linux-kernel - Re: [RFC PATCH 1/4] drm/msm: register a fault handler for display mmu faults

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Date: Fri, 21 Jun 2024 16:35:10 -0700
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
CC: <freedreno@...ts.freedesktop.org>, Rob Clark <robdclark@...il.com>,
        "Sean
 Paul" <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        "David Airlie" <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        <dri-devel@...ts.freedesktop.org>, <seanpaul@...omium.org>,
        <swboyd@...omium.org>, <dianders@...omium.org>,
        <quic_jesszhan@...cinc.com>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/4] drm/msm: register a fault handler for display mmu
 faults



On 5/19/2024 1:38 AM, Dmitry Baryshkov wrote:
> On Fri, May 17, 2024 at 04:37:56PM -0700, Abhinav Kumar wrote:
>> In preparation to register a iommu fault handler for display
>> related modules, register a fault handler for the backing
>> mmu object of msm_kms.
>>
>> Currently, the fault handler only captures the display snapshot
>> but we can expand this later if more information needs to be
>> added to debug display mmu faults.
>>
>> Signed-off-by: Abhinav Kumar <quic_abhinavk@...cinc.com>
>> ---
>>   drivers/gpu/drm/msm/msm_kms.c | 25 +++++++++++++++++++++++++
>>   1 file changed, 25 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/msm/msm_kms.c b/drivers/gpu/drm/msm/msm_kms.c
>> index af6a6fcb1173..62c8e6163e81 100644
>> --- a/drivers/gpu/drm/msm/msm_kms.c
>> +++ b/drivers/gpu/drm/msm/msm_kms.c
>> @@ -200,6 +200,28 @@ struct msm_gem_address_space *msm_kms_init_aspace(struct drm_device *dev)
>>   	return aspace;
>>   }
>>   
>> +static int msm_kms_fault_handler(void *arg, unsigned long iova, int flags, void *data)
>> +{
>> +	struct msm_kms *kms = arg;
>> +	struct msm_disp_state *state;
>> +	int ret;
>> +
>> +	ret = mutex_lock_interruptible(&kms->dump_mutex);
>> +	if (ret)
>> +		return ret;
>> +
>> +	state = msm_disp_snapshot_state_sync(kms);
>> +
>> +	mutex_unlock(&kms->dump_mutex);
>> +
>> +	if (IS_ERR(state)) {
>> +		DRM_DEV_ERROR(kms->dev->dev, "failed to capture snapshot\n");
>> +		return PTR_ERR(state);
>> +	}
>> +
>> +	return 0;
> 
> Hmm, after reading the rest of the code, this means that we won't get
> the error on the console. Could you please change this to -ENOSYS?
> 

I am perhaps missing something here. Are you referring to this 
documentation?

  * Specifically, -ENOSYS is returned if a fault handler isn't installed
  * (though fault handlers can also return -ENOSYS, in case they want to
  * elicit the default behavior of the IOMMU drivers).
  */
int report_iommu_fault(struct iommu_domain *domain, struct device *dev,
                        unsigned long iova, int flags)

Doesnt this mean ENOSYS is when fault handler is not installed?

Because the console print will come if we return 0?

         if (!report_iommu_fault(ctx->domain, ctx->dev, iova, 0)) {
                 dev_err_ratelimited(ctx->dev,
                                     "Unhandled context fault: fsr=0x%x, "
                                     "iova=0x%016llx, fsynr=0x%x, cb=%d\n",
                                     fsr, iova, fsynr, ctx->asid);
         }

>> +}
>> +
>>   void msm_drm_kms_uninit(struct device *dev)
>>   {
>>   	struct platform_device *pdev = to_platform_device(dev);
>> @@ -261,6 +283,9 @@ int msm_drm_kms_init(struct device *dev, const struct drm_driver *drv)
>>   		goto err_msm_uninit;
>>   	}
>>   
>> +	if (kms->aspace)
>> +		msm_mmu_set_fault_handler(kms->aspace->mmu, kms, msm_kms_fault_handler);
>> +
>>   	drm_helper_move_panel_connectors_to_head(ddev);
>>   
>>   	drm_for_each_crtc(crtc, ddev) {
>> -- 
>> 2.44.0
>>
>