Re: [patch 03/12] USB: serial: keyspan_pda: Consolidate room query

[Alan Stern]

On Wed, Oct 14, 2020 at 04:52:18PM +0200, Thomas Gleixner wrote:
> From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> 
> Having two copies of the same code doesn't make the code more readable and
> allocating a buffer of 1 byte for a synchronous operation is a pointless
> exercise.

Not so.  In fact, it is required, because a portion of a structure 
cannot be mapped for DMA unless it is aligned at a cache line boundary.

> Add a byte buffer to struct keyspan_pda_private which can be used
> instead. The buffer is only used in open() and tty->write().

This won't work.

>  Console writes
> are not calling into the query. open() obviously happens before write() and
> the writes are serialized by bit 0 of port->write_urbs_free which protects
> also the transaction itself.
> 
> Move the actual query into a helper function and cleanup the usage sites in
> keyspan_pda_write() and keyspan_pda_open().
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Johan Hovold <johan@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: linux-usb@xxxxxxxxxxxxxxx
> ---
>  drivers/usb/serial/keyspan_pda.c |  102 ++++++++++++++++-----------------------
>  1 file changed, 43 insertions(+), 59 deletions(-)
> 
> --- a/drivers/usb/serial/keyspan_pda.c
> +++ b/drivers/usb/serial/keyspan_pda.c
> @@ -47,6 +47,7 @@ struct keyspan_pda_private {
>  	struct work_struct			unthrottle_work;
>  	struct usb_serial	*serial;
>  	struct usb_serial_port	*port;
> +	u8			query_buf;
>  };
>  
>  
> @@ -436,6 +437,31 @@ static int keyspan_pda_tiocmset(struct t
>  	return rc;
>  }
>  
> +/*
> + * Using priv->query_buf is safe here because this is only called for TTY
> + * operations open() and write(). write() comes post open() obviously and
> + * write() itself is serialized via bit 0 of port->write_urbs_free. Console
> + * writes are never calling into this.
> + */
> +static int keyspan_pda_query_room(struct usb_serial *serial,
> +				  struct keyspan_pda_private *priv)
> +{
> +	int res;
> +
> +	res = usb_control_msg(serial->dev, usb_rcvctrlpipe(serial->dev, 0),
> +			      6, /* write_room */
> +			      USB_TYPE_VENDOR | USB_RECIP_INTERFACE | USB_DIR_IN,
> +			      0, /* value */
> +			      0, /* index */
> +			      &priv->query_buf,
> +			      1,
> +			      2000);

Instead, consider using the new usb_control_msg_recv() API.  But it 
might be better to allocate the buffer once and for all.

Alan Stern