[Intel-gfx] [PATCH v2 06/19] drm/i915/dsb: Avoid corrupting the first register write

[Manna, Animesh]

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Ville
> Syrjala
> Sent: Wednesday, June 7, 2023 12:45 AM
> To: intel-gfx at lists.freedesktop.org
> Subject: [Intel-gfx] [PATCH v2 06/19] drm/i915/dsb: Avoid corrupting the first
> register write
> 
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> i915_gem_object_create_internal() does not hand out zeroed memory. Thus
> we may confuse whatever stale garbage is in there as a previous register
> write and mistakenly handle the first actual register write as an indexed
> write. This can end up corrupting the instruction sufficiently well to lose the
> entire register write.
> 
> Make sure we've actually emitted a previous instruction before attemting
> indexed register write merging.
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

LGTM.
Signed-off-by: Animesh Manna <animesh.manna at intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_dsb.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c
> b/drivers/gpu/drm/i915/display/intel_dsb.c
> index 093b2567883d..a20ae5313d23 100644
> --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> @@ -141,6 +141,14 @@ static bool intel_dsb_prev_ins_is_write(struct
> intel_dsb *dsb,
>  	const u32 *buf = dsb->cmd_buf;
>  	u32 prev_opcode, prev_reg;
> 
> +	/*
> +	 * Nothing emitted yet? Must check before looking
> +	 * at the actual data since i915_gem_object_create_internal()
> +	 * does *not* give you zeroed memory!
> +	 */
> +	if (dsb->free_pos == 0)
> +		return false;
> +
>  	prev_opcode = buf[dsb->ins_start_offset + 1] >>
> DSB_OPCODE_SHIFT;
>  	prev_reg = buf[dsb->ins_start_offset + 1] & DSB_REG_VALUE_MASK;
> 
> --
> 2.39.3