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

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Date:   Fri, 3 Dec 2021 16:57:47 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Bjørn Mork <bjorn@...k.no>,
        Lee Jones <lee.jones@...aro.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Oliver Neukum <oliver@...kum.org>,
        "David S. Miller" <davem@...emloft.net>, linux-usb@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH 1/1] net: cdc_ncm: Allow for dwNtbOutMaxSize to be unset
 or zero

On Fri, 03 Dec 2021 15:52:48 +0100 Bjørn Mork wrote:
> Lee Jones <lee.jones@...aro.org> writes:
> 
> > diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
> > index 24753a4da7e60..e303b522efb50 100644
> > --- a/drivers/net/usb/cdc_ncm.c
> > +++ b/drivers/net/usb/cdc_ncm.c
> > @@ -181,6 +181,8 @@ static u32 cdc_ncm_check_tx_max(struct usbnet *dev, u32 new_tx)
> >  		min = ctx->max_datagram_size + ctx->max_ndp_size + sizeof(struct usb_cdc_ncm_nth32);
> >  
> >  	max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_TX, le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize));
> > +	if (max == 0)
> > +		max = CDC_NCM_NTB_MAX_SIZE_TX; /* dwNtbOutMaxSize not set */
> >  
> >  	/* some devices set dwNtbOutMaxSize too low for the above default */
> >  	min = min(min, max);  
> 
> I believe this is the best possible fix, considering the regressions
> anything stricter might cause.
> 
> We know of at least one MBIM device where dwNtbOutMaxSize is as low as
> 2048.
> 
> According to the MBIM spec, the minimum and default value for
> wMaxSegmentSize is also 2048.  This implies that the calculated "min"
> value is at least 2076, which is why we need that odd looking
> 
>   min = min(min, max);
> 
> So let's just fix this specific zero case without breaking the
> non-conforming devices.
> 
> 
> Reviewed-by: Bjørn Mork <bjorn@...k.no>

Applied to net, thanks!