Re: [PATCH v2 1/1] usb: gadget: f_uac2: finalize wMaxPacketSize according to bandwidth

[Peter Chen <peter.chen@freescale.com>]

On Wed, Jul 22, 2015 at 10:04:30AM +0200, Daniel Mack wrote:
> On 07/22/2015 08:45 AM, Peter Chen wrote:
> > According to USB Audio Device 2.0 Spec, Ch4.10.1.1:
> > wMaxPacketSize is defined as follows:
> > Maximum packet size this endpoint is capable of sending or receiving
> > when this configuration is selected.
> > This is determined by the audio bandwidth constraints of the endpoint.
> > 
> > In current code, the wMaxPacketSize is defined as the maximum packet size
> > for ISO endpoint, and it will let the host reserve much more space than
> > it really needs, so that we can't let more endpoints work together at
> > one frame.
> > 
> > We find this issue when we try to let 4 f_uac2 gadgets work together [1]
> > at FS connection.
> > 
> > [1]http://www.spinics.net/lists/linux-usb/msg123478.html
> > 
> > Cc: andrzej.p@samsung.com
> > Cc: zonque@gmail.com
> > Cc: tiwai@suse.de
> > Cc: <stable@vger.kernel.org> #v3.18+
> > Cc: Alan Stern <stern@rowland.harvard.edu>
> > Signed-off-by: Peter Chen <peter.chen@freescale.com>
> > ---
> > 
> > Changes for v2:
> > - Using DIV_ROUND_UP to calculate max packet size
> > 
> >  drivers/usb/gadget/function/f_uac2.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/drivers/usb/gadget/function/f_uac2.c b/drivers/usb/gadget/function/f_uac2.c
> > index 6d3eb8b..6eaa4c4 100644
> > --- a/drivers/usb/gadget/function/f_uac2.c
> > +++ b/drivers/usb/gadget/function/f_uac2.c
> > @@ -987,6 +987,7 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
> >  	struct f_uac2_opts *uac2_opts;
> >  	struct usb_string *us;
> >  	int ret;
> > +	u16 c_max_packet_size, p_max_packet_size;
> >  
> >  	uac2_opts = container_of(fn->fi, struct f_uac2_opts, func_inst);
> >  
> > @@ -1070,6 +1071,19 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
> >  	uac2->p_prm.uac2 = uac2;
> >  	uac2->c_prm.uac2 = uac2;
> >  
> > +	/* Calculate wMaxPacketSize according to audio bandwidth */
> > +	c_max_packet_size = uac2_opts->c_chmask * uac2_opts->c_ssize
> > +		* DIV_ROUND_UP(uac2_opts->c_srate, 1000);
> > +	p_max_packet_size = uac2_opts->p_chmask * uac2_opts->p_ssize
> > +		* DIV_ROUND_UP(uac2_opts->p_srate, 1000);
> > +	if ((c_max_packet_size > fs_epout_desc.wMaxPacketSize) ||
> > +		(p_max_packet_size > fs_epin_desc.wMaxPacketSize)) {
> > +		dev_err(dev, "parameters are incorrect\n");
> > +		goto err;
> > +	}
> > +	fs_epin_desc.wMaxPacketSize = cpu_to_le16(c_max_packet_size);
> > +	fs_epout_desc.wMaxPacketSize = cpu_to_le16(p_max_packet_size);
> > +
> >  	hs_epout_desc.bEndpointAddress = fs_epout_desc.bEndpointAddress;
> >  	hs_epout_desc.wMaxPacketSize = fs_epout_desc.wMaxPacketSize;
> 
> Your calculation still doesn't take into account the endpoint's
> 'bInterval', and for HS, the value is still wrong.
> 

I still not understand why I need to consider 'bInterval' for packet
size, per my understanding, 'bInterval' is the interval time for sending
each packet. At current code, it defines wMaxPacketSize as max value
(1023/1024) for one packet, it may cause problem for audio driver,
so you have the patch (9bb87f168931 usb: gadget: f_uac2: send reasonably
sized packets) for reducing packet size according to its 'bInterval', but
with my change, the wMaxPacketSize will be smaller than its max value,
do we still need to reduce packet size for each transfer?

-- 

Best Regards,
Peter Chen