[Mesa-dev] [PATCH] i965: Extend the negative 32-bit deltas to 64-bits

[Chris Wilson]

Quoting Sergii Romantsov (2018-03-26 13:16:24)
> Negative deltas are used to fake a range in a large buffer.
> See 900a5c91eeb3
> "i965: Use negative relocation deltas to minimise vertex uploads"
> 
> Gen8+ use 48-bit address relocations so need to extend the sign

Note that 48-bit relocations were only switched on in
commit cee9f3890351 ("i965: Allow 48-bit addressing on Gen8+.")
to save having to backport too far (although the patch is trivial).

> to 64-bit return value. Without it we have higher bits zeroed
> and missing the negavive values.
> Haswell and older use 32-bit deltas so are unaffected by this issue.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101408
> Signed-off-by: Sergii Romantsov <sergii.romantsov at globallogic.com>
> Tested-by: Andriy Khulap <andriy.khulap at globallogic.com>
> ---
>  src/mesa/drivers/dri/i965/intel_batchbuffer.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/intel_batchbuffer.c b/src/mesa/drivers/dri/i965/intel_batchbuffer.c
> index d824ff2..128da77 100644
> --- a/src/mesa/drivers/dri/i965/intel_batchbuffer.c
> +++ b/src/mesa/drivers/dri/i965/intel_batchbuffer.c
> @@ -1124,8 +1124,10 @@ emit_reloc(struct intel_batchbuffer *batch,
>     /* Using the old buffer offset, write in what the right data would be, in
>      * case the buffer doesn't move and we can short-circuit the relocation
>      * processing in the kernel
> +    *
> +    * Some target_offsets may be negative, so extend the sign to 64 bits.
>      */
> -   return entry->offset + target_offset;
> +   return entry->offset + (int64_t)((int32_t)target_offset);

Although just changing s/uint32_t target_offset/int32_t target_offset/
may be cleaner.
-Chris