Re: [PATCH 3/6] USB: serial: ftdi_sio: only allow valid latency timer values

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On 02/03/17 14:22, Johan Hovold wrote:
On Tue, Feb 28, 2017 at 12:51:25PM +0000, Ian Abbott wrote:
Valid latency timer values are between 1 ms and 255 ms in 1 ms steps.
The store function for the "latency_timer" device attribute currently
allows any value, although only the lower 8-bits will be written to the
latency timer.  Return an error for out-of-range values.

And in fact, 0 is currently used (and accepted by the device) if a
negative value is provided.

Yes, although for the same reason that writing the value 'foo' to the "latency_timer" device attribute results in 0 being used!


Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
---
 drivers/usb/serial/ftdi_sio.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
index a1b90f4184a7..2da99875cecb 100644
--- a/drivers/usb/serial/ftdi_sio.c
+++ b/drivers/usb/serial/ftdi_sio.c
@@ -1716,6 +1716,9 @@ static ssize_t latency_timer_store(struct device *dev,
 	int v = simple_strtoul(valbuf, NULL, 10);
 	int rv;

+	if (v < 1 || v > 255)
+		return -EINVAL;
+

We probably still need to accept 0 here, which seems to give a timer of
1 ms, as someone may be relying on that behaviour already.

Yes, I just partially confirmed that 0 gives a short latency timeout like 1, although I never measured it accurately. (I only judged it from responsiveness of typed characters being echoed back through a serial loop-back connector.)

I'll fix that in v2 patch series.


 	priv->latency = v;
 	rv = write_latency_timer(port);
 	if (rv < 0)

Johan



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-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@xxxxxxxxx> )=-
-=(                          Web: http://www.mev.co.uk/  )=-
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