linux-kernel - Re: [PATCH v5 04/21] usb: dwc2: host: Set host_perio_tx_fifo

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Date:	Thu, 28 Jan 2016 15:25:35 -0800
From:	Doug Anderson <dianders@...omium.org>
To:	Kever Yang <kever.yang@...k-chips.com>
Cc:	John Youn <John.Youn@...opsys.com>, Felipe Balbi <balbi@...com>,
	Tao Huang <huangtao@...k-chips.com>,
	"Herrero, Gregory" <gregory.herrero@...el.com>,
	Heiko Stübner <heiko@...ech.de>,
	John Youn <johnyoun@...opsys.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ming Lei <ming.lei@...onical.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"Kaukab, Yousaf" <yousaf.kaukab@...el.com>,
	Alan Stern <stern@...land.harvard.edu>,
	吴良峰 <william.wu@...k-chips.com>,
	Julius Werner <jwerner@...omium.org>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>
Subject: Re: [PATCH v5 04/21] usb: dwc2: host: Set host_perio_tx_fifo_size to
 304 for rk3066

Hi,

On Thu, Jan 28, 2016 at 10:16 AM, Doug Anderson <dianders@...omium.org> wrote:
> Kever,
>
>
> On Wed, Jan 27, 2016 at 10:41 PM, Kever Yang <kever.yang@...k-chips.com> wrote:
>> Hi Doug,
>>
>> We are in HOST mode, we only need to receive data from USB camera
>> with RX FIFO, and no need to use TX FIFO for USB webcam, right? :)
>>
>> Any way, I think we need to NAK this patch after look into the design
>> of dwc2 controller. Because all the dwc2 controller inside the Rockchip
>> chips don't support the thresholding FIFO mode, in this case, there is
>> no more transaction before a whole packet is send out and the dwc2 only
>> care if the available FIFO is enough for next packet or not.
>>
>> So, the addition 48 words won't help to shorten the latency for data prepare
>> in this case.
>
> Ah ha!  I see where I messed up.  It wasn't lack of FIFO space that I
> was running into, it was lack of queue space.  :-P  I had conflated
> the two of them in my mind.  We still use the TX queue to transmit
> small packets so that we can receive our data...
>
> OK, so it's pretty sane to assume that exhausting the periodic TX FIFO
> isn't terribly common, I think.  Audio won't do it by itself and you
> probably won't have more than one audio device.  I guess if you had a
> video _output_ device hooked over USB then it could possible exhaust
> things.  ...but I think dwc2 still has a ways to do before I'd suggest
> anyone hooking that up.  :-P  At some point in time you could imagine
> the driver being more dynamic.
>
> OK, so the non periodic FIFO it is, then.

Actually, in my simple tests adding it to the non periodic FIFO didn't
help at all.  I'll just drop this patch.

-Doug