Re: [RFC PATCH v4 04/10] iommu/riscv: use data structure instead of individual values

[Gong Shuai <gsh517025@gmail.com>]

Hi Fangyu,

On 9/20/2026 11:08 AM, fangyu.yu@linux.alibaba.com wrote:
>>Hi Fangyu,
>>
>>On 9/15/2026 11:28 AM, fangyu.yu@linux.alibaba.com wrote:
>>> From: Zong Li <zong.li@sifive.com>
>>>
>>> The parameter will be increased when we need to set up more
>>> bit fields in the device context. Use a data structure to
>>> wrap them up.
>>>
>>> Signed-off-by: Zong Li <zong.li@sifive.com>
>>> Signed-off-by: Fangyu Yu <fangyu.yu@linux.alibaba.com>
>>> ---
>>>   drivers/iommu/riscv/iommu.c | 27 +++++++++++++++++----------
>>>   1 file changed, 17 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
>>> index a6c8307e82ea..c5b6214ee672 100644
>>> --- a/drivers/iommu/riscv/iommu.c
>>> +++ b/drivers/iommu/riscv/iommu.c
>>> @@ -1165,7 +1165,7 @@ static void riscv_iommu_iodir_iotinval(struct riscv_iommu_device *iommu,
>>>    * interim translation faults.
>>>    */
>>>   static void riscv_iommu_iodir_update(struct riscv_iommu_device *iommu,
>>> -				     struct device *dev, u64 fsc, u64 ta)
>>> +				     struct device *dev, struct riscv_iommu_dc *new_dc)
>>>   {
>>>   	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
>>>   	struct riscv_iommu_dc *dc;
>>> @@ -1204,10 +1204,10 @@ static void riscv_iommu_iodir_update(struct riscv_iommu_device *iommu,
>>>   	for (i = 0; i < fwspec->num_ids; i++) {
>>>   		dc = riscv_iommu_get_dc(iommu, fwspec->ids[i]);
>>>   		tc = READ_ONCE(dc->tc);
>>> -		tc |= ta & RISCV_IOMMU_DC_TC_V;
>>> +		tc |= new_dc->ta & RISCV_IOMMU_DC_TC_V;
>>
>>According to SPEC 3.1.3.3, the "V bit" does not exist in DC.ta.
>>This should be a convention from when the iodir_update() function
>>did not yet use `struct riscv_iommu_dc` as a parameter, using bit 0
>>of DC.ta to indicate that the DC is valid. Now that iodir_update()
>>receives the complete dc, it can directly access DC.tc.V, so this
>>convention should no longer be necessary and will cause confusion.
>>
> 
> Hi Shuai:
> 
> Thanks for the clarification.
> 
> My understanding is that `ta` here is used as a
> combined concept, not only for `DC.ta`, but also
> for `PC.ta` in this flow.
> 
> The `V` bit is only initialized when `ta` is first
> set up, so the current usage still follows the
> original design, without changing most of the
> existing logic.
> 
> The original code also has comments describing
> this behavior.

I got your point.

When fsc and ta were passed as separate raw parameters, treating
them as a combined concept was arguably reasonable, since the same
bit layout is shared between DC and PC in this path. But now that
iodir_update() takes a struct riscv_iommu_dc, I think it would be
better to keep the semantics explicit.

If PDT/PC support is added later, we can introduce a separate
struct riscv_iommu_pc parameter for the PC context, instead of
trying to make one ta/fsc cover both cases.


> Thanks,
> Fangyu
> 
>>>
>>> -		WRITE_ONCE(dc->fsc, fsc);
>>> -		WRITE_ONCE(dc->ta, ta & RISCV_IOMMU_PC_TA_PSCID);
>>> +		WRITE_ONCE(dc->fsc, new_dc->fsc);
>>> +		WRITE_ONCE(dc->ta, new_dc->ta & RISCV_IOMMU_PC_TA_PSCID);
>>
>>`RISCV_IOMMU_PC_*` should be replaced with `RISCV_IOMMU_DC_*` for
>>readability. There are several other similar places in this file
>>that are unrelated to Process Context or PASID but use the *PC* macros.
>>
> 
> My understanding is similar to the `ta` case: `fsc`
> likely refers to both `dc.fsc` and `pc.fsc` in this
> path, since some of their fields share the same bit
> layout.
Yeah, exactly, they share the same layout, so logically it's fine.

