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Date:   Fri, 18 Dec 2020 13:17:59 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     netdev@...r.kernel.org, andrew@...n.ch,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Murali Krishna Policharla <murali.policharla@...adcom.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        "open list:BROADCOM SYSTEMPORT ETHERNET DRIVER" 
        <bcm-kernel-feedback-list@...adcom.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: systemport: set dev->max_mtu to
 UMAC_MAX_MTU_SIZE

On 12/18/20 1:14 PM, Vladimir Oltean wrote:
> On Fri, Dec 18, 2020 at 01:08:58PM -0800, Florian Fainelli wrote:
>> On 12/18/20 1:02 PM, Vladimir Oltean wrote:
>>> On Fri, Dec 18, 2020 at 12:54:33PM -0800, Florian Fainelli wrote:
>>>> On 12/18/20 12:52 PM, Vladimir Oltean wrote:
>>>>> On Fri, Dec 18, 2020 at 12:30:20PM -0800, Florian Fainelli wrote:
>>>>>> On 12/18/20 12:24 PM, Vladimir Oltean wrote:
>>>>>>> Hi Florian,
>>>>>>>
>>>>>>> On Fri, Dec 18, 2020 at 09:38:43AM -0800, Florian Fainelli wrote:
>>>>>>>> The driver is already allocating receive buffers of 2KiB and the
>>>>>>>> Ethernet MAC is configured to accept frames up to UMAC_MAX_MTU_SIZE.
>>>>>>>>
>>>>>>>> Fixes: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
>>>>>>>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>>>>>>>> ---
>>>>>>>>  drivers/net/ethernet/broadcom/bcmsysport.c | 1 +
>>>>>>>>  1 file changed, 1 insertion(+)
>>>>>>>>
>>>>>>>> diff --git a/drivers/net/ethernet/broadcom/bcmsysport.c b/drivers/net/ethernet/broadcom/bcmsysport.c
>>>>>>>> index 0fdd19d99d99..b1ae9eb8f247 100644
>>>>>>>> --- a/drivers/net/ethernet/broadcom/bcmsysport.c
>>>>>>>> +++ b/drivers/net/ethernet/broadcom/bcmsysport.c
>>>>>>>> @@ -2577,6 +2577,7 @@ static int bcm_sysport_probe(struct platform_device *pdev)
>>>>>>>>  			 NETIF_F_HW_VLAN_CTAG_TX;
>>>>>>>>  	dev->hw_features |= dev->features;
>>>>>>>>  	dev->vlan_features |= dev->features;
>>>>>>>> +	dev->max_mtu = UMAC_MAX_MTU_SIZE;
>>>>>>>>  
>>>>>>>>  	/* Request the WOL interrupt and advertise suspend if available */
>>>>>>>>  	priv->wol_irq_disabled = 1;
>>>>>>>> -- 
>>>>>>>> 2.25.1
>>>>>>>>
>>>>>>>
>>>>>>> Do you want to treat the SYSTEMPORT Lite differently?
>>>>>>>
>>>>>>> 	/* Set maximum frame length */
>>>>>>> 	if (!priv->is_lite)
>>>>>>> 		umac_writel(priv, UMAC_MAX_MTU_SIZE, UMAC_MAX_FRAME_LEN);
>>>>>>> 	else
>>>>>>> 		gib_set_pad_extension(priv);
>>>>>>
>>>>>> SYSTEMPORT Lite does not actually validate the frame length, so setting
>>>>>> a maximum number to the buffer size we allocate could work, but I don't
>>>>>> see a reason to differentiate the two types of MACs here.
>>>>>
>>>>> And if the Lite doesn't validate the frame length, then shouldn't it
>>>>> report a max_mtu equal to the max_mtu of the attached DSA switch, plus
>>>>> the Broadcom tag length? Doesn't the b53 driver support jumbo frames?
>>>>
>>>> And how would I do that without create a horrible layering violation in
>>>> either the systemport driver or DSA? Yes the b53 driver supports jumbo
>>>> frames.
>>>
>>> Sorry, I don't understand where is the layering violation (maybe it doesn't
>>> help me either that I'm not familiar with Broadcom architectures).
>>>
>>> Is the SYSTEMPORT Lite always used as a DSA master, or could it also be
>>> used standalone? What would be the issue with hardcoding a max_mtu value
>>> which is large enough for b53 to use jumbo frames?
>>
>> SYSTEMPORT Lite is always used as a DSA master AFAICT given its GMII
>> Integration Block (GIB) was specifically designed with another MAC and
>> particularly that of a switch on the other side.
>>
>> The layering violation I am concerned with is that we do not know ahead
>> of time which b53 switch we are going to be interfaced with, and they
>> have various limitations on the sizes they support. Right now b53 only
>> concerns itself with returning JMS_MAX_SIZE, but I am fairly positive
>> this needs fixing given the existing switches supported by the driver.
> 
> Maybe we don't need to over-engineer this. As long as you report a large
> enough max_mtu in the SYSTEMPORT Lite driver to accomodate for all
> possible revisions of embedded switches, and the max_mtu of the switch
> itself is still accurate and representative of the switch revision limits,
> I think that's good enough.

I suppose that is fair, v2 coming, thanks!
-- 
Florian

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