[RFC] em28xx: fix analog streaming with USB bulk transfers

  With the conversion to videobuf2, some unnecessary calls of
em28xx_set_alternate() have been removed. It is now called at analog streaming
start only.
This has unveiled a bug that causes USB bulk transfers to fail with all urbs
having status -EVOERFLOW.
The reason is, that for bulk transfers usb_set_interface() needs to be called
even if the previous alt setting was the same (side note: bulk transfers seem
to work only with alt=0).
While it seems to be NOT necessary for isoc transfers, it's reasonable to just
call usb_set_interface() unconditionally in em28xx_set_alternate().
Also add a comment that explains the issue to prevent regressions in the future.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
---
 drivers/media/usb/em28xx/em28xx-core.c |   43 ++++++++++++++++----------------
 1 Datei geändert, 22 Zeilen hinzugefügt(+), 21 Zeilen entfernt(-)
  

Am 09.01.2013 21:52, schrieb Frank Schäfer:
> With the conversion to videobuf2, some unnecessary calls of
> em28xx_set_alternate() have been removed. It is now called at analog streaming
> start only.
> This has unveiled a bug that causes USB bulk transfers to fail with all urbs
> having status -EVOERFLOW.
> The reason is, that for bulk transfers usb_set_interface() needs to be called
> even if the previous alt setting was the same (side note: bulk transfers seem
> to work only with alt=0).
> While it seems to be NOT necessary for isoc transfers, it's reasonable to just
> call usb_set_interface() unconditionally in em28xx_set_alternate().
> Also add a comment that explains the issue to prevent regressions in the future.

The problem is, that I would really like to understand why the old code
was working...
Maybe the reason is hidden somewhere in videobuf2 or it's a firmware
issue or i'm just too tired at the moment.

Regards,
Frank



>
> Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
> ---
>  drivers/media/usb/em28xx/em28xx-core.c |   43 ++++++++++++++++----------------
>  1 Datei geändert, 22 Zeilen hinzugefügt(+), 21 Zeilen entfernt(-)
>
> diff --git a/drivers/media/usb/em28xx/em28xx-core.c b/drivers/media/usb/em28xx/em28xx-core.c
> index 80f87bb..ce4f252 100644
> --- a/drivers/media/usb/em28xx/em28xx-core.c
> +++ b/drivers/media/usb/em28xx/em28xx-core.c
> @@ -811,12 +811,12 @@ int em28xx_resolution_set(struct em28xx *dev)
>  /* Set USB alternate setting for analog video */
>  int em28xx_set_alternate(struct em28xx *dev)
>  {
> -	int errCode, prev_alt = dev->alt;
> +	int errCode;
>  	int i;
>  	unsigned int min_pkt_size = dev->width * 2 + 4;
>  
>  	/* NOTE: for isoc transfers, only alt settings > 0 are allowed
> -		 for bulk transfers, use alt=0 as default value */
> +		 bulk transfers seem to work only with alt=0 ! */
>  	dev->alt = 0;
>  	if ((alt > 0) && (alt < dev->num_alt)) {
>  		em28xx_coredbg("alternate forced to %d\n", dev->alt);
> @@ -847,25 +847,26 @@ int em28xx_set_alternate(struct em28xx *dev)
>  	}
>  
>  set_alt:
> -	if (dev->alt != prev_alt) {
> -		if (dev->analog_xfer_bulk) {
> -			dev->max_pkt_size = 512; /* USB 2.0 spec */
> -			dev->packet_multiplier = EM28XX_BULK_PACKET_MULTIPLIER;
> -		} else { /* isoc */
> -			em28xx_coredbg("minimum isoc packet size: %u (alt=%d)\n",
> -				       min_pkt_size, dev->alt);
> -			dev->max_pkt_size =
> -					  dev->alt_max_pkt_size_isoc[dev->alt];
> -			dev->packet_multiplier = EM28XX_NUM_ISOC_PACKETS;
> -		}
> -		em28xx_coredbg("setting alternate %d with wMaxPacketSize=%u\n",
> -			       dev->alt, dev->max_pkt_size);
> -		errCode = usb_set_interface(dev->udev, 0, dev->alt);
> -		if (errCode < 0) {
> -			em28xx_errdev("cannot change alternate number to %d (error=%i)\n",
> -					dev->alt, errCode);
> -			return errCode;
> -		}
> +	/* NOTE: for bulk transfers, we need to call usb_set_interface()
> +	 * even if the previous settings were the same. Otherwise streaming
> +	 * fails with all urbs having status = -EOVERFLOW ! */
> +	if (dev->analog_xfer_bulk) {
> +		dev->max_pkt_size = 512; /* USB 2.0 spec */
> +		dev->packet_multiplier = EM28XX_BULK_PACKET_MULTIPLIER;
> +	} else { /* isoc */
> +		em28xx_coredbg("minimum isoc packet size: %u (alt=%d)\n",
> +			       min_pkt_size, dev->alt);
> +		dev->max_pkt_size =
> +				  dev->alt_max_pkt_size_isoc[dev->alt];
> +		dev->packet_multiplier = EM28XX_NUM_ISOC_PACKETS;
> +	}
> +	em28xx_coredbg("setting alternate %d with wMaxPacketSize=%u\n",
> +		       dev->alt, dev->max_pkt_size);
> +	errCode = usb_set_interface(dev->udev, 0, dev->alt);
> +	if (errCode < 0) {
> +		em28xx_errdev("cannot change alternate number to %d (error=%i)\n",
> +				dev->alt, errCode);
> +		return errCode;
>  	}
>  	return 0;
>  }

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