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

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Date:   Fri, 26 Aug 2022 19:05:15 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dmitry Bogdanov <d.bogdanov@...ro.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "target-devel@...r.kernel.org" <target-devel@...r.kernel.org>
Subject: Re: [PATCH v2 21/25] usb: gadget: f_tcm: Get stream by tag

On Fri, Aug 26, 2022, Sebastian Andrzej Siewior wrote:
> 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.
> 

Sure, I can look into it.

Thanks,
Thinh