linux-kernel - Re: [RFC PATCH 2/2] ACPI/IORT: use swiotlb_dma

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Date:   Wed, 4 Apr 2018 19:57:25 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Wang Dongsheng <dongsheng.wang@...-semitech.com>
Cc:     rjw@...ysocki.net, gregkh@...uxfoundation.org,
        hanjun.guo@...aro.org, sudeep.holla@....com,
        yu.zheng@...-semitech.com, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] ACPI/IORT: use swiotlb_dma_ops when smmu probe
 failed

On 04/04/18 17:01, Lorenzo Pieralisi wrote:
> [+cc Robin]
> 
> On Thu, Mar 29, 2018 at 03:01:00AM -0700, Wang Dongsheng wrote:
>> If SMMU probe failed, master should use swiotlb as dma ops.
>> SMMU may probe failed with specified environment, so there
>> are not any iommu resources in iommu_device_list.
>>
>> The master will always get EPROBE_DEFER from really_probe
>> (dma_configure) but in fact SMMU has probe failed. The issue
>> causes all of masters failed to be driven.

Let's just take a step back - why is SMMU probe failing? That seems to 
be the primary issue here, because it implies that either your hardware, 
firmware or kernel is broken, any of which would make boot failure 
somewhat unsurprising anyway.

> I added Robin to pick his brain. An alternative would consist
> in using a bus notifier to prevent deferred probing once the SMMU
> driver probing failed but that seems backwards given that a major
> reason to move to deferred probing was to remove the bus notifiers
> dependency in the first place.
> 
> It seems to me this is both an OF/ACPI issue - it is not an IORT
> only problem.

Yes, this is just an instance of the general probe-deferral problem, 
e.g. once you have multiple dependencies it's possible to end up in a 
stalemate where everything including the IOMMU ends up on the deferred 
probe list with nothing to kick it and make progress again.

Furthermore it seems to me that the whole premise in this patch is 
flawed, since even genuine probe failure may well be transient - just 
because one attempt failed doesn't mean a later attempt can't succeed. 
Thus "the most recent probe attempt failed" cannot be considered a 
fundamentally different state from "no driver is currently bound".

Robin.

> 
> Lorenzo
> 
>> Signed-off-by: Wang Dongsheng <dongsheng.wang@...-semitech.com>
>> ---
>>   drivers/acpi/arm64/iort.c | 39 +++++++++++++++++++++++++++++++++------
>>   1 file changed, 33 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
>> index e2f7bdd..a6f4c27 100644
>> --- a/drivers/acpi/arm64/iort.c
>> +++ b/drivers/acpi/arm64/iort.c
>> @@ -774,17 +774,45 @@ static int arm_smmu_iort_xlate(struct device *dev, u32 streamid,
>>   	return ret;
>>   }
>>   
>> -static inline bool iort_iommu_driver_enabled(u8 type)
>> +static int iort_check_dev_dl_status(struct device *dev, void *data)
>>   {
>> +	struct fwnode_handle *fwnode = data;
>> +
>> +	if (dev->fwnode != fwnode)
>> +		return 0;
>> +
>> +	if (dev->links.status == DL_DEV_PROBE_FAILED)
>> +		return -ENODEV;
>> +
>> +	return -EPROBE_DEFER;
>> +}
>> +
>> +static int iort_iommu_driver_enabled(u8 type, struct fwnode_handle *fwnode)
>> +{
>> +	bool buildin;
>> +	int ret;
>> +
>>   	switch (type) {
>>   	case ACPI_IORT_NODE_SMMU_V3:
>> -		return IS_BUILTIN(CONFIG_ARM_SMMU_V3);
>> +		buildin = IS_BUILTIN(CONFIG_ARM_SMMU_V3);
>> +		break;
>>   	case ACPI_IORT_NODE_SMMU:
>> -		return IS_BUILTIN(CONFIG_ARM_SMMU);
>> +		buildin = IS_BUILTIN(CONFIG_ARM_SMMU);
>> +		break;
>>   	default:
>>   		pr_warn("IORT node type %u does not describe an SMMU\n", type);
>> -		return false;
>> +		buildin = false;
>>   	}
>> +
>> +	if (!buildin)
>> +		return -ENODEV;
>> +
>> +	ret = bus_for_each_dev(&platform_bus_type, NULL, fwnode,
>> +			       iort_check_dev_dl_status);
>> +	if (!ret)
>> +		return -EPROBE_DEFER;
>> +
>> +	return ret;
>>   }
>>   
>>   #ifdef CONFIG_IOMMU_API
>> @@ -919,8 +947,7 @@ static int iort_iommu_xlate(struct device *dev, struct acpi_iort_node *node,
>>   	 */
>>   	ops = iommu_ops_from_fwnode(iort_fwnode);
>>   	if (!ops)
>> -		return iort_iommu_driver_enabled(node->type) ?
>> -		       -EPROBE_DEFER : -ENODEV;
>> +		return iort_iommu_driver_enabled(node->type, iort_fwnode);
>>   
>>   	return arm_smmu_iort_xlate(dev, streamid, iort_fwnode, ops);
>>   }
>> -- 
>> 2.7.4
>>