linux-kernel - Re: [PATCH net-next v3 1/7] dma: compile-out DMA sync op calls when not used

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Date: Mon, 19 Feb 2024 13:53:36 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Robin Murphy <robin.murphy@....com>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Christoph Hellwig <hch@....de>, Marek Szyprowski
	<m.szyprowski@...sung.com>, Joerg Roedel <joro@...tes.org>, Will Deacon
	<will@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael
 J. Wysocki" <rafael@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Alexander Duyck
	<alexanderduyck@...com>, <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v3 1/7] dma: compile-out DMA sync op calls when
 not used

From: Robin Murphy <robin.murphy@....com>
Date: Wed, 14 Feb 2024 17:20:50 +0000

> On 2024-02-14 4:21 pm, Alexander Lobakin wrote:

[...]

>> -static inline void dma_sync_single_for_cpu(struct device *dev,
>> dma_addr_t addr,
>> -        size_t size, enum dma_data_direction dir)
>> +static inline void __dma_sync_single_for_cpu(struct device *dev,
>> +        dma_addr_t addr, size_t size, enum dma_data_direction dir)
> 
> To me it would feel more logical to put all the wrappers inside the
> #ifdef CONFIG_HAS_DMA and not touch these stubs at all (what does it
> mean to skip an inline no-op?). Or in fact, if dma_skip_sync() is
> constant false for !HAS_DMA, then we could also just make the external
> function declarations unconditional and remove the stubs. Not a critical
> matter though, and I defer to whatever Christoph thinks is most
> maintainable.

It's done like that due to that I'm adding a runtime check in the second
patch. I don't feel like touching this twice makes sense.

[...]

>> @@ -348,18 +348,72 @@ static inline void dma_unmap_single_attrs(struct
>> device *dev, dma_addr_t addr,
>>       return dma_unmap_page_attrs(dev, addr, size, dir, attrs);
>>   }
>>   +static inline void __dma_sync_single_range_for_cpu(struct device *dev,
>> +        dma_addr_t addr, unsigned long offset, size_t size,
>> +        enum dma_data_direction dir)
>> +{
>> +    __dma_sync_single_for_cpu(dev, addr + offset, size, dir);
>> +}
>> +
>> +static inline void __dma_sync_single_range_for_device(struct device
>> *dev,
>> +        dma_addr_t addr, unsigned long offset, size_t size,
>> +        enum dma_data_direction dir)
>> +{
>> +    __dma_sync_single_for_device(dev, addr + offset, size, dir);
>> +}
> 
> There is no need to introduce these two.

I already replied to this in the previous thread. Some subsys may want
to check for the shortcut earlier to avoid call ladders of their own
functions. See patch 6 for example where I use this one.

> 
>> +
>> +static inline bool dma_skip_sync(const struct device *dev)
>> +{
>> +    return !IS_ENABLED(CONFIG_DMA_NEED_SYNC);
>> +}
>> +
>> +static inline bool dma_need_sync(struct device *dev, dma_addr_t
>> dma_addr)
>> +{
>> +    return !dma_skip_sync(dev) ? __dma_need_sync(dev, dma_addr) : false;
>> +}
> 
> That's a bit of a mind-bender... is it actually just
> 
>     return !dma_skip_sync(dev) && __dma_need_sync(dev, dma_addr);

Oh, indeed ._.

> 
> ?
> 
> (I do still think the negative flag makes it all a little harder to
> follow in general than a positive "device needs to consider syncs" flag
> would.)

I think it was in the original Eric's idea and I kept this.
I'm fine with inverting it.

[...]

> Thanks,
> Robin.

Thanks,
Olek