Re: [PATCH v1 4/4] usb: serial: mxuport: add sysfs control for UART FIFO

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, May 13, 2026 at 10:20:21AM +0200, Johan Hovold wrote:
> On Tue, May 12, 2026 at 07:28:19PM +0800, Crescent Hsieh wrote:
> > On Thu, May 07, 2026 at 05:59:05PM +0200, Johan Hovold wrote:
> > > On Tue, Mar 24, 2026 at 11:50:41AM +0800, Crescent Hsieh wrote:
> > > > Add a per-port sysfs attribute, uart_fifo, to allow userspace to enable
> > > > or disable the UART FIFO at runtime.
> > > 
> > > Why would you want to do that?
> > > 
> > > We should try to avoid driver specific sysfs knobs.
> > 
> > The motivation is to allow userspace to choose between lower latency and
> > higher throughput at runtime. Some latency-sensitive request/response
> > deployments become unreliable if the UART FIFO is enabled due to the
> > added buffering delay.
> 
> Please mention this in the commit message.

Wait, no one should EVER be relying on USB for latency-sensitive
situations.  USB does not guarantee this at all, and if you put a uart
in the way, like you are doing here, that just makes it even worse and
even more non-determinisitic.

Please don't think that this is even a viable solution, it is not.  And
no papering over it in the kernel will ever give you the requirements
that you need, so please do not do this.

Just use a "real" uart on a PCI bus if you expect anything close to
determinism.

thanks,

greg k-h