``DMA_F_CAN_SKIP_SYNC`` is advertised,
>> +		 * the conditions for synchronizing are the same as with
>> +		 * the direct DMA.
>> +		 */
>> +		skip = dev_is_dma_coherent(dev);
>> +	else
>> +		skip = false;
>> +
>> +	dma_set_skip_sync(dev, skip);
> 
> I'd just assign directly to dev->dma_skip_sync instead of using a
> local variable and the dma_set_skip_sync call - we are under
> ifdef CONFIG_DMA_NEED_SYNC here and thus know is is available.
> 
>> +static inline void swiotlb_disable_dma_skip_sync(struct device *dev)
>> +{
>> +	/*
>> +	 * If dma_skip_sync was set, reset it to false on first SWIOTLB buffer
>> +	 * mapping/allocation to always sync SWIOTLB buffers.
>> +	 */
>> +	if (unlikely(dma_skip_sync(dev)))
>> +		dma_set_skip_sync(dev, false);
>> +}
> 
> Nothing really swiotlb-specific here.  Also the naming is a bit odd.
> Maybe have a dma_set_skip_sync helper without the bool to enable
> skipping, and a dma_clear_skip_sync that clear the flag.  The optimization
> to first check the flag here could just move into that latter
> helper.

Sounds good!

Thanks,
Olek