linux-kernel - Re: [PATCH net] net: systemport: set dev->max_mtu to UMAC_MAX_MTU

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Date:   Fri, 18 Dec 2020 23:02:50 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...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 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?