> 
> If we want to rename these for readability, I think
> that should be done in a separate patch set, rather
> than as part of this one.

Agree that it should be done in a separate patch.

Thanks,
Shuai

> 
>>
>>Thanks,
>>Shuai
>>
>>>   		/* Update device context, write TC.V as the last step. */
>>>   		dma_wmb();
>>>   		WRITE_ONCE(dc->tc, tc);
>>> @@ -1288,22 +1288,22 @@ static int riscv_iommu_attach_paging_domain(struct iommu_domain *iommu_domain,
>>>   	struct riscv_iommu_device *iommu = dev_to_iommu(dev);
>>>   	struct riscv_iommu_info *info = dev_iommu_priv_get(dev);
>>>   	struct pt_iommu_riscv_64_hw_info pt_info;
>>> -	u64 fsc, ta;
>>> +	struct riscv_iommu_dc dc = {0};
>>>
>>>   	pt_iommu_riscv_64_hw_info(&domain->riscvpt, &pt_info);
>>>
>>>   	if (!riscv_iommu_pt_supported(iommu, pt_info.fsc_iosatp_mode))
>>>   		return -ENODEV;
>>>
>>> -	fsc = FIELD_PREP(RISCV_IOMMU_PC_FSC_MODE, pt_info.fsc_iosatp_mode) |
>>> +	dc.fsc = FIELD_PREP(RISCV_IOMMU_PC_FSC_MODE, pt_info.fsc_iosatp_mode) |
>>>   	      FIELD_PREP(RISCV_IOMMU_PC_FSC_PPN, pt_info.ppn);
>>> -	ta = FIELD_PREP(RISCV_IOMMU_PC_TA_PSCID, domain->pscid) |
>>> +	dc.ta = FIELD_PREP(RISCV_IOMMU_PC_TA_PSCID, domain->pscid) |
>>>   	     RISCV_IOMMU_PC_TA_V;
>>>
>>>   	if (riscv_iommu_bond_link(domain, dev))
>>>   		return -ENOMEM;
>>>
>>> -	riscv_iommu_iodir_update(iommu, dev, fsc, ta);
>>> +	riscv_iommu_iodir_update(iommu, dev, &dc);
>>>   	riscv_iommu_bond_unlink(info->domain, dev);
>>>   	info->domain = domain;
>>>
>>> @@ -1378,9 +1378,12 @@ static int riscv_iommu_attach_blocking_domain(struct iommu_domain *iommu_domain,
>>>   {
>>>   	struct riscv_iommu_device *iommu = dev_to_iommu(dev);
>>>   	struct riscv_iommu_info *info = dev_iommu_priv_get(dev);
>>> +	struct riscv_iommu_dc dc = {0};
>>> +
>>> +	dc.fsc = RISCV_IOMMU_FSC_BARE;
>>>
>>>   	/* Make device context invalid, translation requests will fault w/ #258 */
>>> -	riscv_iommu_iodir_update(iommu, dev, RISCV_IOMMU_FSC_BARE, 0);
>>> +	riscv_iommu_iodir_update(iommu, dev, &dc);
>>>   	riscv_iommu_bond_unlink(info->domain, dev);
>>>   	info->domain = NULL;
>>>
>>> @@ -1400,8 +1403,12 @@ static int riscv_iommu_attach_identity_domain(struct iommu_domain *iommu_domain,
>>>   {
>>>   	struct riscv_iommu_device *iommu = dev_to_iommu(dev);
>>>   	struct riscv_iommu_info *info = dev_iommu_priv_get(dev);
>>> +	struct riscv_iommu_dc dc = {0};
>>> +
>>> +	dc.fsc = RISCV_IOMMU_FSC_BARE;
>>> +	dc.ta = RISCV_IOMMU_PC_TA_V;
>>>
>>> -	riscv_iommu_iodir_update(iommu, dev, RISCV_IOMMU_FSC_BARE, RISCV_IOMMU_PC_TA_V);
>>> +	riscv_iommu_iodir_update(iommu, dev, &dc);
>>>   	riscv_iommu_bond_unlink(info->domain, dev);
>>>   	info->domain = NULL;
>>>