[PATCH v4 02/10] mei: late_bind: add late binding component driver

[Usyskin, Alexander]

> Subject: Re: [PATCH v4 02/10] mei: late_bind: add late binding component
> driver
> 
> On Tue, Jul 01, 2025 at 10:11:54PM +0530, Nilawar, Badal wrote:
> >
> > On 01-07-2025 18:04, Nilawar, Badal wrote:
> > >
> > > On 01-07-2025 15:15, Greg KH wrote:
> > > > On Tue, Jul 01, 2025 at 02:02:15PM +0530, Nilawar, Badal wrote:
> > > > > On 28-06-2025 17:48, Greg KH wrote:
> > > > > > > + * @payload_size: size of the payload data in bytes
> > > > > > > + * @payload: data to be sent to the firmware
> > > > > > > + */
> > > > > > > +struct csc_heci_late_bind_req {
> > > > > > > +    struct mkhi_msg_hdr header;
> > > > > > > +    u32 type;
> > > > > > > +    u32 flags;
> > > > > > What is the endian of these fields?  And as this crosses the
> > > > > > kernel/hardware boundry, shouldn't these be __u32?
> > > > > endian of these fields is little endian, all the headers are
> > > > > little endian.
> > > > > I will add comment at top.
> > > > No, use the proper types if this is little endian.  Don't rely on a
> > > > comment to catch things when it goes wrong.
> > > >
> > > > > On __u32 I doubt we need to do it as csc send copy it to
> > > > > internal buffer.
> > > > If this crosses the kernel boundry, it needs to use the proper type.
> > >
> > > Understood. I will proceed with using __le32 in this context, provided
> > > that Sasha agrees.
> >
> > I believe __le{32, 16} is used only when the byte order is fixed and matches
> > the host system's native endianness. Since the CSC controller is
> > little-endian, is it necessary to specify the endianness here?
> > If it is mandatory to use the __le{32, 16} endian type, then is there need
> > to convert endianness using cpu_to_le and le_to_cpu?
> 
> I honestly don't believe that specifying endianness here is **needed**.
> I mean, it might be future safe to use the __le32 and
> flags = cpu_to_le32(1 << 0) just in case someone decide to port all the
> GPU code to run in big-endian CPU. Very unlikely I'd say, and much more cases
> to resolve before we get to this gpu use case here I'm afraid.
> 
> Weel, unless this mei here can be used outside of GPU context?!
> 

There is nothing useful in this outside of GPU context.
This module is tailored for GPU use-case.
If Xe driver is bound to be little-endian, this one should be too.
Other similar modules use u32.

- - 
Thanks,
Sasha


> >
> > >
> > > >
> > > > > > > +{
> > > > > > > +    struct mei_cl_device *cldev;
> > > > > > > +    struct csc_heci_late_bind_req *req = NULL;
> > > > > > > +    struct csc_heci_late_bind_rsp rsp;
> > > > > > > +    size_t req_size;
> > > > > > > +    ssize_t ret;
> > > > > > > +
> > > > > > > +    if (!dev || !payload || !payload_size)
> > > > > > > +        return -EINVAL;
> > > > > > How can any of these ever happen as you control the callers of this
> > > > > > function?
> > > > > I will add WARN here.
> > > > So you will end up crashing the machine and getting a CVE assigned for
> > > > it?
> > > >
> > > > Please no.  If it can't happen, then don't check for it.  If it can
> > > > happen, great, handle it properly.  Don't just give up and cause a
> > > > system to reboot, that's a horrible way to write kernel code.
> 
> I agree here that the WARN is not a good way to handle that.
> We either don't check (remove it) or handle properly (keep as is).
> 
> With the context of where this driver is used I'd say it can't happen.
> Since xe is properly setting it right now and I don't believe we have
> other usages of this mei driver here.
> 
> But if there's a chance of this getting used outside of xe, then
> we need to keep the check...
> 
> But if you keep the check, then also use __lb32() because we need
> some consistency in the reasoning, one way or the other.
> 
> > >
> > > Fine, will drop the idea of WARN here.
> > >
> > > Thanks,
> > > Badal
> > >
> > > >
> > > > thanks,
> > > >
> > > > greg k-h