Re: [PATCH] dma-mapping: Relax warnings for per-device areas

[Fredrik Noring]

Thank you Robin,

On Tue, Jul 03, 2018 at 02:08:30PM +0100, Robin Murphy wrote:
> The reasons why dma_free_attrs() should not be called from IRQ context
> are not necessarily obvious and somewhat buried in the development
> history, so let's start by documenting the warning itself to help anyone
> who does happen to hit it and wonder what the deal is.
> 
> However, this check turns out to be slightly over-restrictive for the
> way that per-device memory has been spliced into the general API, since
> for that case we know that dma_declare_coherent_memory() has created an
> appropriate CPU mapping for the entire area and nothing dynamic should
> be happening. Given that the usage model for per-device memory is often
> more akin to streaming DMA than 'real' coherent DMA (e.g. allocating and
> freeing space to copy short-lived packets in and out), it is also
> somewhat more reasonable for those operations to happen in IRQ handlers
> for such devices.
> 
> A somewhat similar line of reasoning also applies at the other end for
> the mask check in dma_alloc_attrs() too - indeed, a device which cannot
> access anything other than its own local memory probably *shouldn't*
> have a valid mask for the general coherent DMA API.
> 
> Therefore, let's move the per-device area hooks up ahead of the assorted
> checks, so that they get a chance to resolve the request before we get
> as far as definite "you're doing it wrong" territory.

I have tested this patch and it corrects both problems with the PS2 OHCI
driver. I believe there is a fair chance that drivers/usb/host/ohci-sm501.c
and drivers/usb/host/ohci-tmio.c are fixed as well, since they are similar.

Tested-by: Fredrik Noring <noring@xxxxxxxxxx>

Fredrik

> Reported-by: Fredrik Noring <noring@xxxxxxxxxx>
> Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> ---
>  include/linux/dma-mapping.h | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index f9cc309507d9..ffeca3ab59c0 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -512,12 +512,12 @@ static inline void *dma_alloc_attrs(struct device *dev, size_t size,
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  	void *cpu_addr;
>  
> -	BUG_ON(!ops);
> -	WARN_ON_ONCE(dev && !dev->coherent_dma_mask);
> -
>  	if (dma_alloc_from_dev_coherent(dev, size, dma_handle, &cpu_addr))
>  		return cpu_addr;
>  
> +	BUG_ON(!ops);
> +	WARN_ON_ONCE(dev && !dev->coherent_dma_mask);
> +
>  	/* let the implementation decide on the zone to allocate from: */
>  	flag &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
>  
> @@ -537,12 +537,19 @@ static inline void dma_free_attrs(struct device *dev, size_t size,
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
> -	BUG_ON(!ops);
> -	WARN_ON(irqs_disabled());
> -
>  	if (dma_release_from_dev_coherent(dev, get_order(size), cpu_addr))
>  		return;
>  
> +	BUG_ON(!ops);
> +	/*
> +	 * On non-coherent platforms which implement DMA-coherent buffers via
> +	 * non-cacheable remaps, ops->free() may call vunmap(). Thus arriving
> +	 * here in IRQ context is a) at risk of a BUG_ON() or trying to sleep
> +	 * on some machines, and b) an indication that the driver is probably
> +	 * misusing the coherent API anyway.
> +	 */
> +	WARN_ON(irqs_disabled());
> +
>  	if (!ops->free || !cpu_addr)
>  		return;
>  
> -- 
> 2.17.1.dirty
>