linux-kernel - Re: [PATCH v3 1/4] Bluetooth: hci_qca: use wait_until

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Date:   Wed, 12 Dec 2018 17:42:51 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Balakrishna Godavarthi <bgodavar@...eaurora.org>
Cc:     Johan Hovold <johan@...nel.org>, marcel@...tmann.org,
        johan.hedberg@...il.com, mka@...omium.org,
        linux-kernel@...r.kernel.org, linux-bluetooth@...r.kernel.org,
        hemantg@...eaurora.org, linux-arm-msm@...r.kernel.org,
        Johan Hovold <jhovold@...il.com>
Subject: Re: [PATCH v3 1/4] Bluetooth: hci_qca: use wait_until_sent() for
 power pulses

On Thu, Dec 06, 2018 at 04:10:07PM +0530, Balakrishna Godavarthi wrote:
> Hi Johan,
> 
> On 2018-12-05 11:55, Johan Hovold wrote:
> > On Fri, Nov 30, 2018 at 08:32:44PM +0530, Balakrishna Godavarthi wrote:

> >> +	ret = serdev_device_write(hu->serdev, &cmd, sizeof(cmd), 0);
> > 
> > You're still using 0 as a timeout here which is broken, as I already
> > told you.
> 
> [Bala]: got the change now will update to timeout to non zero value.
> 
> > From 4.21 this will result in an indefinite timeout, but currently
> > implies not to wait for a full write buffer to drain at all.
> > 
> > As I also mentioned, you need to to make sure to call
> > serdev_device_write_wakeup() in the write_wakup() path if you are going
> > to use serdev_device_write() at all.
> 
> [Bala]: this where i am confused.
>          calling serdev_device_write is calling an wakeup internally. 
> below is the flow
> 
>          ttyport_write_buf:
>                * calling serdev_device_write() will call write_buf() in 
> this call we are enabling bit "TTY_DO_WRITE_WAKEUP" and calling write()
>                  i.e. uart_write() where we call in start_tx. this will 
> go to the vendor specific write where in isr we call uart_write_wakeup()
>                  
> https://elixir.bootlin.com/linux/latest/source/drivers/tty/serial/qcom_geni_serial.c#L756
> 
>          
> uart_write_wakeup()->ttyport_write_wakeup()->serdev_controller_write_wakeup()->hci_uart_write_wakeup()->hci_uart_tx_wakeup()
> 
>          the above is flow when serdev_device_write() is called, it is 
> indirectly calling serdev_write_wakeup().

No, serdev_device_write_wakeup() is currently not called in this path,
which means you cannot use serdev_device_write().

>          Why actual we need to call an serdev_write_wakeup() is this 
> wakeup related to the UART port or for the BT chip.

serdev_device_write_wakeup() is where a writer blocked on a full write
buffer in serdev_device_write() is woken up. 

Johan