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

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Date:   Wed, 8 Aug 2018 17:34:12 +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,
        ", linux-arm-kernel"@lists.infradead.org,
        linux-mediatek@...ts.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 05:10 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.

Mikko