linux-kernel - Re: [PATCH 1/2] iommu/vt-d: Remove the dead code in init_iommu

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Message-ID: <c8c0a8f6-751b-b9e7-ffd8-72859c0e3b12@linux.intel.com>
Date:   Thu, 1 Jun 2023 09:15:37 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Yanfei Xu <yanfei.xu@...el.com>, dwmw2@...radead.org,
        joro@...tes.org, will@...nel.org, robin.murphy@....com
Cc:     baolu.lu@...ux.intel.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iommu/vt-d: Remove the dead code in init_iommu_hw()

On 5/31/23 2:55 PM, Yanfei Xu wrote:
> 
> On 5/31/2023 11:24 AM, Baolu Lu wrote:
>> On 5/30/23 5:25 PM, Yanfei Xu wrote:
>>> After 'commit 2a41ccee2fdc ("iommu/vt-d: Change
>>> iommu_enable/disable_translation to return void")', init_iommu_hw() only
>>> returns 0. If statement for return value of this function is 
>>> meaningless.
>>> Hence change init_iommu_hw() to return viod and remove the dead code of
>>> if statement in init_iommu_hw()
>>>
>>> Signed-off-by: Yanfei Xu<yanfei.xu@...el.com>
>>> ---
>>>   drivers/iommu/intel/iommu.c | 12 ++----------
>>>   1 file changed, 2 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>> index 8096273b034c..e98f1b122b49 100644
>>> --- a/drivers/iommu/intel/iommu.c
>>> +++ b/drivers/iommu/intel/iommu.c
>>> @@ -2963,7 +2963,7 @@ static void __init init_no_remapping_devices(void)
>>>   }
>>>     #ifdef CONFIG_SUSPEND
>>> -static int init_iommu_hw(void)
>>> +static void init_iommu_hw(void)
>>>   {
>>>       struct dmar_drhd_unit *drhd;
>>>       struct intel_iommu *iommu = NULL;
>>> @@ -2988,8 +2988,6 @@ static int init_iommu_hw(void)
>>>           iommu_enable_translation(iommu);
>>>           iommu_disable_protect_mem_regions(iommu);
>>>       }
>>> -
>>> -    return 0;
>>
>> 2966 static int init_iommu_hw(void)
>> 2967 {
>> 2968         struct dmar_drhd_unit *drhd;
>> 2969         struct intel_iommu *iommu = NULL;
>> 2970
>> 2971         for_each_active_iommu(iommu, drhd)
>> 2972                 if (iommu->qi)
>> 2973                         dmar_reenable_qi(iommu);
>>
>> dmar_reenable_qi() still possibly returns an error number. It's better
>> to pass this error number to the caller of init_iommu_hw()?
>>
> Event dmar_reenable_qi can return error number, but there is no caller
> check it. As below, only these two places invoke it:
> 1. init_iommu_hw->dmar_reenable_qi
> 2. reenable_irq_remapping->dmar_reenable_qi
> 
> I think we can also convert dmar_reenable_qi() to return void:
> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
> index a3414afe11b0..1432483c79e8 100644
> --- a/drivers/iommu/intel/dmar.c
> +++ b/drivers/iommu/intel/dmar.c
> @@ -2112,13 +2112,10 @@ int __init enable_drhd_fault_handling(void)
>   /*
>    * Re-enable Queued Invalidation interface.
>    */
> -int dmar_reenable_qi(struct intel_iommu *iommu)
> +void dmar_reenable_qi(struct intel_iommu *iommu)
>   {
> -       if (!ecap_qis(iommu->ecap))
> -               return -ENOENT;
> -
> -       if (!iommu->qi)
> -               return -ENOENT;
> +       WARN_ON(!iommu->qi || !ecap_qis(iommu->ecap))
> +               return;
> 
>          /*
>           * First disable queued invalidation.
> @@ -2130,8 +2127,6 @@ int dmar_reenable_qi(struct intel_iommu *iommu)
>           * invalidation.
>           */
>          __dmar_enable_qi(iommu);
> -
> -       return 0;
>   }
> 
>  From my understanding, dmar_reenable_qi() is used in suspend/resume case,
> so the extended cap of an existing IOMMU hardware is unlikely changed. As
> for the check of iommu->qi, if dmar_reenable_qi() can be invoked all is
> depended on the no-NULL of iommu->qi at first. How about using WARN_ON for
> both of them to simply this function.

This seems to be heading in the opposite direction. Actually any
operation may succeed or fail.

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index b871a6afd803..ecc2007a96f9 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -2967,10 +2967,13 @@ static int init_iommu_hw(void)
  {
         struct dmar_drhd_unit *drhd;
         struct intel_iommu *iommu = NULL;
+       int ret;

-       for_each_active_iommu(iommu, drhd)
-               if (iommu->qi)
-                       dmar_reenable_qi(iommu);
+       for_each_active_iommu(iommu, drhd) {
+               ret = dmar_reenable_qi(iommu);
+               if (ret)
+                       return ret;
+       }

         for_each_iommu(iommu, drhd) {
                 if (drhd->ignored) {

Best regards,
baolu