Re: [PATCH v2] usb: gadget: f_uac2: fixed EP-IN wMaxPacketSize

[Jerome Brunet <jbrunet@baylibre.com>]

On Tue 05 Oct 2021 at 10:44, Pavel Hofman <pavel.hofman@ivitera.com> wrote:

> Async feedback patches broke enumeration on Windows 10 previously fixed
> by commit 789ea77310f0 ("usb: gadget: f_uac2: always increase endpoint
> max_packet_size by one audio slot").
>
> While the existing calculation for EP OUT capture for async mode yields
> size+1 frame due to uac2_opts->fb_max > 0, playback side lost the +1
> feature.  Therefore the +1 frame addition must be re-introduced for
> playback. Win10 enumerates the device only when both EP IN and EP OUT
> max packet sizes are (at least) +1 frame.
>
> Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
> Fixes: e89bb4288378 ("usb: gadget: u_audio: add real feedback
> implementation")
> Cc: stable@vger.kernel.org
> Tested-by: Henrik Enquist <henrik.enquist@gmail.com>
> Tested-by: Jack Pham <jackp@codeaurora.org>
> ---
>  drivers/usb/gadget/function/f_uac2.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/usb/gadget/function/f_uac2.c b/drivers/usb/gadget/function/f_uac2.c
> index 17a7ab2c799c..5c7eddf511e5 100644
> --- a/drivers/usb/gadget/function/f_uac2.c
> +++ b/drivers/usb/gadget/function/f_uac2.c
> @@ -665,11 +665,17 @@ static int set_ep_max_packet_size(const struct f_uac2_opts *uac2_opts,
>  		ssize = uac2_opts->c_ssize;
>  	}
>  
> -	if (!is_playback && (uac2_opts->c_sync == USB_ENDPOINT_SYNC_ASYNC))
> +	if (!is_playback && (uac2_opts->c_sync == USB_ENDPOINT_SYNC_ASYNC)) {
> +	  // Win10 requires max packet size + 1 frame

Actually it is not really about windows 10, but more about the USB Audio
Format specification:
see https://www.usb.org/sites/default/files/USB%20Audio%20v3.0_0.zip 
section 2.3.1.2.1

Windows 10 is just picky about what the SIP size should be.

>  		srate = srate * (1000 + uac2_opts->fb_max) / 1000;
> -
> -	max_size_bw = num_channels(chmask) * ssize *
> -		DIV_ROUND_UP(srate, factor / (1 << (ep_desc->bInterval - 1)));
> +		// updated srate is always bigger, therefore DIV_ROUND_UP always yields +1
> +		max_size_bw = num_channels(chmask) * ssize *
> +			(DIV_ROUND_UP(srate, factor / (1 << (ep_desc->bInterval - 1))));

Doing DIV_ROUND_UP is actually right for 
if (!is_playback && (uac2_opts->c_sync == USB_ENDPOINT_SYNC_ADAPTIVE))

otherwise, it should be rounded down + 1.
I'll reply with my version of the patch to make things more clear

> +	} else {
> +		// adding 1 frame provision for Win10
> +		max_size_bw = num_channels(chmask) * ssize *
> +			(DIV_ROUND_UP(srate, factor / (1 << (ep_desc->bInterval - 1))) + 1);
> +	}
>  	ep_desc->wMaxPacketSize = cpu_to_le16(min_t(u16, max_size_bw,
>  						    max_size_ep));