netdev - Re: [PATCH net-next 02/14] net: cdc_ncm: use device rx

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Date:	Fri, 19 Oct 2012 01:30:01 +0200
From:	Alexey Orishko <alexey.orishko@...il.com>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	Oliver Neukum <oliver@...kum.org>, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Greg Suarez <gpsuarez2512@...il.com>,
	"Fangxiaozhi (Franko)" <fangxiaozhi@...wei.com>,
	Dan Williams <dcbw@...hat.com>,
	Aleksander Morgado <aleksander@...edo.com>
Subject: Re: [PATCH net-next 02/14] net: cdc_ncm: use device rx_max value if
 update failed

On Fri, Oct 19, 2012 at 12:09 AM, Bjørn Mork <bjorn@...k.no> wrote:
> Oliver Neukum <oliver@...kum.org> writes:
>> On Thursday 18 October 2012 22:40:55 Bjørn Mork wrote:
>>> If the device refuses our updated value, then we must be prepared
>>> to receive URBs as big as the device wants to send.  Set rx_max
>>> to the device provided value on error.
>>
>> Problematic in principle. How do you allocate a buffer of arbitrary size?
>
> You cannot of course.  You can only try and give up if it doesn't work.
> rx_submit would end up returning -ENOMEM, but we are not always checking
> that so it will most likely fail silently.
>
> But I don't think we can just continue with the smaller buffer size
> without having the device agree to that either.  That is also likely to
> fail silently.  Note that this patch was added exactly because one of
> the MBIM test devices did refuse the lower rx_max we tried to enforce.
> The device insists on using 128kB buffers.
>
> Maybe we should cap it at some arbitrary reasonable value, and just bail
> out from bind if the device insists on a larger buffer?  Would that be
> OK?  How big buffers are (semi-)reasonable?
>

I recommend to drop this.Vendor has to fix firmware.
Current version of the driver supports 16-bit NTB, which means you can address
(64K only - NTB header). So, how do you plan to use 64K-128K buffer space,
if it can't be addressed by 16 bit offset?
Another angle to big buffers, even while using 64K buffers your TCP connection
will suffer, so what's the point making huge buffers?

/alexey
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