Hello Jakub, On Mon, 16 May 2022 12:20:48 -0700 Jakub Kicinski <kuba@xxxxxxxxxx> wrote: > On Sat, 14 May 2022 17:06:53 +0200 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 out-of-band tagging protocol is using the very beggining of > > the skb headroom to store the tag. The drawback of this approch is > > that the headroom isn't initialized upon allocating it, therefore > > we have a chance that the garbage data that lies there at > > allocation time actually ressembles a valid oob tag. This is only > > problematic if we are sending/receiving traffic on the master port, > > which isn't a valid DSA use-case from the beggining. When dealing > > from traffic to/from a slave port, then the oob tag will be > > initialized properly by the tagger or the mac driver through the > > use of the dsa_oob_tag_push() call. > > > > Signed-off-by: Maxime Chevallier <maxime.chevallier@xxxxxxxxxxx> > > This must had been asked on v1 but there's no trace of it in the > current submission afaict... No you're correct, this wasn't explained. > If the tag is passed in the descriptor how is this not a pure > switchdev driver? The explanation must be preserved somehow. The main reason is that although the MAC and switch are rightly coupled on that platform, the switch is actually a QC8K that can live on it's own, as an external switch. Here, it's just a slightly modified version of this IP. The same goes for the MAC IP, but so far we don't support any other platform that have the MAC as a standalone controller. As far as we can tell, platforms that have this MAC also include a QCA8K, but the datasheet also mentions other modes (like outputing RGMII). Is this valid to have it as a standalone ethernet driver in that situation ? Thanks, Maxime
