Re: [PATCH] USB: serial: add nt124 usb to serial driver

From: George McCollister
Date: Mon Dec 15 2014 - 11:09:28 EST

On Mon, Dec 15, 2014 at 3:52 AM, Johan Hovold <johan@xxxxxxxxxx> wrote:
> On Sun, Dec 14, 2014 at 11:51:11AM -0600, George McCollister wrote:
>> Johan,
>> While working on the tx_empty changes you suggested it occurred to me
>> that it might not be obvious to others that the firmware doesn't send
>> a packet with the NT124_CTRL_TXEMPTY flag cleared when it begins
>> transmitting. The practical implication is that if the driver sets
>> tx_empty = true when it sees NT124_CTRL_TXEMPTY, tx_empty must be
>> reset to false somewhere when more data is transmitted. Perhaps I
>> could add prepare_write_buffer and do it in there before calling
>> usb_serial_generic_prepare_write_buffer(). Does that sound acceptable?
> Hmm. There's no way to query that flag? And the status is sent (as bulk
> in data) periodically or only when data has been received? And not when
> the actual status changes?
The bulk in packets are not sent periodically only on TXEMPTY, other
line change or received data. There's no way to query the flag, though
we're still at the stage we can make modifications to the firmware if
there's justification. One of the design goals is to minimize
unnecessary USB traffic so if there's a place to clear the flag in the
driver without querying it would be preferable.
> A potential problem with using prepare_write_buffer would be on failures
> to submit the write urb, in which case this flag might never be cleared.
Yes, this could be a problem though I wonder how many (if any)
recoverable situations this would happen in that didn't eventually
lead to a transmission. Adding a timer with a long timeout that set
tx_empty = true crossed my mind but that doesn't really seem any less
error prone than the original issue. I could of course duplicate a
bunch of code from generic.c and make a few minor changes to set
tx_empty = true but that obviously isn't desirable either.

If none of the above solutions sound acceptable, let me know I and I
guess I'll change the firmware to allow polling of TXEMPTY with a
control message and remove it from the bulk in packet.

Thanks again,

> Johan
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