[Intel-gfx] [PATCH 2/4] drm/i915: Skip the execlists CSB scan and rewrite if the ring is untouched

[Joonas Lahtinen]

On la, 2017-01-21 at 09:28 +0000, Chris Wilson wrote:
> If the CSB head/tail pointers are unchanged, we can skip the update of
> the CSB register afterwards.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

<SNIP>

> @@ -577,7 +577,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>  
>  	intel_uncore_forcewake_get(dev_priv, engine->fw_domains);
>  
> -	if (!execlists_elsp_idle(engine)) {
> +	if (!execlists_elsp_idle(engine)) do {

Oh dear, not like this. I hope this was a leftover from debugging.

> @@ -587,9 +587,12 @@ static void intel_lrc_irq_handler(unsigned long data)
>  		csb = readl(csb_mmio);
>  		head = GEN8_CSB_READ_PTR(csb);
>  		tail = GEN8_CSB_WRITE_PTR(csb);
> +		if (head == tail)
> +			break;
> +
>  		if (tail < head)
>  			tail += GEN8_CSB_ENTRIES;
> -		while (head < tail) {
> +		do {
>  			unsigned int idx = ++head % GEN8_CSB_ENTRIES;
>  			unsigned int status = readl(buf + 2 * idx);

Theoretically it should not matter, I wonder if this is covered well
enough in CI that we're confident to skip such writes?

Code is;

Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation