On Mon, Dec 18, 2017 at 3:54 PM, Russell King - ARM Linux <li...@armlinux.org.uk> wrote: > On Mon, Dec 18, 2017 at 03:48:17PM +0100, Michał Mirosław wrote: >> On Mon, Dec 18, 2017 at 02:57:37PM +0100, Linus Walleij wrote: >> > On Sat, Dec 16, 2017 at 8:39 PM, Linus Walleij <linus.wall...@linaro.org> >> > wrote: >> > >> > > The Gemini ethernet has been around for years as an out-of-tree >> > > patch used with the NAS boxen and routers built on StorLink >> > > SL3512 and SL3516, later Storm Semiconductor, later Cortina >> > > Systems. These ASICs are still being deployed and brand new >> > > off-the-shelf systems using it can easily be acquired. >> [...] >> > > --- >> > > Changes from v8: >> > > - Remove dependency guards in Kconfig to get a wider compile >> > > coverage for the driver to detect broken APIs etc. >> > >> > I guess we need to hold this off for a while, the code does >> > some weird stuff using the ARM-internal page DMA mapping >> > API. >> > >> > I *think* what happens is that the driver allocates a global queue >> > used for RX and TX on both interfaces, then initializes that with >> > page pointers and gives that to the hardware to play with. >> > >> > When an RX packet comes in, the RX routine needs to figure >> > out from the DMA (physical) address which remapped >> > page/address this random physical address pointer >> > corresponds to. >> > >> > The Linux DMA API assumption is that the driver keeps track >> > of this mapping, not the hardware. So we need to figure out >> > a way to reverse-map this. Preferably quickly, and without >> > using any ARM-internal mapping APIs. >> >> IIRC, the hardware copies descriptors from free queue (FREEQ) >> to RX queues. FREEQ is shared among the two ethernet ports.
Seems like that to me too. I will try to refactor and break it apart a bit. The way freeq works is undocumented, even in the official datasheet for CS3516 (the memory area is just "reserved"), so the code is the only documentation of it. >> This platform is CPU bound, so every additional lookup will >> hit performance here. In my version I had an #ifdef for >> COMPILE_TEST that replaced ARM-specific calls with stubs. >> Since the driver is not expected to work on other platforms, >> this seemed like the best workaround to make it compile >> on other arches. > > Really. No. Stop going beneath the covers and using ARM private > implementation APIs in drivers. > > Take that as a big NAK to that. Don't worry, it won't happen. I am already thinking about better approaches that stay with the public DMA-API. > (I don't seem have the patch in question here to look at though.) I'll put you on CC in future postings. Yours, Linus Walleij