Messages in this thread | | | Date | Tue, 23 Jan 2018 23:18:06 -0500 | From | Jon Mason <> | Subject | Re: [PATCH] NTB: ntb_perf: fix cast to restricted __le32 |
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On Fri, Jan 19, 2018 at 10:26:37PM +0100, Arnd Bergmann wrote: > On Fri, Jan 19, 2018 at 10:03 PM, Serge Semin <fancer.lancer@gmail.com> wrote: > > > > Actually the provided patch is the best solution I could come up with. > > The thing is, that the methods can't be changed. Those functions are > > the part of the NTB API methods used by many drivers. So basically they > > are like pci_{read,write}_config_{byte,word,dword}() methods. We can't > > change their prototypes only because it's suit some driver. The methods > > give an access to the NTB device dummy u32-sized registers, nothing > > else. So endianness is the transmitted data settings in this case. > > > > NTB is the technology to interconnect some two systems with possibly > > different endianness (unlike PCI, which interconnect CPU with LE devices). > > In this case I'd need to set some agreement up between two systems about > > the endianness of the exchanged data like host and network types in > > Linux networking. I've chosen the network data to be little-endian, > > that's why I needed first to convert them from CPU to le32, then on > > remote side convert them back from le32 to CPU. > > > > If you have any better suggestion how the warning can be fixed, I'd > > be glad to stick to it. > > I don't think your description matches what you actually do: The > underlying ntb hardware drivers (amd, idt, intel, mscc) all treat the > incoming data as CPU-endian and convert it to little-endian on > the register side, so the framework already assumes that whatever > you do here uses a little-endian wire-level protocol. > > On a little-endian kernel/CPU, nothing is ever swapped here, neither > in the ntb_perf front-end nor in the back-ends. On a big-endian > kernel/CPU, they both swap, so you end up with CPU-endian > data on the wire, so it should be impossible for a big-endian > system to talk to a little-endian one. Have you actually tried that > combination with the current code?
I do not believe anyone has every tried NTB on a big endien system, let alone tried it with one side LE and the other BE. To my knowledge, this has only ever been used on x86 to x86.
> If my interpretation is correct, then the best solution would be to > completely remove the cpu_to_le32/le32_to_cpu conversions > from ntb_perf, and just define that it works like any other PCI > device, exchanging little-endian data.
Yes, this would be the best solution. Thank you for the insight.
Serge, when you get a chance, please make this change and resumbit.
Thanks, Jon
> > There are two interesting cases to consider though: > > - if someone wants to implement an NTB based protocol > using big-endian data on the wire, you probably want to add > a ntb_peer_spad_read_be()/ntb_peer_msg_write_be() > set of interfaces, to go along with ioread32_be()/iowrite32_be() > the same way that ntb_peer_spad_read()/ntb_peer_msg_write() > ends up doing ioread32()/iowrite32() with the implied little-endian > behavior. > > - memcpy_toio()/memcpy_fromio() and ioread32_rep()/iowrite32_rep > importantly do not do any byteswap, they are meant to > transfer byte streams. > > Arnd
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