Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node

From: Johan Hovold <johan@kernel.org>
To: Ikjoon Jang <ikjn@chromium.org>
Cc: linux-usb@vger.kernel.org,
	GregKroah-Hartman <gregkh@linuxfoundation.org>,
	RobHerring <robh+dt@kernel.org>,
	MarkRutland <mark.rutland@arm.com>,
	AlanStern <stern@rowland.harvard.edu>,
	SuwanKim <suwan.kim027@gmail.com>,
	"GustavoA . R . Silva" <gustavo@embeddedor.com>,
	JohanHovold <johan@kernel.org>,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	drinkcat@chromium.org
Subject: Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node
Date: Tue, 3 Dec 2019 17:53:01 +0100	[thread overview]
Message-ID: <20191203165301.GH10631@localhost> (raw)
In-Reply-To: <20191203101552.199339-1-ikjn@chromium.org>

On Tue, Dec 03, 2019 at 06:15:52PM +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.
> 
> When we know reducing autosuspend delay for built-in HIDs is better for
> power saving, we can reduce it to the optimal value. But if a parent hub
> has a long bInterval, mouse lags a lot from more frequent autosuspend.
> So this enables overriding bInterval for a hard wired hub device only
> when we know that reduces the power consumption.

I think I saw you argue about why this shouldn't simply be configured at
runtime. Please include that here too, I can't seem to remember why...

> Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
> Acked-by: Alan Stern <stern@rowland.harvard.edu>
> ---
>  drivers/usb/core/config.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/usb/core/config.c b/drivers/usb/core/config.c
> index 5f40117e68e7..95ec5af42a1c 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 +258,14 @@ 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 (usb_of_has_combined_node(to_usb_device(ddev))) {

Not only hubs have combined nodes so you probably need to check
bDeviceClass here instead.

> +		u32 interval = 0;
> +		if (!of_property_read_u32(ddev->of_node, "hub,interval",
> +				    &interval))
> +			d->bInterval = min_t(u8, interval, 255);

You want min_t(u32, ...) here to avoid surprises when someone specifies
a value > 255.

> +	}
> +
>  	/*
>  	 * Fix up bInterval values outside the legal range.
>  	 * Use 10 or 8 ms if no proper value can be guessed.

Johan