linux-kernel - Re: [PATCH 1/2] mailbox: switch to hrtimer for tx

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Message-ID: <CABb+yY3=4z8+u+e+EgzRh3YH0RjWs1-nE_KjJ7f6AWmONuUV-g@mail.gmail.com>
Date:	Thu, 30 Jul 2015 23:41:45 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Sudeep Holla <sudeep.holla@....com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Juri Lelli <Juri.Lelli@....com>
Subject: Re: [PATCH 1/2] mailbox: switch to hrtimer for tx_complete polling

On Wed, Jul 29, 2015 at 2:18 PM, Sudeep Holla <sudeep.holla@....com> wrote:
>
>
> On 29/07/15 09:33, Jassi Brar wrote:
>
>> On Mon, Jul 27, 2015 at 3:18 PM, Sudeep Holla <sudeep.holla@....com>
>> wrote:
>>>
>>> On 27/07/15 04:26, Jassi Brar wrote:
>>>>
>>>>
>>>>>>
>>>>>>> we might end-up waiting
>>>>>>> for atleast a jiffy even though the response for that message from
>>>>>>> the
>>>>>>> remote is received via interrupt and processed in relatively smaller
>>>>>>> time granularity.
>>>>>>>
>>>>>> That is wrong.
>>>>>
>>>>>
>>>>>
>>>>> No see below.
>>>>>
>>>>>>     If the controller supports TX interrupt it should set txdone_irq,
>>>>>> which prevents polling i.e, controller driver calls mbox_chan_txdone.
>>>>>>
>>>>>>     If the controller doesn't support TX interrupt but the client
>>>>>> receives some ack packet, then the client should set knows_txdone and
>>>>>> call mbox_client_txdone. Again you don't have to wait on polling.
>>>>>>
>>>>>
>>>>> Sorry if I was not clear in the commit message, but I thought I did
>>>>> mention TXDONE_BY_POLL. The case I am referring is definitely not
>>>>> TXDONE_BY_IRQ or TXDONE_BY_ACK.
>>>>>
>>>> That statement is still wrong. The TXDONE_BY_POLL modifier does't make
>>>> it
>>>> right.
>>>>
>>>
>>> I am fine to modify/clarify that statement.
>>>
>>>> Anyways, I see you meant the 3rd case of neither IRQ nor ACK.
>>>>
>>>
>>> Yes the remote indicates by setting a flag in status register.
>>>
>> However, looking at the arm_scpi.c the protocol does support
>> TXDONE_BY_ACK that is, every command has a reply packet telling if the
>> command was successful or failure. When you receive a reply, obviously
>> the command has already been received by the remote. Which is
>> mbox_client.knows_txdone or TXDONE_BY_ACK.
>>
>
> I do understand TXDONE_BY_ACK, but SCPI protocol doesn't support that.
> You can verify the SCPI specification document.
>
>>>> It seems your remote doesn't send some protocol level 'ack' packet
>>>> replying if the command was successfully executed or not. That means
>>>> Linux can't differentiate successful execution of the command from a
>>>> silent failure (remote still has to set the TX_done flag to make way
>>>> for next messages).
>>>
>>>
>>> Agreed and again I confirm the remote processor in question just sets
>>> the flag always and correctly and doesn't use a protocol ACK.
>>>
>> As I note above, the arm_scpi.c tells a different story.
>>
>
> You are just concluding this from my stupid comment.
>
> [..]
>
>>>>> Hope this clarifies the reasons for switching to hrtimer.
>>>>>
>>>> I am not against using hrtimer, just need to make sure we don't simply
>>>> suppress the symptoms of wrong implementation.
>>>
>>>
>>> Agreed, and that's a valid concern. So far based on the testing and
>>> benchmarking done so far, we don't think this patch is suppressing
>>> anything incorrectly.
>>>
>>> If you still have concerns with this solution, please explain them here
>>> so that we can discuss and come to conclusion and the issue is fixed.
>>>
>> I just replied on the patch where you set
>>       cl->knows_txdone = false;
>> and which is not the case.
>>
>> We may use hrtimer for polling, but your platform doesn't have to rely on
>> that.
>>
>
> Again, sorry for misleading comment, we do need hrtimer as replied on
> scpi thread. Any other concern with this patch ?
>
Polling by hrtimers is OK. Not to mean this is the best solution for
your platform. Please revise the changelog completely.

Thanks.
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