Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
To:     Peter Chen <hzpeterchen@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>
Cc:     "WeitaoWang-oc@zhaoxin.com" <WeitaoWang-oc@zhaoxin.com>,
        mathias.nyman@intel.com, Alan Stern <stern@rowland.harvard.edu>,
        USB list <linux-usb@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>, CobeChen@zhaoxin.com,
        TimGuo@zhaoxin.com, tonywwang@zhaoxin.com, weitaowang@zhaoxin.com
References: <3c576edf-89c3-ccf3-a43f-4ce2c1ced18d@zhaoxin.com>
 <Yi7xJy70XZCA8RyN@kroah.com>
 <CAL411-o_2PSndEVXfa+ciLukSr5u5w8G9T63d2MpSm2Fpn5QTQ@mail.gmail.com>
From:   Mathias Nyman <mathias.nyman@linux.intel.com>
Subject: Re: [PATCH] USB: Fix xhci ERDP update issue
Message-ID: <d95ca5f6-221b-1a22-abbe-10621e2fb219@linux.intel.com>
Date:   Wed, 16 Mar 2022 14:45:27 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Firefox/78.0 Thunderbird/78.14.0
MIME-Version: 1.0
In-Reply-To: <CAL411-o_2PSndEVXfa+ciLukSr5u5w8G9T63d2MpSm2Fpn5QTQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 16.3.2022 13.57, Peter Chen wrote:
> On Mon, Mar 14, 2022 at 10:34 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>>
>> On Mon, Mar 14, 2022 at 03:25:23PM +0800, WeitaoWang-oc@zhaoxin.com wrote:
>>> On some situations, software handles TRB events slower than adding TRBs,
>>> xhci_irq will not exit until all events are handled. If xhci_irq just
>>> handles 256 TRBs and exit, the temp variable(event_ring_deq) driver records
>>> in xhci irq is equal to driver current dequeue pointer. It will cause driver
>>> not update ERDP and software dequeue pointer lost sync with ERDP. On the
>>> next xhci_irq, the event ring is full but driver will not update ERDP as
>>> software dequeue pointer is equal to ERDP.
> 
> At the current driver, the ERDP is updated at most 128 TRBs, how is
> the above condition
> triggered?
> 
> Peter
> 

Before, and during _one_ interrupt handling xHC hardware writes exactly 256 events
to event ring. ring buffer size is 256 so buffer position 0 and 256 point
to the same place.

Interrupt handler stores software dequeue in a local variable "event_ring_deq".
Handler start handling events, it updates software dequeue, but not local variable.
After 128 events handler updates hardware ERDP.

So at event 128 we got:
Hardware ERDP = 128
software dequeue = 128
event_ring_deq = 0

Handler continue handling events, at event 256 try to update HW ERDP again, but fail due
to this condition in update_erst_dequeue():
      if (event_ring_deq != xhci->event_ring->dequeue)

This fails because event_ring_deq is still 0, and software deq is 256,
pointing to the same place in the event ring.

So at the end of the interrupt handler we have:
HW ERDP = 128
software dequeue = 256 (same as 0)

So in this specific case we fail to update ERDP correctly

-Mathias