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From: Vladimir Oltean <olteanv@gmail.com>
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: netdev@vger.kernel.org, andrew@lunn.ch,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Murali Krishna Policharla <murali.policharla@broadcom.com>,
	Vladimir Oltean <vladimir.oltean@nxp.com>,
	"open list:BROADCOM SYSTEMPORT ETHERNET DRIVER" 
	<bcm-kernel-feedback-list@broadcom.com>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net] net: systemport: set dev->max_mtu to UMAC_MAX_MTU_SIZE
Date: Fri, 18 Dec 2020 23:14:35 +0200	[thread overview]
Message-ID: <20201218211435.mjdknhltolu4gvqr@skbuf> (raw)
In-Reply-To: <cf2daa3c-8f64-0f58-5a42-2182cec5ba4a@gmail.com>

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@gmail.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.

  reply	other threads:[~2020-12-18 21:15 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-18 17:38 Florian Fainelli
2020-12-18 20:24 ` Vladimir Oltean
2020-12-18 20:30   ` Florian Fainelli
2020-12-18 20:52     ` Vladimir Oltean
2020-12-18 20:54       ` Florian Fainelli
2020-12-18 21:02         ` Vladimir Oltean
2020-12-18 21:08           ` Florian Fainelli
2020-12-18 21:14             ` Vladimir Oltean [this message]
2020-12-18 21:17               ` Florian Fainelli
2020-12-21 21:49                 ` Florian Fainelli
2020-12-21 22:25                   ` Vladimir Oltean
2020-12-21 22:55                     ` Florian Fainelli

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