Re: [PATCH 1/1] net: cdc_ncm: Allow for dwNtbOutMaxSize to be unset or zero

[Lee Jones]

On Fri, 03 Dec 2021, Bjørn Mork wrote:

> Lee Jones <lee.jones@xxxxxxxxxx> writes:
> > On Fri, 03 Dec 2021, Bjørn Mork wrote:
> 
> >> This I don't understand.  If we have for example
> >> 
> >>  new_tx = 0
> >>  max = 0
> >>  min = 1514(=datagram) + 8(=ndp) + 2(=1+1) * 4(=dpe) + 12(=nth) = 1542
> >> 
> >> then
> >> 
> >>  max = max(min, max) = 1542
> >>  val = clamp_t(u32, new_tx, min, max) = 1542
> >> 
> >> so we return 1542 and everything is fine.
> >
> > I don't believe so.
> >
> > #define clamp_t(type, val, lo, hi) \
> >               min_t(type, max_t(type, val, lo), hi)
> >
> > So:
> >               min_t(u32, max_t(u32, 0, 1542), 0)
> 
> 
> I don't think so.  If we have:
> 
>  new_tx = 0
>  max = 0
>  min = 1514(=datagram) + 8(=ndp) + 2(=1+1) * 4(=dpe) + 12(=nth) = 1542
>  max = max(min, max) = 1542
> 
> Then we have
> 
>   min_t(u32, max_t(u32, 0, 1542), 1542)
> 
> 
> If it wasn't clear - My proposal was to change this:
> 
>   - min = min(min, max);
>   + max = max(min, max);
> 
> in the original code.

Oh, I see.  Yes, I missed the reallocation of 'max'.

I thought we were using original values and just changing min() to max().

> But looking further I don't think that's a good idea either.  I searched
> through old email and found this commit:
> 
> commit a6fe67087d7cb916e41b4ad1b3a57c91150edb88
> Author: Bjørn Mork <bjorn@xxxxxxx>
> Date:   Fri Nov 1 11:17:01 2013 +0100
> 
>     net: cdc_ncm: no not set tx_max higher than the device supports
>     
>     There are MBIM devices out there reporting
>     
>       dwNtbInMaxSize=2048 dwNtbOutMaxSize=2048
>     
>     and since the spec require a datagram max size of at least
>     2048, this means that a full sized datagram will never fit.
>     
>     Still, sending larger NTBs than the device supports is not
>     going to help.  We do not have any other options than either
>      a) refusing to bindi, or
>      b) respect the insanely low value.
>     
>     Alternative b will at least make these devices work, so go
>     for it.
>     
>     Cc: Alexey Orishko <alexey.orishko@xxxxxxxxx>
>     Signed-off-by: Bjørn Mork <bjorn@xxxxxxx>
>     Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> 
> diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
> index 4531f38fc0e5..11c703337577 100644
> --- a/drivers/net/usb/cdc_ncm.c
> +++ b/drivers/net/usb/cdc_ncm.c
> @@ -159,8 +159,7 @@ static u8 cdc_ncm_setup(struct usbnet *dev)
>         }
>  
>         /* verify maximum size of transmitted NTB in bytes */
> -       if ((ctx->tx_max < (CDC_NCM_MIN_HDR_SIZE + ctx->max_datagram_size)) ||
> -           (ctx->tx_max > CDC_NCM_NTB_MAX_SIZE_TX)) {
> +       if (ctx->tx_max > CDC_NCM_NTB_MAX_SIZE_TX) {
>                 dev_dbg(&dev->intf->dev, "Using default maximum transmit length=%d\n",
>                         CDC_NCM_NTB_MAX_SIZE_TX);
>                 ctx->tx_max = CDC_NCM_NTB_MAX_SIZE_TX;
> 
> 
> 
> 
> 
> So there are real devices depending on a dwNtbOutMaxSize which is too
> low.  Our calculated minimum for MBIM will not fit.
> 
> So let's go back your original test for zero.  It's better than
> nothing.  I'll just ack that.

Sure, no problem.

Thanks for conversing with me.

> > Perhaps we should use max_t() here instead of clamp?
> 
> No.  That would allow userspace to set an unlimited buffer size.

Right, I see.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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