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From:   Jassi Brar <jassisinghbrar@gmail.com>
Date:   Wed, 8 Aug 2018 19:40:11 +0530
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Subject: Re: [PATCH v3 3/8] mailbox: Add transmit done by blocking option
To:     Mikko Perttunen <cyndis@kapsi.fi>
Cc:     Mikko Perttunen <mperttunen@nvidia.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Thierry Reding <thierry.reding@gmail.com>,
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On Wed, Aug 8, 2018 at 5:08 PM, Mikko Perttunen <cyndis@kapsi.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@nvidia.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.