linux-kernel - Re: [PATCH v1 14/15] scsi: ufs: commit descriptors before setting the doorbell

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Date:	Sun, 30 Aug 2015 09:50:55 -0000
From:	ygardi@...eaurora.org
To:	"Rob Herring" <robherring2@...il.com>
Cc:	"Yaniv Gardi" <ygardi@...eaurora.org>,
	"linux-arm-msm" <linux-arm-msm@...r.kernel.org>,
	linux-scsi@...r.kernel.org, "Dolev Raviv" <draviv@...eaurora.org>,
	"Gilad Broner" <gbroner@...eaurora.org>,
	linux-scsi-owner@...r.kernel.org,
	"Jej B" <james.bottomley@...senpartnership.com>,
	"Santosh Y" <santoshsy@...il.com>,
	"Subhash Jadavani" <subhashj@...eaurora.org>, hch@...radead.org,
	"Paul Bolle" <pebolle@...cali.nl>,
	"James E.J. Bottomley" <jbottomley@...n.com>,
	"Vinayak Holikatti" <vinholikatti@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 14/15] scsi: ufs: commit descriptors before setting
 the doorbell

> On Thu, Aug 27, 2015 at 7:11 AM,  <ygardi@...eaurora.org> wrote:
>>> On Tue, Aug 25, 2015 at 7:36 AM,  <ygardi@...eaurora.org> wrote:
>>>>> On Aug 21, 2015 3:10 PM, "Yaniv Gardi" <ygardi@...eaurora.org> wrote:
>>>>>>
>>>>>> Add a write memory barrier to make sure descriptors prepared are
>>>>>> actually
>>>>>> written to memory before ringing the doorbell. We have also added
>>>>>> the
>>>>>> write memory barrier after ringing the doorbell register so that
>>>>>> controller sees the new request immediately.
>>>>>>
>>>>>> Signed-off-by: Yaniv Gardi <ygardi@...eaurora.org>
>>>>>>
>>>>>> ---
>>>>>>  drivers/scsi/ufs/ufshcd.c | 6 ++++++
>>>>>>  1 file changed, 6 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
>>>>>> index fef0660..876148b 100644
>>>>>> --- a/drivers/scsi/ufs/ufshcd.c
>>>>>> +++ b/drivers/scsi/ufs/ufshcd.c
>>>>>> @@ -833,6 +833,8 @@ void ufshcd_send_command(struct ufs_hba *hba,
>>>>>> unsigned int task_tag)
>>>>>>         ufshcd_clk_scaling_start_busy(hba);
>>>>>>         __set_bit(task_tag, &hba->outstanding_reqs);
>>>>>>         ufshcd_writel(hba, 1 << task_tag,
>>>>>> REG_UTP_TRANSFER_REQ_DOOR_BELL);
>>>>>> +       /* Make sure that doorbell is committed immediately */
>>>>>> +       wmb();
>>>>>
>>>>> Is this really necessary? Is there a measurable difference?
>>>>
>>>> I'm not sure if there is a measurable difference, but as the Door-Bell
>>>> register is the one that actually responsible for the HW execution of
>>>> the
>>>> requests, anyhow, it's recommended to its value will be written
>>>> instantly to the memory.
>>>
>>> A barrier doesn't guarantee speed, only ordering. Unless you can
>>> measure the difference, you should not have it.
>>
>> Rob,
>> let me have an example:
>> context#1 updates outstanding_reqs variable and write(DOOR_BELL)
>> context#2 upon interrupt of a request completion the following happens:
>>   report completion on each one of the bits in:
>>   outstanding_reqs ^ read(DOOR_BELL);
>>
>> 0. let's assume the DOOR_BELL = 0x1 (which means 1 active request in
>> slot 0)
>> 1. context#1: update the DOOR_BELL to be 0x3; (2 active requests: in
>> slot
>> 0 and 1)
>> 2. the new value 0x3 is still not written to the DR so DORR_BELL is
>> still
>> 0x1, but outstanding_reqs is already updated = 0x3
>> 3. the request in slot 0 just completed, and interrupt happens, so
>> DORR_BELL is now 0 (request in slot 0 completed)
>> 4. context#2: outstanding_reqs ^ read(DOOR_BELL) = 0x3 ^ 0x0 = 0x3 =>
>> wrong conclusion since the request in slot 1 never completed, and
>> actually
>> never started.
>
> Barriers alone will never solve this problem. They may narrow the
> window possibly, but the problem is still there. What you have to have
> is a spinlock around all accesses to both outstanding_reqs and
> doorbell register. And guess what, spinlocks have appropriate barriers
> to ensure visibility of what they protect. Or perhaps the h/w provides
> another way to signal what slots have completed. Using the same
> register for doorbell and completion status is not ideal.
>

can i assume spin_lock_irqsave() and spin_unlock_irqrestore()
both provide barriers ? i couldn't find the barrier instruction
when following the call chain...


> Rob
> --
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