linux-kernel - RE: [PATCH v7 09/10] usb: dwc3: Check for IOC/LST bit in both event->status and TRB->ctrl fields

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Message-ID: <87efap21pw.fsf@linux.intel.com>
Date:   Mon, 10 Dec 2018 11:03:07 +0200
From:   Felipe Balbi <balbi@...nel.org>
To:     Anurag Kumar Vulisha <anuragku@...inx.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Johan Hovold <johan@...nel.org>,
        Jaejoong Kim <climbbb.kim@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Roger Quadros <rogerq@...com>,
        Manu Gautam <mgautam@...eaurora.org>,
        "martin.petersen\@oracle.com" <martin.petersen@...cle.com>,
        Bart Van Assche <bvanassche@....org>,
        Mike Christie <mchristi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Colin Ian King <colin.king@...onical.com>
Cc:     "linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "v.anuragkumar\@gmail.com" <v.anuragkumar@...il.com>,
        Thinh Nguyen <thinhn@...opsys.com>,
        Tejas Joglekar <tejas.joglekar@...opsys.com>,
        Ajay Yugalkishore Pandey <APANDEY@...inx.com>
Subject: RE: [PATCH v7 09/10] usb: dwc3: Check for IOC/LST bit in both event->status and TRB->ctrl fields


Hi,

Anurag Kumar Vulisha <anuragku@...inx.com> writes:
>>>>> Thanks for reviewing this patch. Lets consider an example where a
>>>>> request has num_sgs > 0 and each sg is mapped to a TRB and the last
>>>>> TRB has the IOC bit set. Once the controller is done with the
>>>>> transfer, it  generates XferInProgress for the last TRB (since IOC bit
>>>>> is set). As a part of trb reclaim process
>>>>> dwc3_gadget_ep_reclaim_trb_sg() calls
>>>>> dwc3_gadget_ep_reclaim_completed_trb() for req->num_sgs times. Since
>>>>> the event already has the IOC bit set, the loop is exited from the
>>>>> loop at the very first TRB and the remaining TRBs (mapped to the sglist) are left
>>>>unhandled.
>>>>> To avoid this we modified the code to exit only if both TRB & event
>>>>> has the IOC bit set.
>>>>
>>>>Seems like IOC case should just test for chain flag as well:
>>>>
>>>
>>> Okay. Along with this logic the code for updating chain bit should also be modified I
>>guess.
>>
>>not really
>>
>>> Since the IOC bit is also set when there are not enough TRBs available, the code
>>should be
>>> modified to not set DWC3_TRB_CTRL_CHN bit when the IOC bit is set. I will update
>>below
>>> changes along with your suggestions and resend the patches.
>>
>>no. Actually I don't think we're allowed to split a scatter/gather like
>>that. I did that quite a while ago, but I don't think we're allowed to
>>do so. What we should do, in that case, is not even queue that request
>>until we have enough for all members of the scatter/gather. But that's a
>>separate patch, anyway.
>>
>
> Okay. I have a doubt here, not pushing the request until all sgs are mapped to enough TRBs
> might remove the driver complexity but reduce the performance (since we are waiting
> until enough TRBs are available). Are we okay with that?  

The only other way would be to copy the buffer over to a contiguous
buffer. That will also reduce performance. I think we need to consider
how frequently this may actually happen. I dare to say we don't have any
usb function in kernel as of today that can, easily and frequently, fall
into such a situation. Besides, the performance loss can be amortized by
a deeper request queue.

IMO, this is a minor problem. But, certainly, if you have the setup,
_do_ run some benchmarking and report your findings :-)

-- 
balbi

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