linux-kernel - Re: [PATCH] iommu: of: skip iommu_device_list traversal in of_iommu

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Message-ID: <352d1518-8191-19e0-b9f3-29dfe91a2ad4@codeaurora.org>
Date:   Thu, 24 Sep 2020 10:21:31 +0530
From:   Charan Teja Kalla <charante@...eaurora.org>
To:     Robin Murphy <robin.murphy@....com>, joro@...tes.org,
        iommu@...ts.linux-foundation.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu: of: skip iommu_device_list traversal in
 of_iommu_xlate()



On 9/23/2020 9:54 PM, Robin Murphy wrote:
> On 2020-09-23 15:53, Charan Teja Reddy wrote:
>> In of_iommu_xlate(), check if iommu device is enabled before traversing
>> the iommu_device_list through iommu_ops_from_fwnode(). It is of no use
>> in traversing the iommu_device_list only to return NO_IOMMU because of
>> iommu device node is disabled.
> 
> Well, the "use" is that it keeps the code that much smaller and simpler
> to have a single path for returning this condition. This whole callstack
> isn't exactly a high-performance code path to begin with, and we've
> always assumed that IOMMUs present but disabled in DT would be a pretty
> rare exception. 

Fine..I thought that it is logical to return when IOMMU DT node is
disabled over code simplicity. And agree that it is not high-performance
path.

> Do you have a system that challenges those assumptions
> and shows any benefit from this change?

No, I don't have a system that challenges these assumptions.

> 
> Robin.
> 
>> Signed-off-by: Charan Teja Reddy <charante@...eaurora.org>
>> ---
>>   drivers/iommu/of_iommu.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
>> index e505b91..225598c 100644
>> --- a/drivers/iommu/of_iommu.c
>> +++ b/drivers/iommu/of_iommu.c
>> @@ -94,9 +94,10 @@ static int of_iommu_xlate(struct device *dev,
>>       struct fwnode_handle *fwnode = &iommu_spec->np->fwnode;
>>       int ret;
>>   +    if (!of_device_is_available(iommu_spec->np))
>> +        return NO_IOMMU;
>>       ops = iommu_ops_from_fwnode(fwnode);
>> -    if ((ops && !ops->of_xlate) ||
>> -        !of_device_is_available(iommu_spec->np))
>> +    if (ops && !ops->of_xlate)
>>           return NO_IOMMU;
>>         ret = iommu_fwspec_init(dev, &iommu_spec->np->fwnode, ops);
>>

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