linux-kernel - Re: [PATCH 2/2] usb: overridable hub bInterval by device node

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Date:   Sun, 17 Nov 2019 08:13:45 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Ikjoon Jang <ikjn@...omium.org>
Cc:     linux-usb@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Suwan Kim <suwan.kim027@...il.com>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Johan Hovold <johan@...nel.org>,
        Nicolas Boitchat <drinkcat@...omium.org>
Subject: Re: [PATCH 2/2] usb: overridable hub bInterval by device node

On Sun, Nov 17, 2019 at 11:33:05AM +0800, Ikjoon Jang wrote:
> This patch enables hub device to override its own endpoint descriptor's
> bInterval when the hub has a device node with "hub,interval" property,
> 
> Some existing hub devices have adjustable interval so the device is
> allowed to use different bInterval. This is useful when the hub's default
> bInterval is too big, so child device's waking up from autosuspend
> takes much time.
> 
> Signed-off-by: Ikjoon Jang <ikjn@...omium.org>
> ---
>  drivers/usb/core/config.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/core/config.c b/drivers/usb/core/config.c
> index 5f40117e68e7..234ca6124c98 100644
> --- a/drivers/usb/core/config.c
> +++ b/drivers/usb/core/config.c
> @@ -6,6 +6,7 @@
>  #include <linux/usb.h>
>  #include <linux/usb/ch9.h>
>  #include <linux/usb/hcd.h>
> +#include <linux/usb/of.h>
>  #include <linux/usb/quirks.h>
>  #include <linux/module.h>
>  #include <linux/slab.h>
> @@ -257,6 +258,11 @@ static int usb_parse_endpoint(struct device *ddev, int cfgno, int inum,
>  	memcpy(&endpoint->desc, d, n);
>  	INIT_LIST_HEAD(&endpoint->urb_list);
>  
> +	/* device node property overrides bInterval */
> +	if (unlikely(usb_of_has_combined_node(to_usb_device(ddev))))

Only ever use likely/unlikely if you can measure the difference with a
benchmark.  If not, then never use it as the compiler and CPU will
almost always get it right and more correct than you can.

And for code that has no performance issues/impact like this one, never
do it.

thanks,

greg k-h