linux-kernel - Re: [PATCH 1/2 v2] dma-iommu: use static-key to minimize the impact in the fast-path

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Message-ID: <c961822e-e768-9d20-87cf-85db95b6aab6@arm.com>
Date:   Tue, 19 Jan 2021 15:52:41 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@...radead.org>,
        Lianbo Jiang <lijiang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, thomas.lendacky@....com,
        will@...nel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 1/2 v2] dma-iommu: use static-key to minimize the impact
 in the fast-path

On 2021-01-19 15:26, Christoph Hellwig wrote:
> On Tue, Jan 19, 2021 at 07:16:15PM +0800, Lianbo Jiang wrote:
>> +static DEFINE_STATIC_KEY_FALSE(__deferred_attach);
> 
> Why the strange underscores?  Wouldn't iommu_deferred_attach_enabled
> be a better name?
> 
>> -	if (unlikely(iommu_dma_deferred_attach(dev, domain)))
>> +	if (static_branch_unlikely(&__deferred_attach) &&
>> +	    iommu_dma_deferred_attach(dev, domain))
> 
> Also insted of duplicating this logic in three places, maybe rename
> iommu_dma_deferred_attach to __iommu_dma_deferred_attach and create
> a small inline wrapper for it?

Once patch #2 is in place, I really don't see any point. The "helper" 
would add a minimum of 5 lines to save at most 3, and would have to be 
annotated as always_inline - which a whole other camp of people would 
probably object to - in order for the static branch to be properly useful.

It's not as if this is a complex or hard-to-read expression, so IMO 
having 3 lines repeated 3 times is objectively better than having 2 
lines necessarily repeated 3 times plus having to scroll up and find 
several more lines to follow what it's doing.

Robin.