On Thu, May 05, 2022 at 01:19:08PM -0400, Farhan Khan wrote:
> Hi all,
>
> Summary Question:
>
> Broadly, I am trying to understand where a interrupt callback is specified if
> not already specified by usbd_open_pipe_intr(9). Specifically, for the
> athn(4)
> driver, I am trying to understand if/how athn_usb_intr() is executed for Tx
> interrupts. This seems necessary yet I do not see a callback specified by
> usbd_setup_xfer(9) or by usbd_open_pipe_intr(9).
>
> All code is located in sys/dev/usb/if_athn_usb.c.
>
> Question Walk-through:
>
> >From reading the code, it seems that the athn_usb_intr() function is called
> whenever a Tx interrupt is triggered. The reason I think this is because
> there
> is a tsleep_nsec(9) for a Tx interrupt that awaits for a wakeup(9) that only
> happens in athn_usb_intr().
>
> The 3 relevant steps are listed below in athn_usb_htc_setup() under the
> comment "Set credits for WLAN Tx pipe":
>
> 1. athn_usb_htc_msg(), which runs usbd_setup_xfer(9) and usbd_transfer(9) for
> a Tx interrupt. The callback is set to NULL.
> 2. usc->wait_msg_id is set to AR_HTC_MSG_CONF_PIPE_RSP.
> 3. A tsleep_nsec() on &usc->wait_msg_id
>
> The only place I see a wakeup(9) on &usc->wait_msg_id is within
> athn_usb_intr(), on condition that usc->wait_msg_id is set to
> AR_HTC_MSG_CONF_PIPE_RSP. Seems like a perfect match. Additionally, I do not
> see an Rx interrupt anywhere else. But even if it does happen somewhere and I
> am just missing it, the only place AR_HTC_MSG_CONF_PIPE_RSP is used is step 2.
>
> Rx interrupt callbacks to athn_usb_intr() are specified by the
> usbd_open_pipe_intr(9) call in athn_usb_open_pipes(). That seems explicit.
> But
> for the Tx interrupt, I do not see where the mapping to athn_usb_intr() is.
>
> Please assist, thank you.
Everything related to Tx is happening in athn_usb_tx() and athn_usb_txoef().
usbd_setup_xfer() gets a function pointer to call when the USB transfer
has completed. This function pointer is athn_usb_txeof():
usbd_setup_xfer(data->xfer, usc->tx_data_pipe, data, data->buf,
xferlen, USBD_FORCE_SHORT_XFER | USBD_NO_COPY, ATHN_USB_TX_TIMEOUT,
athn_usb_txeof);
athn_usb_txeof() puts the buffer associated with the Tx attempt back onto
the list of free buffers, and schedules more Tx attempts if needed by
calling if_start().
The associated mbuf is freed quite early, before the Tx attempt is even made,
because the entire packet gets copied into the Tx command sent to the device:
/* Copy payload. */
m_copydata(m, 0, m->m_pkthdr.len, frm);
frm += m->m_pkthdr.len;
m_freem(m);
