linux-kernel - Re: [PATCH v2] usb: serial: defer URB submission for ftdi

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Date:   Mon, 15 Apr 2019 10:00:58 +0200
From:   Johan Hovold <johan@...nel.org>
To:     "Ramachandran Srinivasan (BRT-SG)" <srinivasan.r@...chip.com>
Cc:     Johan Hovold <johan@...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>
Subject: Re: [PATCH v2] usb: serial: defer URB submission for ftdi_sio

On Tue, Apr 09, 2019 at 10:47:42AM +0000, Ramachandran Srinivasan (BRT-SG) wrote:
> From: Srinivasan R <srinivasan.r@...chip.com>
> 
> Deferring the URB resubmission, this can help time share the available
> DMA chanels of DWC_OTG host controller in RaspberryPi.This change can
> fix the problem, failed to enumerate the USB device when all the 8
> instance of ftdi_sio serial driver is open by application. many bugs
> had been rasied for RaspberryPi around this problem can be solved

Thanks for resending with a commit message. Your patch is still
white-space damaged (check your mail setup, and make sure you can send a
patch to yourself and apply it with git am), but with the above
description I think I can give some feedback already.

First of all, if there's a problem here, it would need to be fixed in
the host-controller driver of the rpi as we don't want to add
workarounds for host-controller issues to every USB driver.

I suggest you write up an even more detailed bug report describing the
problem you're seeing and send it to the list.

Meanwhile, you can try increasing the latency timer of the connected
ftdi devices to reduce the amount of traffic they generate when they're
otherwise idle (16 ms default, can be increased up to 255 ms I think).
That may possibly allow you to push the limits of the rpi.

Johan