Re: [PATCH v3] usb: gadget: epautoconf: claim smallest endpoints first

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Mar 12, 2023 at 06:48:36PM -0400, Ruslan Bilovol wrote:
> UDC hardware may have endpoints with different maxpacket
> size. Current endpoint matching code takes first matching
> endpoint from the list.
> 
> It's always possible that gadget allocates endpoints for
> small transfers (maxpacket size) first, then larger ones.
> That works fine if all matching UDC endpoints have same
> maxpacket size or are big enough to serve that allocation.
> 
> However, some UDCs have first endpoints in the list with
> bigger maxpacket size, whereas last endpoints are much
> smaller. In this case endpoint allocation will fail for
> the gadget (which allocates smaller endpoints first) on
> final endpoint allocations.

Note, please use all 72 columns in your changelog text if possible.

> 
> To make endpoint allocation fair, pick up smallest
> matching endpoints first, leaving bigger ones for
> heavier applications.
> 
> Keel old behavior when "wMaxPacketSize == 0" because

"Keel"?

> it's a special case. In this case a gadget driver wants
> to use a whole available MaxPacketSize of claimed
> endpoint. Since it doesn't know what MaxPacketSize
> may be in a particular UDC endpoint, it just
> relies on epautoconf core and gets what's available

I don't see the wMaxPacketSize == 0 case in the code today, so why are
you adding that?

And this really isn't "smallest endpoints", it is "find the best fit"

> 
> Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
> ---
> 
> v3: updated commit msg, rebased onto latest gregkh/usb-next
> v2: rebased onto latest balbi/next branch
> v1: https://lore.kernel.org/lkml/20200629200551.27040-1-ruslan.bilovol@gmail.com/
> 
>  drivers/usb/gadget/epautoconf.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/usb/gadget/epautoconf.c b/drivers/usb/gadget/epautoconf.c
> index ed5a92c474e5..086bb46e3f5a 100644
> --- a/drivers/usb/gadget/epautoconf.c
> +++ b/drivers/usb/gadget/epautoconf.c
> @@ -66,7 +66,7 @@ struct usb_ep *usb_ep_autoconfig_ss(
>  	struct usb_ss_ep_comp_descriptor *ep_comp
>  )
>  {
> -	struct usb_ep	*ep;
> +	struct usb_ep	*ep, *ep_min = NULL;
>  
>  	if (gadget->ops->match_ep) {
>  		ep = gadget->ops->match_ep(gadget, desc, ep_comp);
> @@ -74,14 +74,27 @@ struct usb_ep *usb_ep_autoconfig_ss(
>  			goto found_ep;
>  	}
>  
> -	/* Second, look at endpoints until an unclaimed one looks usable */
> +	/*
> +	 * Second, look at endpoints until an unclaimed one looks usable.
> +	 * Try to find one with smallest maxpacket limit, leaving larger
> +	 * endpoints for heavier applications

What do you mean by "heavier"?

This is a "find the smallest endpoint to fit the request" type of logic,
right?  If so, please say just that.


> +	 */
>  	list_for_each_entry (ep, &gadget->ep_list, ep_list) {
> -		if (usb_gadget_ep_match_desc(gadget, ep, desc, ep_comp))
> -			goto found_ep;
> +		if (usb_gadget_ep_match_desc(gadget, ep, desc, ep_comp)) {
> +			if (desc->wMaxPacketSize == 0)
> +				goto found_ep;
> +			else if (!ep_min)
> +				ep_min = ep;
> +			else if (ep->maxpacket_limit < ep_min->maxpacket_limit)
> +				ep_min = ep;
> +		}
>  	}
>  
>  	/* Fail */
> -	return NULL;
> +	if (!ep_min)
> +		return NULL;

The fail comment should be on the return NULL line, or better yet,
rewritten to say:
	/* If we found no endpoint that fits, then fail the request */


thanks,

greg k-h