Re: [PATCH 05/16] bus: mhi: core: Add support for ringing channel/event ring doorbells

Manivannan Sadhasivam
2020-01-27T07:11:03Z

On Sat, Jan 25, 2020 at 02:46:31PM +0100, Greg KH wrote:
> On Fri, Jan 24, 2020 at 03:51:12PM -0700, Jeffrey Hugo wrote:
> > > +struct mhi_event_ctxt {
> > > + u32 reserved : 8;
> > > + u32 intmodc : 8;
> > > + u32 intmodt : 16;
> > > + u32 ertype;
> > > + u32 msivec;
> > > +
> > > + u64 rbase __packed __aligned(4);
> > > + u64 rlen __packed __aligned(4);
> > > + u64 rp __packed __aligned(4);
> > > + u64 wp __packed __aligned(4);
> > > +};
> >
> > This is the struct that is shared with the device, correct? Surely it needs
> > to be packed then? Seems like you'd expect some padding between msivec and
> > rbase on a 64-bit system otherwise, which is probably not intended.
> >
> > Also I strongly dislike bitfields in structures which are shared with
> > another system since the C specification doesn't define how they are
> > implemented, therefore you can run into issues where different compilers
> > decide to implement the actual backing memory differently. I know its less
> > convinent, but I would prefer the use of bitmasks for these fields.
>
> You have to use bitmasks in order for all endian cpus to work properly
> here, so that needs to be fixed.
>

Okay.

> Oh, and if these values are in hardware, then the correct types also
> need to be used (i.e. __u32 and __u64).
>

I thought the __* prefix types are only for sharing with userspace...
Could you please clarify why it is needed here?

Thanks,
Mani

> good catch!
>
> greg k-h