On 3/20/2018 10:00 AM, Sinan Kaya wrote:
> On 3/20/2018 9:48 AM, Jason Gunthorpe wrote:
>> On Mon, Mar 19, 2018 at 10:47:43PM -0400, Sinan Kaya wrote:
>>> Code includes wmb() followed by writel(). writel() already has a barrier on
>>> some architectures like arm64.
>>>
>>> This ends up CPU observing two barriers back to back before executing the
>>> register write.
>>>
>>> Since code already has an explicit barrier call, changing writel() to
>>> writel_relaxed().
>>>
>>> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
>>> drivers/infiniband/hw/bnxt_re/qplib_rcfw.c | 8 ++++----
>>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c
>>> index 8329ec6..4a6b981 100644
>>> +++ b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c
>>> @@ -181,10 +181,10 @@ static int __send_message(struct bnxt_qplib_rcfw *rcfw, struct cmdq_base *req,
>>>
>>> /* ring CMDQ DB */
>>> wmb();
>>> - writel(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
>>> - rcfw->cmdq_bar_reg_prod_off);
>>> - writel(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
>>> - rcfw->cmdq_bar_reg_trig_off);
>>> + writel_relaxed(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
>>> + rcfw->cmdq_bar_reg_prod_off);
>>> + writel_relaxed(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
>>> + rcfw->cmdq_bar_reg_trig_off);
>>
>> Woah, this may not be safe..
>>
>> The definition of writel_relaxed() is that it is fully unordered, so
>> the above two writes may change order now. Broadcom guys would have to
>> ack if that it is OK or not for their hardware.
>>
>> In general this is not an OK approach for a mechanical
>> conversion.. Only the first writel can be convereted.
>>
>> You need to check all your patches to make sure there are no
>> subsequent writel's in the places touched.
>
> I paid special attention to this one and went to check the barriers
> document. According to the document, writes (whether it is relaxed or not)
> are always observed by the HW inorder with respect to each other.
> It just doesn't guarantee anything above writel_relaxed() to be observed.
> Since we already have a wmb(), this is taken care of.
>
> If somebody wants things to be observed after register write, there should
> have been a wmb() or mmiowb() afterwards.
Never mind, it will break some architectures. I'll only change the first one.
(1) On some systems, I/O stores are not strongly ordered across all CPUs, and
so for _all_ general drivers locks should be used and mmiowb() must be
issued prior to unlocking the critical section.
(2) If the accessor functions are used to refer to an I/O memory window with
relaxed memory access properties, then _mandatory_ memory barriers are
required to enforce ordering.
>
>
>>
>> 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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