+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/
