[3/9] em28xx: USB bulk packet size fix

  Hauppauge em28xx bulk devices exhibit continuity errors and corrupted
packets, when run in VMWare virtual machines. Unknown if other
manufacturers bulk models exhibit the same issue. KVM/Qemu is unaffected.

According to documentation the maximum packet multiplier for em28xx in bulk
transfer mode is 256 * 188 bytes. This changes the size of bulk transfers
to maximum supported value and have a bonus beneficial alignment.

Before:
# 512 * 384 = 196608
## 196608 % 188 != 0

After:
# 512 * 47 * 2 = 48128    (188 * 128 * 2)
## 48128 % 188 = 0

This sets up USB to expect just as many bytes as the em28xx is set to emit.

Successful usage under load afterwards natively and in both VMWare
and KVM/Qemu virtual machines.

Signed-off-by: Brad Love <brad@nextdimension.cc>
---
 drivers/media/usb/em28xx/em28xx.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
  

On Thu, Jan 4, 2018 at 7:04 PM, Brad Love <brad@nextdimension.cc> wrote:
> Hauppauge em28xx bulk devices exhibit continuity errors and corrupted
> packets, when run in VMWare virtual machines. Unknown if other
> manufacturers bulk models exhibit the same issue. KVM/Qemu is unaffected.
>
> According to documentation the maximum packet multiplier for em28xx in bulk
> transfer mode is 256 * 188 bytes. This changes the size of bulk transfers
> to maximum supported value and have a bonus beneficial alignment.
>
> Before:
> # 512 * 384 = 196608
> ## 196608 % 188 != 0
>
> After:
> # 512 * 47 * 2 = 48128    (188 * 128 * 2)
> ## 48128 % 188 = 0
>
> This sets up USB to expect just as many bytes as the em28xx is set to emit.
>
> Successful usage under load afterwards natively and in both VMWare
> and KVM/Qemu virtual machines.
>
> Signed-off-by: Brad Love <brad@nextdimension.cc>

:+1

Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>

> ---
>  drivers/media/usb/em28xx/em28xx.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/media/usb/em28xx/em28xx.h b/drivers/media/usb/em28xx/em28xx.h
> index c85292c..7be8ac9 100644
> --- a/drivers/media/usb/em28xx/em28xx.h
> +++ b/drivers/media/usb/em28xx/em28xx.h
> @@ -191,7 +191,7 @@
>     USB 2.0 spec says bulk packet size is always 512 bytes
>   */
>  #define EM28XX_BULK_PACKET_MULTIPLIER 384
> -#define EM28XX_DVB_BULK_PACKET_MULTIPLIER 384
> +#define EM28XX_DVB_BULK_PACKET_MULTIPLIER 94
>
>  #define EM28XX_INTERLACED_DEFAULT 1
>
> --
> 2.7.4
>