linux-kernel - Re: [PATCH v2 21/25] usb: gadget: f

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Date:   Fri, 26 Aug 2022 11:06:32 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dmitry Bogdanov <d.bogdanov@...ro.com>,
        linux-scsi@...r.kernel.org, target-devel@...r.kernel.org
Subject: Re: [PATCH v2 21/25] usb: gadget: f_tcm: Get stream by tag

On 2022-07-18 18:28:16 [-0700], Thinh Nguyen wrote:
> diff --git a/drivers/usb/gadget/function/f_tcm.c b/drivers/usb/gadget/function/f_tcm.c
> index 084143213176..a10e74290664 100644
> --- a/drivers/usb/gadget/function/f_tcm.c
> +++ b/drivers/usb/gadget/function/f_tcm.c
> @@ -506,6 +506,22 @@ static void uasp_cleanup_old_alt(struct f_uas *fu)
>  	uasp_free_cmdreq(fu);
>  }
>  
> +static struct uas_stream *uasp_get_stream_by_tag(struct f_uas *fu, u16 tag)
> +{
> +	/*
> +	 * For simplicity, we use mod operation to quickly find an in-progress
> +	 * matching command tag to check for overlapped command. The assumption
> +	 * is that the UASP class driver will limit to using tag id from 1 to
> +	 * USBG_NUM_CMDS. This is based on observation from the Windows and
> +	 * Linux UASP storage class driver behavior. If an unusual UASP class
> +	 * driver uses a tag greater than USBG_NUM_CMDS, then this method may no
> +	 * longer work due to possible stream id collision. In that case, we
> +	 * need to use a proper algorithm to fetch the stream (or simply walk
> +	 * through all active streams to check for overlap).
> +	 */
> +	return &fu->stream[tag % USBG_NUM_CMDS];

Could you please avoid the assumption what tag actually is?
Please take a look at hashtable.h, hash_add(), hash_del(),
hash_for_each_possible_safe() is probably all you need.
That % looks efficient but gcc will try and remove the div operation
which is something the hash implementation (as of hash_min()) avoids. So
the only additional costs here is the additional hashtable which worth
the price given that you don't assume what tag can be.

Sebastian