On Thu, Aug 31, 2023 at 03:34:37PM +1000, Michael Ellerman wrote: > Michal Suchánek <msucha...@suse.de> writes: > > Hello, > > > > thanks for working on this. > > > > On Tue, Aug 22, 2023 at 04:33:39PM -0500, Nathan Lynch via B4 Relay wrote: > >> From: Nathan Lynch <nath...@linux.ibm.com> > >> > >> PowerVM LPARs may retrieve Vital Product Data (VPD) for system > >> components using the ibm,get-vpd RTAS function. > >> > >> We can expose this to user space with a /dev/papr-vpd character > >> device, where the programming model is: > >> > >> struct papr_location_code plc = { .str = "", }; /* obtain all VPD */ > >> int devfd = open("/dev/papr-vpd", O_WRONLY); > >> int vpdfd = ioctl(devfd, PAPR_VPD_CREATE_HANDLE, &plc); > >> size_t size = lseek(vpdfd, 0, SEEK_END); > >> char *buf = malloc(size); > >> pread(devfd, buf, size, 0); > >> > >> When a file descriptor is obtained from ioctl(PAPR_VPD_CREATE_HANDLE), > >> the file contains the result of a complete ibm,get-vpd sequence. The > > > > Could this be somewhat less obfuscated? > > > > What the caller wants is the result of "ibm,get-vpd", which is a > > well-known string identifier of the rtas call. > > Not really. What the caller wants is *the VPD*. Currently that's done > by calling the RTAS "ibm,get-vpd" function, but that could change in > future. There's RTAS calls that have been replaced with a "version 2" in > the past, that could happen here too. Or the RTAS call could be replaced > by a hypercall (though unlikely). > > But hopefully if the underlying mechanism changed the kernel would be > able to hide that detail behind this new API, and users would not need > to change at all. > > > Yet this identifier is never passed in. Instead we have this new > > PAPR_VPD_CREATE_HANDLE. This is a completely new identifier, specific to > > this call only as is the /dev/papr-vpd device name, another new > > identifier. > > > > Maybe the interface could provide a way to specify the service name? > > > >> file contents are immutable from the POV of user space. To get a new > >> view of VPD, clients must create a new handle. > > > > Which is basically the same as creating a file descriptor with open(). > > Sort of. But much cleaner becuase you don't need to create a file in the > filesystem and tell userspace how to find it.
You very much do. There is the /dev/papr-vpd and PAPR_VPD_CREATE_HANDLE which userspace has to know about, the PAPR_VPD_CREATE_HANDLE is not even possible to find at all. Thanks Michal