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, 26 Feb 2024 16:27:16 +0000
From: Robin Murphy <robin.murphy@....com>
To: Alexander Lobakin <aleksander.lobakin@...el.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

On 19/02/2024 12:53 pm, Alexander Lobakin wrote:
> 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.

Huh? Why would anything need touching twice? All I'm saying is that it's 
pretty pointless to add any invocations of dma_skip_sync() in !HAS_DMA 
paths where we already know the whole API is stubbed out anyway. The 
only cases which are worth differentiating here are HAS_DMA + 
DMA_NEED_SYNC vs. HAS_DMA + !DMA_NEED_SYNC (with the subsequent runtime 
check then just subdividing the former).

> 
> [...]
> 
>>> @@ -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.

Ugh, no. If the page pool code wants to be clever poking around and 
sidestepping parts of the documented API, it can flippin' well open-code 
a single addition to call __dma_sync_single_for_device() directly 
itself. I'm not at all keen on having to maintain "common" APIs for such 
niche trickery.

Thanks,
Robin.