Running on bare hardware

Dave Airlie airlied at
Sun Feb 6 13:28:35 PST 2011

On Mon, Feb 7, 2011 at 6:48 AM, Marty Jack <martyj19 at> wrote:
> Well, I have over 40 years experience, a lot of it on Unix and more recently Linux and a lot of it doing compilers, so data typing is second nature.
> But, I'm getting a little older, I could be mistaken and I'm always happy to learn something new.  Maybe sometime when you have a moment you would be so kind as to explain how this parameter block works successfully for DRM_IOCTL_VERSION and the modesetting ones wouldn't if they had pointer types in the 32/64 scenario you raised.  (We did have the exact same situation on Tru64 Unix running a userspace program compiled with 32-bit pointers.)
> struct drm_version {
>        int version_major;        /**< Major version */
>        int version_minor;        /**< Minor version */
>        int version_patchlevel;   /**< Patch level */
>        size_t name_len;          /**< Length of name buffer */
>        char *name;       /**< Name of driver */
>        size_t date_len;          /**< Length of date buffer */
>        char *date;       /**< User-space buffer to hold date */
>        size_t desc_len;          /**< Length of desc buffer */
>        char *desc;       /**< User-space buffer to hold desc */
> };

Look at that structs built on a 32-bit compiler and a 64-bit compiler,
notice the alignment differences and size differences, so when a 32-bit
struct gets passed to the kernel we need to add an additional layer of
compatiblity, have a look at drm_ioc32.c, this is an extra copy of stuff
to fix up all the 32/64 issues.

The proper way is to design ioctls so compat layers aren't needed, to work
across all 32/64 combos, so that means using 64-bit aligned types as much
as possible, and padding to make sure 64-bit types don't end up unaligned.

We then use libdrm in userspace to abstract away the internal details
of the interface,
you shouldn't ever be directly talking to drm ioctls.

x86-64 and ppc have different requirements on 64-bit alignments just
to make things more interesting.


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