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SubjectRe: [PATCH v3 18/18] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs
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+linuxppc-dev@lists.ozlabs.org

On 3/17/2018 11:05 AM, Jason Gunthorpe wrote:
> On Sat, Mar 17, 2018 at 12:25:14AM -0400, Sinan Kaya wrote:
>> On 3/17/2018 12:03 AM, Sinan Kaya wrote:
>>> On 3/16/2018 11:40 PM, Sinan Kaya wrote:
>>>> I'll change writel_relaxed() with __raw_writel() in the series like you suggested
>>>> and also look at your other comments.
>>>
>>> I spoke too soon.
>>>
>>> Now that I realized, code needs to follow one of the following patterns for correctness
>>>
>>> 1)
>>> wmb()
>>> writel()/writel_relaxed()
>>>
>>> or
>>>
>>> 2)
>>> wmb()
>>> __raw_wrltel()
>>> mmiowb()
>>>
>>> but definitely not
>>>
>>> wmb()
>>> __raw_wrltel()
>>>
>>> Since #1 == #2, I'll stick to my current implementation of writel_relaxed()
>>>
>>> Changing writel() to writel_relaxed() or __raw_writel() isn't enough. PowerPC needs mmiowb()
>>> for correctness. ARM's mmiowb() implementation is empty.
>>>
>>> So, there is no one size fits all solution with the current state of affairs.
>>>
>>>
>>
>> I think I finally got what you mean.
>>
>> Code seems to have
>>
>> wmb()
>> writel()/writeq()
>> wmb()
>>
>> this can be safely replaced with
>>
>> wmb()
>> __raw_writel()/__raw_writeq()
>> wmb()
>>
>> This will work on all arches. Below is the new version. Let me know if this is OK.
>>
>> +++ b/drivers/infiniband/hw/cxgb4/t4.h
>> @@ -457,7 +457,7 @@ static inline void pio_copy(u64 __iomem *dst, u64 *src)
>> int count = 8;
>>
>> while (count) {
>> - writeq(*src, dst);
>> + __raw_writeq(*src, dst);
>> src++;
>> dst++;
>> count--;
>> @@ -477,15 +477,16 @@ static inline void t4_ring_sq_db(struct t4_wq *wq, u16 inc, union t4_wr *wqe)
>> (u64 *)wqe);
>> } else {
>> pr_debug("DB wq->sq.pidx = %d\n", wq->sq.pidx);
>> - writel(PIDX_T5_V(inc) | QID_V(wq->sq.bar2_qid),
>> - wq->sq.bar2_va + SGE_UDB_KDOORBELL);
>> + __raw_writel(PIDX_T5_V(inc) | QID_V(wq->sq.bar2_qid),
>> + wq->sq.bar2_va + SGE_UDB_KDOORBELL);
>> }
>>
>> /* Flush user doorbell area writes. */
>> wmb();
>> return;
>> }
>> - writel(QID_V(wq->sq.qid) | PIDX_V(inc), wq->db);
>> + __raw_writel(QID_V(wq->sq.qid) | PIDX_V(inc), wq->db);
>> + mmiowmb()
>> }
>>
>> static inline void t4_ring_rq_db(struct t4_wq *wq, u16 inc,
>> @@ -502,15 +503,16 @@ static inline void t4_ring_rq_db(struct t4_wq *wq, u16 inc,
>> (void *)wqe);
>> } else {
>> pr_debug("DB wq->rq.pidx = %d\n", wq->rq.pidx);
>> - writel(PIDX_T5_V(inc) | QID_V(wq->rq.bar2_qid),
>> - wq->rq.bar2_va + SGE_UDB_KDOORBELL);
>> + __raw_writel(PIDX_T5_V(inc) | QID_V(wq->rq.bar2_qid),
>> + wq->rq.bar2_va + SGE_UDB_KDOORBELL);
>> }
>>
>> /* Flush user doorbell area writes. */
>> wmb();
>> return;
>> }
>> - writel(QID_V(wq->rq.qid) | PIDX_V(inc), wq->db);
>> + __raw_writel(QID_V(wq->rq.qid) | PIDX_V(inc), wq->db);
>> + mmiowmb();
>
> No! NAK! Adding *new* barriers to use the *wrong* accessor is crazy!
>
> Your first patch was right, replacing
> wmb()
> writel()
>
> With wmb(); writel_relaxed() is fine, and helps ARM.
>
> The problem with PPC has nothing to do with the writel, it is with the
> spinlock, and we can't improve it without adding some new
> infrastructure. Certainly don't hack around the problem by using
> __raw_writel in multi-arch drivers!
>
> In userspace we settled on something like this as a pattern:
>
> mmio_wc_spin_lock()
> writel_wc_mmio()
> mmio_wc_spin_unlock()

>
> Where mmio_wc_spin_unlock incorporates the mmiowmb and is defined to
> fully contain all MMIO access to WC and UC memory.
>
> Due to our userspace the pattern is specifically used with MMIO writes
> to WC BAR memory, but it could be generalized..
>
> This allows all known arches to use the best code in all cases. x86
> can even optimize a lot and combine the mmiowmb() into the
> spin_unlock, apparently.

Given both Dave [1] and Jason's feedback, we have to ask PowerPC developers
to fix writel_relaxed().

Otherwise, PowerPC won't be able to take advantage of the optimizations
introduced in this series.

I don't think we need writel_really_relaxed() API.

Somebody also has to take a task and work very hard to get rid of __raw_writeX()
APIs in drivers/net directory. It looked like a very common practice though
it clearly violates multiarch portability concerns Jason and Deve highlighted.

This will be the next version:

iff --git a/drivers/infiniband/hw/cxgb4/t4.h b/drivers/infiniband/hw/cxgb4/t4.h
index 8369c7c..6e5658a 100644
--- a/drivers/infiniband/hw/cxgb4/t4.h
+++ b/drivers/infiniband/hw/cxgb4/t4.h
@@ -457,7 +457,7 @@ static inline void pio_copy(u64 __iomem *dst, u64 *src)
int count = 8;

while (count) {
- writeq(*src, dst);
+ writeq_relaxed(*src, dst);
src++;
dst++;
count--;
@@ -477,15 +477,15 @@ static inline void t4_ring_sq_db(struct t4_wq *wq, u16 inc, union t4_wr *wqe)
(u64 *)wqe);
} else {
pr_debug("DB wq->sq.pidx = %d\n", wq->sq.pidx);
- writel(PIDX_T5_V(inc) | QID_V(wq->sq.bar2_qid),
- wq->sq.bar2_va + SGE_UDB_KDOORBELL);
+ writel_relaxed(PIDX_T5_V(inc) | QID_V(wq->sq.bar2_qid),
+ wq->sq.bar2_va + SGE_UDB_KDOORBELL);
}

/* Flush user doorbell area writes. */
wmb();
return;
}
- writel(QID_V(wq->sq.qid) | PIDX_V(inc), wq->db);
+ writel_relaxed(QID_V(wq->sq.qid) | PIDX_V(inc), wq->db);
}

static inline void t4_ring_rq_db(struct t4_wq *wq, u16 inc,
@@ -502,15 +502,15 @@ static inline void t4_ring_rq_db(struct t4_wq *wq, u16 inc,
(void *)wqe);
} else {
pr_debug("DB wq->rq.pidx = %d\n", wq->rq.pidx);
- writel(PIDX_T5_V(inc) | QID_V(wq->rq.bar2_qid),
- wq->rq.bar2_va + SGE_UDB_KDOORBELL);
+ writel_relaxed(PIDX_T5_V(inc) | QID_V(wq->rq.bar2_qid),
+ wq->rq.bar2_va + SGE_UDB_KDOORBELL);
}

/* Flush user doorbell area writes. */
wmb();
return;
}
- writel(QID_V(wq->rq.qid) | PIDX_V(inc), wq->db);
+ writel_relaxed(QID_V(wq->rq.qid) | PIDX_V(inc), wq->db);
}


[1] https://lkml.org/lkml/2018/3/17/100
>
> Jason
>


--
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

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