linux-kernel - Re: [PATCH v3 3/8] mailbox: Add transmit done by blocking option

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Date:   Wed, 8 Aug 2018 17:46:27 +0300
From:   Mikko Perttunen <cyndis@...si.fi>
To:     Jassi Brar <jassisinghbrar@...il.com>
Cc:     Mikko Perttunen <mperttunen@...dia.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jon Hunter <jonathanh@...dia.com>,
        Devicetree List <devicetree@...r.kernel.org>,
        linux-serial@...r.kernel.org, linux-tegra@...r.kernel.org,
        ", linux-arm-kernel"@lists.infradead.org,
        srv_heupstream <linux-mediatek@...ts.infradead.org>,
        srv_heupstream <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/8] mailbox: Add transmit done by blocking option

On 08/08/2018 05:39 PM, Jassi Brar wrote:
> On Wed, Aug 8, 2018 at 8:04 PM, Mikko Perttunen <cyndis@...si.fi> wrote:
>>
>>
>> On 08/08/2018 05:10 PM, Jassi Brar wrote:
>>>
>>> On Wed, Aug 8, 2018 at 5:08 PM, Mikko Perttunen <cyndis@...si.fi> wrote:
>>>>
>>>>
>>>>
>>>> On 04.08.2018 13:45, Mikko Perttunen wrote:
>>>>>
>>>>>
>>>>> On 08/03/2018 03:54 PM, Jassi Brar wrote:
>>>>>>
>>>>>>
>>>>>> On Mon, Jul 2, 2018 at 5:10 PM, Mikko Perttunen <mperttunen@...dia.com>
>>>>>> wrote:
>>>>>>>
>>>>>>>
>>>>>>> Add a new TXDONE option, TXDONE_BY_BLOCK. With this option, the
>>>>>>> send_data function of the mailbox driver is expected to block until
>>>>>>> the message has been sent. The new option is used with the Tegra
>>>>>>> Combined UART driver to minimize unnecessary overhead when
>>>>>>> transmitting
>>>>>>> data.
>>>>>>>
>>>>>> 1) TXDONE_BY_BLOCK flag :-
>>>>>>            Have you tried setting the flag
>>>>>> mbox_chan->mbox_client->tx_block
>>>>>> ?
>>>>>
>>>>>
>>>>>
>>>>> No - I suppose I should have done that. I'm a bit concerned about
>>>>> overhead
>>>>> as send_data may be called thousands of times per second, so I tried to
>>>>> make
>>>>> it as close as possible to the downstream driver that just pokes the
>>>>> mailbox
>>>>> register directly.
>>>>
>>>>
>>>>
>>>> I tried using polling in the mailbox framework. Some printing is done
>>>> from
>>>> atomic context so it seems tx_block cannot be used -
>>>> wait_for_completion_timeout understandably does not work in atomic
>>>> context.
>>>> I also tried without tx_block, in which case I got some horribly garbled
>>>> output, but "Try increasing MBOX_TX_QUEUE_LEN" was readable there.
>>>>
>>>> Any opinions?
>>>>
>>> The problems arise because your hardware (SM) supports TXDONE_BY_POLL,
>>> but your client drives it by TXDONE_BY_ACK because the older DB
>>> channels are so.
>>>
>>> Please populate SM channels as a separate controller than DB.
>>> The DB controller, as is, run by ACK method.
>>> The SM controller should be run by polling, i.e, set txdone_poll =
>>> true and the poll period small enough. The virtual tty client driver
>>> should be able to safely set tx_block from appropriate context.
>>>
>>
>> Sorry, I should have clarified that I already split up the controllers. The
>> SM controller has txdone_poll = true. I didn't adjust txpoll_period so I
>> guess it's zero.
>>
> Can you please share your code (controller and client) ? Maybe offline
> if you wish.
> 

I'll upload a git branch tomorrow -- I'm not at the machine with the 
code now.

Mikko