RE: [PATCH 1/6] USB: serial: ftdi_sio: add configurable inter-batch defer for read URBs

[Chinna Mopurigari Naveen Kumar Reddy (FTDI-SG)]

Hi Greg,

On Mon, Jun 22, 2026 at 05:28:00PM +0000, Greg Kroah-Hartman wrote:
> As this is an out-of-tree issue, there's nothing we can do in our
> in-tree drivers for that.

Understood, thanks Greg. I will drop the urb_defer_timer_ns patch and
the low_latency patches that depended on it, and take the host
controller starvation issue to the Raspberry Pi / BCM2835 USB folks
separately.

I will shortly post a v2 with just the two standalone ftdi_sio fixes
(transient control-transfer retry, and making an explicit latency_timer
sysfs write authoritative), which are independent of the host
controller issue.

Thanks,
Naveen

-----Original Message-----
From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 
Sent: Monday, 22 June 2026 5:28 pm
To: Chinna Mopurigari Naveen Kumar Reddy (FTDI-SG) <naveen.reddy@xxxxxxxxxxxx>
Cc: Johan Hovold <johan@xxxxxxxxxx>; linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Arun Pappan (FTDI-SG) <arun.pappan@xxxxxxxxxxxx>
Subject: Re: [PATCH 1/6] USB: serial: ftdi_sio: add configurable inter-batch defer for read URBs

On Mon, Jun 22, 2026 at 09:05:53AM +0000, Chinna Mopurigari Naveen Kumar Reddy (FTDI-SG) wrote:
> Hi Greg,
> 
> On Mon, Jun 22, 2026 at 04:52:00PM +0000, Greg Kroah-Hartman wrote:
> > This is not the 1990's, module parameters should not be used anymore...
> > Why not just fix this at the root of the problem?
> 
> Thank you for the review, Greg.
> 
> You are right on both points.
> 
> On the module parameter: I will switch to sysfs if the per-driver 
> approach is accepted, or drop it entirely.
> 
> On the location: your core point is well-taken. The DMA channel 
> starvation is a BCM2835 DWC_OTG host controller deficiency, and the 
> correct fix belongs in the host controller driver, not in each USB device driver.
> I will investigate fixing it there instead. If that is not feasible 
> (the
> BCM2835 DWC_OTG is a vendor tree that Broadcom/RPi maintain outside 
> mainline), I will report back before proposing any per-driver workaround.

As this is an out-of-tree issue, there's nothing we can do in our in-tree drivers for that.  Please work with that team to either get their code merged properly upstream with us, where we can all work on the issue together, or just work with them out-of-tree.

thanks,

greg k-h