Date:   Sun, 26 Dec 2021 23:17:21 +0100
From:   Matthias-Christian Ott <ott@...ix.org>
To:     petkan@...leusys.com
Cc:     linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] net: usb: pegasus: Request Ethernet FCS from hardware

On 26/12/2021 23:12, Petko Manolov wrote:
> On 21-12-26 17:12:24, Matthias-Christian Ott wrote:
>> On 26/12/2021 17:02, Petko Manolov wrote:
>>> On 21-12-26 14:25:02, Matthias-Christian Ott wrote:
>>>> Commit 1a8deec09d12 ("pegasus: fixes reported packet length") tried to
>>>> configure the hardware to not include the FCS/CRC of Ethernet frames.
>>>> Unfortunately, this does not work with the D-Link DSB-650TX (USB IDs
>>>> 2001:4002 and 2001:400b): the transferred "packets" (in the terminology
>>>> of the hardware) still contain 4 additional octets. For IP packets in
>>>> Ethernet this is not a problem as IP packets contain their own lengths
>>>> fields but other protocols also see Ethernet frames that include the FCS
>>>> in the payload which might be a problem for some protocols.
>>>>
>>>> I was not able to open the D-Link DSB-650TX as the case is a very tight
>>>> press fit and opening it would likely destroy it. However, according to
>>>> the source code the earlier revision of the D-Link DSB-650TX (USB ID
>>>> 2001:4002) is a Pegasus (possibly AN986) and not Pegasus II (AN8511)
>>>> device. I also tried it with the later revision of the D-Link DSB-650TX
>>>> (USB ID 2001:400b) which is a Pegasus II device according to the source
>>>> code and had the same results. Therefore, I'm not sure whether the RXCS
>>>> (rx_crc_sent) field of the EC0 (Ethernet control_0) register has any
>>>> effect or in which revision of the hardware it is usable and has an
>>>> effect. As a result, it seems best to me to revert commit
>>>> 1a8deec09d12 ("pegasus: fixes reported packet length") and to set the
>>>> RXCS (rx_crc_sent) field of the EC0 (Ethernet control_0) register so
>>>> that the FCS/CRC is always included.
>>>>
>>>> Fixes: 1a8deec09d12 ("pegasus: fixes reported packet length")
>>>> Signed-off-by: Matthias-Christian Ott <ott@...ix.org>
>>>> ---
>>>>  drivers/net/usb/pegasus.c | 15 ++++++++++++++-
>>>>  1 file changed, 14 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/net/usb/pegasus.c b/drivers/net/usb/pegasus.c
>>>> index c4cd40b090fd..140d11ae6688 100644
>>>> --- a/drivers/net/usb/pegasus.c
>>>> +++ b/drivers/net/usb/pegasus.c
>>>> @@ -422,7 +422,13 @@ static int enable_net_traffic(struct net_device *dev, struct usb_device *usb)
>>>>  	ret = read_mii_word(pegasus, pegasus->phy, MII_LPA, &linkpart);
>>>>  	if (ret < 0)
>>>>  		goto fail;
>>>> -	data[0] = 0xc8; /* TX & RX enable, append status, no CRC */
>>>> +	/* At least two hardware revisions of the D-Link DSB-650TX (USB IDs
>>>> +	 * 2001:4002 and 2001:400b) include the Ethernet FCS in the packets,
>>>> +	 * even if RXCS is set to 0 in the EC0 register and the hardware is
>>>> +	 * instructed to not include the Ethernet FCS in the packet.Therefore,
>>>> +	 * it seems best to set RXCS to 1 and later ignore the Ethernet FCS.
>>>> +	 */
>>>> +	data[0] = 0xc9; /* TX & RX enable, append status, CRC */
>>>>  	data[1] = 0;
>>>>  	if (linkpart & (ADVERTISE_100FULL | ADVERTISE_10FULL))
>>>>  		data[1] |= 0x20;	/* set full duplex */
>>>> @@ -513,6 +519,13 @@ static void read_bulk_callback(struct urb *urb)
>>>>  		pkt_len = buf[count - 3] << 8;
>>>>  		pkt_len += buf[count - 4];
>>>>  		pkt_len &= 0xfff;
>>>> +		/* The FCS at the end of the packet is ignored. So subtract
>>>> +		 * its length to ignore it.
>>>> +		 */
>>>> +		pkt_len -= ETH_FCS_LEN;
>>>> +		/* Subtract the length of the received status at the end of the
>>>> +		 * packet as it is not part of the Ethernet frame.
>>>> +		 */
>>>>  		pkt_len -= 4;
>>>>  	}
>>>
>>> Nice catch.  However, changing these constants for all devices isn't such a
>>> good idea.  I'd rather use vendor and device IDs to distinguish these two
>>> cases in the above code.
>>
>> I don't think that it would hurt to include the FCS for all devices. I only
>> have the datasheets for the ADM8511/X and the ADM8513 but it seems that all
>> devices that are supported by the driver also include the RXCS field in EC0.
>> This was also the previous behaviour before commit 1a8deec09d12 and seemed to
>> have worked. It also only adds four octet that have to be transferred and it
>> seems to avoid exceptions for different devices which seems to be a good idea,
>> in particular, because it is not easy to acquire all of the supported devices
>> as they are no longer sold or manufactured.
> 
> The fix that commit 1a8deec09d12 introduces is real (the commit message makes
> sense) and i don't feel confident to revert it so lightly.  I think i have all
> relevant datasheets somewhere, along with a couple of old "pegasus I" devices,
> which i could use for testing. Not at home right now, the aforementioned testing
> will have to wait a couple of days.
> 
>> That being said, if you are going to veto this change otherwise, I can of
>> course just add the FCS back for the two USB IDs, even though it likely
>> affects other devices as well.
> 
> Like i said, i don't want to hurry up and revert something that looks like a
> valid fix.  Especially after five years worth of kernel releases and no
> complaints related to 1a8deec09d12.  This should mean two things: a) the driver
> isn't used anymore, or b) this commit fixes a real problem.
> 
> However, if it turn out that your fix is the right one, it goes in without fuss.
> So lets see what it is...

I agree. It is not my intention to break something. Take your time to
test it when you find the time and let me know of the results. We are
not in a hurry. I have my private fork of the driver for the longterm
kernel.

Kind regards,
Matthias-Christian Ott

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