Hello Florian, On Fri, 22 Apr 2022 11:28:30 -0700 Florian Fainelli <f.fainelli@xxxxxxxxx> wrote: Thanks for the review :) > On 4/22/22 11:03, Maxime Chevallier wrote: > > This tagging protocol is designed for the situation where the link > > between the MAC and the Switch is designed such that the Destination > > Port, which is usually embedded in some part of the Ethernet > > Header, is sent out-of-band, and isn't present at all in the > > Ethernet frame. > > > > This can happen when the MAC and Switch are tightly integrated on an > > SoC, as is the case with the Qualcomm IPQ4019 for example, where > > the DSA tag is inserted directly into the DMA descriptors. In that > > case, the MAC driver is responsible for sending the tag to the > > switch using the out-of-band medium. To do so, the MAC driver needs > > to have the information of the destination port for that skb. > > > > This tagging protocol relies on a new set of fields in skb->shinfo > > to transmit the dsa tagging information to and from the MAC driver. > > > > Signed-off-by: Maxime Chevallier <maxime.chevallier@xxxxxxxxxxx> > > First off, I am not a big fan of expanding skb::shared_info because > it is sensitive to cache line sizes and is critical for performance > at much higher speeds, I would expect Eric and Jakub to not be > terribly happy about it. No problem, I'm testing with the skb->cb approach as you suggested and see how it goes. > The Broadcom systemport (bcmsysport.c) has a mode where it can > extract the Broadcom tag and put it in front of the actual packet > contents which appears to be very similar here. From there on, you > can have two strategies: > > - have the Ethernet controller mangle the packet contents such that > the QCA tag is located in front of the actual Ethernet frame and > create a new tagging protocol variant for QCA, similar to the > TAG_BRCM versus TAG_BRCM_PREPEND > > - provide the necessary information for the tagger to work using an > out of band mechanism, which is what you have done, in which case, > maybe you can use skb->cb[] instead of using skb::shared_info? One of the reason why I chose the second is to support possible future cases where another controller would face a similar situation, and also make use of the out-of-band tagger. I understand that it's not very elegant in the sense that this breaks the nice tagging model we have, but adding/removing data before the payload also seems convoluted to achieve the same thing :) It seems that this approach comes with a bit of an overhead since it implies mangling the skb a bit, but I've yet to test this myself. That's actually what I wanted your opinion on, it also seems like Andrew likes the idea of putting the tag ahead of the frame to stick with the actual model. I don't have strong feelings myself on the way of doing this, I'm looking for an approach that is efficient but yet easily maintainable. Thanks, Maxime
