Re: [PATCH] usb: gadget: f_fs: Add the missing get_alt callback

[John Keeping]

On Fri, Nov 24, 2023 at 05:44:35PM +0100, Hardik Gajjar wrote:
> Some USB OTG hubs have multiple alternate configurations to offer,
> such as one for Carplay and another for Carlife.
> 
> This patch implements and sets the get_alt callback to retrieve the
> currently used alternate setting. The new function allows dynamic
> retrieval of the current alternate setting for a specific interface. The
> current alternate setting values are stored in the 'cur_alt' array
> within the 'ffs_function' structure.

Doesn't the alt setting need to be forwarded to userspace?

What happens if the available endpoints change - doesn't that mean the
available endpoint files change?

It's not sufficient to just blindly accept any alt setting and assume it
will work, that may be the case in one specific constrained scenario,
but it's not true in general.  At the very least we must not accept an
alt setting that is not defined in the descriptors.

> Signed-off-by: Hardik Gajjar <hgajjar@xxxxxxxxxxxxxx>
> ---
>  drivers/usb/gadget/function/f_fs.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index efe3e3b85769..37c47c11f57a 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -75,6 +75,7 @@ struct ffs_function {
>  	short				*interfaces_nums;
>  
>  	struct usb_function		function;
> +	int				cur_alt[MAX_CONFIG_INTERFACES];
>  };
>  
>  
> @@ -98,6 +99,7 @@ static int __must_check ffs_func_eps_enable(struct ffs_function *func);
>  static int ffs_func_bind(struct usb_configuration *,
>  			 struct usb_function *);
>  static int ffs_func_set_alt(struct usb_function *, unsigned, unsigned);
> +static int ffs_func_get_alt(struct usb_function *f, unsigned int intf);
>  static void ffs_func_disable(struct usb_function *);
>  static int ffs_func_setup(struct usb_function *,
>  			  const struct usb_ctrlrequest *);
> @@ -3232,6 +3234,15 @@ static void ffs_reset_work(struct work_struct *work)
>  	ffs_data_reset(ffs);
>  }
>  
> +static int ffs_func_get_alt(struct usb_function *f,
> +			    unsigned int interface)
> +{
> +	struct ffs_function *func = ffs_func_from_usb(f);
> +	int intf = ffs_func_revmap_intf(func, interface);
> +
> +	return (intf < 0) ? intf : func->cur_alt[interface];
> +}
> +
>  static int ffs_func_set_alt(struct usb_function *f,
>  			    unsigned interface, unsigned alt)
>  {
> @@ -3266,8 +3277,10 @@ static int ffs_func_set_alt(struct usb_function *f,
>  
>  	ffs->func = func;
>  	ret = ffs_func_eps_enable(func);
> -	if (ret >= 0)
> +	if (ret >= 0) {
>  		ffs_event_add(ffs, FUNCTIONFS_ENABLE);
> +		func->cur_alt[interface] = alt;
> +	}
>  	return ret;
>  }
>  
> @@ -3574,6 +3587,7 @@ static struct usb_function *ffs_alloc(struct usb_function_instance *fi)
>  	func->function.bind    = ffs_func_bind;
>  	func->function.unbind  = ffs_func_unbind;
>  	func->function.set_alt = ffs_func_set_alt;
> +	func->function.get_alt = ffs_func_get_alt;
>  	func->function.disable = ffs_func_disable;
>  	func->function.setup   = ffs_func_setup;
>  	func->function.req_match = ffs_func_req_match;
> -- 
> 2.17.1
>