linux-kernel - Re: [PATCH 2/2] usb: xhci: ext-caps: Use cpu

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Message-ID: <cd0cc5d1-0776-4d5f-9f3f-8261deb0e3bb@linux.intel.com>
Date: Wed, 21 Aug 2024 12:40:15 +0300
From: Mathias Nyman <mathias.nyman@...ux.intel.com>
To: Zhongqiu Han <quic_zhonhan@...cinc.com>, Thinh.Nguyen@...opsys.com,
 gregkh@...uxfoundation.org, mathias.nyman@...el.com
Cc: linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] usb: xhci: ext-caps: Use cpu_relax() when polling
 registers

On 20.8.2024 15.15, Zhongqiu Han wrote:
> It is considered good practice to call cpu_relax() in busy loops, see
> Documentation/process/volatile-considered-harmful.rst. This can lower
> CPU power consumption or yield to a hyperthreaded twin processor and
> also serve as a compiler barrier. In addition, if something goes wrong
> in the busy loop at least it can prevent things from getting worse.
> 
> Signed-off-by: Zhongqiu Han <quic_zhonhan@...cinc.com>
> ---
>   drivers/usb/host/xhci-ext-caps.h | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/usb/host/xhci-ext-caps.h b/drivers/usb/host/xhci-ext-caps.h
> index 96eb36a58738..25d148d60ab0 100644
> --- a/drivers/usb/host/xhci-ext-caps.h
> +++ b/drivers/usb/host/xhci-ext-caps.h
> @@ -144,6 +144,8 @@ static inline int xhci_find_next_ext_cap(void __iomem *base, u32 start, int id)
>   		if (offset != start && (id == 0 || XHCI_EXT_CAPS_ID(val) == id))
>   			return offset;
>   
> +		cpu_relax();
> +
>   		next = XHCI_EXT_CAPS_NEXT(val);
>   		offset += next << 2;
>   	} while (next);

Similar case as with PATCH 1/2

This isn't a busy loop polling for some value.
We traverse xhci extended capabilities until the one we are looking for is found.

Thanks
Mathias