Re: [Intel-gfx] [PATCH v2 6/6] drm/i915/gt: Remove bogus comment on IVB_FBC_RT_BASE_UPPER

[Kenneth Graunke <kenneth@whitecape.org>]

[-- Attachment #1: Type: text/plain, Size: 1937 bytes --]

On Saturday, June 24, 2023 10:17:57 AM PDT Lucas De Marchi wrote:
> The comment on the parameter being 0 to avoid the read back doesn't
> apply as this is not a call to wa_mcr_add(), but rather to
> wa_mcr_clr_set(). So, this register is actually checked and it's
> according to the Bspec that the register is RW, not RO.

I think you mean wa_add and wa_write_clr_set here (not mcr).

One thing I've been confused about while reading this code:

static void
wa_write_clr_set(struct i915_wa_list *wal, i915_reg_t reg, u32 clear, u32 set)
{
        wa_add(wal, reg, clear, set, clear, false);
}

The second to last parameter is read_mask aka wa->read.  We're
initializing it to the...bits to clear.  (I would think it should be
(clear | set) to pick up all modified bits.)

wa_verify seems to balk at ((cur ^ wa->set) & wa->read).  But...if
wa->read is just the clear mask, that wouldn't actually verify that
any bits were set at all.  Or am I misunderstanding something?

If not, we may be failing to verify the majority of our workarounds :(

> Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_workarounds.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> index 848519b58e45..5fe85fad91c1 100644
> --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> @@ -666,7 +666,7 @@ static void icl_ctx_workarounds_init(struct intel_engine_cs *engine,
>  	/* Wa_1604278689:icl,ehl */
>  	wa_write(wal, IVB_FBC_RT_BASE, 0xFFFFFFFF & ~ILK_FBC_RT_VALID);
>  	wa_write_clr_set(wal, IVB_FBC_RT_BASE_UPPER,
> -			 0, /* write-only register; skip validation */
> +			 0,
>  			 0xFFFFFFFF);
>  
>  	/* Wa_1406306137:icl,ehl */

In this particular example, since clear bits are 0, I don't think any
verification is happening at all.

--Ken

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 833 bytes --]