netdev - Re: [PATCH net-next 01/11] net: cdc_ncm: split out rx_max/tx

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Date:	Tue, 13 May 2014 11:25:28 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	Alexey Orishko <alexey.orishko@...il.com>,
	Enrico Mioso <mrkiko.rs@...il.com>,
	David Laight <David.Laight@...LAB.COM>
Subject: Re: [PATCH net-next 01/11] net: cdc_ncm: split out rx_max/tx_max update of setup

Oliver Neukum <oneukum@...e.de> writes:

>> The reason I put that min_t() there instead was an attempt to deal with
>> the (not unlikely) event that some buggy device set dwNtbInMaxSize lower
>> than this required minimum value.  We then have the choices:
>> 
>>  a) fail to support the buggy device
>>  b) attempt to set a larger buffer size than the device supports
>>  c) accept the lower size
>
> My preference would be b) > a) > c)
> It seems to me that would should respect the spec and if the spec sets
> a lower limit then we don't go lower.
>
>> So I chose c) in an attempt to be as gentle as possible.  But I am open
>> to go for a) instead if you think that is better. After all
>> USB_CDC_NCM_NTB_MIN_IN_SIZE is as low as 2048, so it doesn't fit much
>> more than the headers and a single full size ethernet frame.  And I see
>> now that we fail to do further sanity checking after this.  What if
>> dwNtbInMaxSize is 0? Or smaller than the necessary headers?
>
> Exactly. Some fool may simply overlook setting it at all.
>
>> Should I rewrite the above to do a) instead?  I.e.
>> 
>> 	min = USB_CDC_NCM_NTB_MIN_IN_SIZE;
>> 	max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_RX, le32_to_cpu(ctx->ncm_parm.dwNtbInMaxSize));
>>         if (min > max)
>>            fail;
>> 
>> I don't think b) is a good idea.  It might work, but it might also fail
>> in surprising ways making it hard to debug.
>
> Users may prefer working devices to clean failures, but
> I primarily care about conforming to spec. We just shouldn't
> do such violations in a general case.

Yes, I agree.  Will change this. Let's try to go for b) then. I.e.

 	min = USB_CDC_NCM_NTB_MIN_IN_SIZE;
 	max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_RX, le32_to_cpu(ctx->ncm_parm.dwNtbInMaxSize));
        if (max < min)
              max = min;


Bjørn
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