Re: [PATCH v5 04/21] usb: dwc2: host: Set host_perio_tx_fifo_size to 304 for rk3066

From: Kever Yang
Date: Thu Jan 28 2016 - 01:41:38 EST

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.

- Kever
On 01/28/2016 11:28 AM, Doug Anderson wrote:

On Wed, Jan 27, 2016 at 7:10 PM, Kever Yang <kever.yang@xxxxxxxxxxxxxx> wrote:
Hi Doug,

We are using the minimum FIFO size mode for TX now, which only
equal to one max packet size.

The addition FIFO size may help shorten the inter-packet data
prepare latency when the bus/DRAM is busy.

For the actual usage in TX, we have very little change to use the
period TX FIFO with more than one max packet size in host mode.
So far as I know, usb audio use the isochronous tx FIFO, but this
king of device won't have much data payload and won't, I haven't
see a usb audio have more data than 1024byte/ms.

So I suggest we assign this 48 words to host_nperio_tx_fifi_size instead
if we have to do this. Because we are using device base on bulk transaction
like U-disk very frequently.
Try using a USB webcam. With that plus a USB audio device it's easy
to overwhelm the periodic TX FIFO.

If we overwhelm the periodic TX FIFO we might actually fail to
transmit ISO or INT packets at the scheduled time. That seems more
serious of a problem to try to fix than eeking out a tiny bit
performance on a USB disk. ...but of course, it all depends on what
you consider important. ;)

We could split the difference, I suppose and put half on each?