Re: [PATCH] drm/i915: Use a cached mapping for the physical HWS

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

On ke, 2017-05-17 at 14:02 +0100, Chris Wilson wrote:
> Older gen use a physical address for the hardware status page, for which
> we use cache-coherent writes. As the writes are into the cpu cache, we use
> a normal WB mapped page to read the HWS, used for our seqno tracking.
> 
> Anecdotally, I observed lost breadcrumbs writes into the HWS on i965gm,
> which so far have not reoccurred with this patch. How reliable that
> evidence is remains to be seen.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

<SNIP>

> @@ -1091,17 +1094,22 @@ static int init_status_page(struct intel_engine_cs *engine)
>  
>  static int init_phys_status_page(struct intel_engine_cs *engine)
>  {
> -	struct drm_i915_private *dev_priv = engine->i915;
> +	struct page *page;
>  
> -	GEM_BUG_ON(engine->id != RCS);

Was this removal deliberate?

> +	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> +	if (!page)
> +		return -ENOMEM;
>  
> -	dev_priv->status_page_dmah =
> -		drm_pci_alloc(&dev_priv->drm, PAGE_SIZE, PAGE_SIZE);
> -	if (!dev_priv->status_page_dmah)
> +	engine->status_page.dma_addr =
> +		dma_map_page(engine->i915->drm.dev, page, 0, PAGE_SIZE,
> +			     PCI_DMA_BIDIRECTIONAL);
> +	if (dma_mapping_error(engine->i915->drm.dev,
> +			      engine->status_page.dma_addr)) {
> +		__free_page(page);
>  		return -ENOMEM;

Nitpicking, but -ENOSPC would be more accurate?

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

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