Hi Florian, On Sat, 14 May 2022 09:33:44 -0700 Florian Fainelli <f.fainelli@xxxxxxxxx> wrote: > Hi Maxime, > > On 5/14/2022 8:06 AM, 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. > > What I like about your approach is that you have aligned the way an > out of band switch tag is communicated to the networking stack the > same way that an "in-band" switch tag would be communicated. I think > this is a good way forward to provide the out of band tag and I don't > think it creates a performance problem because the Ethernet frame is > hot in the cache (dma_unmap_single()) and we already have an > "expensive" read of the DMA descriptor in coherent memory anyway. > > You could possibly optimize the data flow a bit to limit the amount > of sk_buff data movement by asking your Ethernet controller to DMA > into the data buffer N bytes into the beginning of the data buffer. > That way, if you have reserved say, 2 bytes at the front data buffer > you can deposit the QCA tag there and you do not need to push, > process the tag, then pop it, just process and pop. Consider using > the 2byte stuffing that the Ethernet controller might be adding to > the beginning of the Ethernet frame to align the IP header on a > 4-byte boundary to provide the tag in there? > > If we want to have a generic out of band tagger like you propose, it > seems to me that we will need to invent a synthetic DSA tagging > format which is the largest common denominator of the out of band > tags that we want to support. We could imagine being more compact in > the representation for instance by using an u8 for storing a bitmask > of ports (works for both RX and TX then) and another u8 for various > packet forwarding reasons. Thanks, that was my initial idea indeed. Having a generic tagger that can be re-used would be great IMO. I'll modify the format as you propose, and also give a try to you approach of DMA'ing 2 bytes forward so that the tag location is already allocated, that's a nice idea. > Then we would request the various Ethernet MAC drivers to marshall > their proprietary tag into the DSA synthetic one on receive, and > unmarshall it on transmit. > > Another approach IMHO which maybe helps the maintainability of the > code moving forward as well as ensuring that all Ethernet switch > tagging code lives in one place, is to teach each tagger driver how > to optimize their data paths to minimize the amount of data movements > and checksum re-calculations, this is what I had in mind a few years > ago: > > https://lore.kernel.org/lkml/1438322920.20182.144.camel@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/ > > This might scale a little less well, and maybe this makes too many > assumptions as to where and how the checksums are calculated on the > packet contents, but at least, you don't have logic processing the > same type of switch tag scattered between the Ethernet MAC drivers > (beyond copying/pushing) and DSA switch taggers. That would definitely fit well with this tagger, I didn't know about that series ! Thanks for the review, Maxime > I would like to hear other's opinion on this.
